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Setting Bias on Music Man Amps


Steve Kennedy
Posted on Wednesday, March 20, 2002 - 02:08 pm:   

Music Man Amplifiers


A. This applies to all models containing the following circuit boards:
DB-2, DB-3, DB-4, GP-1, GP-2, GP-3, GP-3A,GD-1,GD-2 AND GD-2A.

B. Adjustment is as follows:

1. Turn the amplifier to "ON" with the HI / LO Standby switch in the HI
position. No Signal.

2. Using a voltmeter measure the voltage from emitter to ground on each of
of the two driver transistors. Across the 3.9 OHM emitter resistors is a
convenient measuring point.

3. Adjust the bias trimpot (TR-1) until you read 25mv DC across the 3.9
OHM emitter resistors. If there is a difference in voltage between the
emitters of the two driver transistors, set the lower of the two to 25mv.
The higher of the two should not exceed 55mv DC.

NOTE: Use only a 1458 op amp to drive the output section.
This applies to all models "not using 12AX7 driver tube".


Section II GRID BIAS CALIBRATION (This applies to all models "using 12AX7 DRIVER TUBE")

A. This applies to all models using a grid drive. (12AX7 driver tube).

B. Adjustment is as follows:

1. Turn the amplifier to "on" with the HI/LO Switch in the HI position.
No signal.

2. Using a volt meter, measure the voltage from cathode (pin 8) to ground
on the output tubes. Across the 10 OHM cathode resistor is a
convenient measuring point.

3. Adjust the bias trimpot located farthest to the right (looking from the
front) on the GB-1 and BB-1 circuit boards until you read .5 volts DC
across the 10 OHM cathode resistor.

NOTE: Be sure tubes are warmed up before adjusting the bias voltage.

David Hecht
Posted on Tuesday, April 23, 2002 - 05:59 pm:   

Is there anything special to know regarding the bias on my MM 110RD Fifty? (I assume its fixed and non-adjustable) How about tubes that work well (or poorly) in these amps? I'm not a tech, just a player and MM afficionado. I love my amp and want to keep her happy for a LONG, LONG TIME.

David Hecht
Posted on Tuesday, April 23, 2002 - 06:29 pm:   

Is there anything special to know regarding the bias on my MM 110RD Fifty? (I assume its fixed and non-adjustable) How about tubes that work well (or poorly) in these amps? I'm not a tech, just a player and MM afficionado. I love my amp and want to keep her happy for a LONG, LONG TIME.

Steve Kennedy
Posted on Sunday, April 28, 2002 - 11:57 pm:   

The 1650-RD chassis (110RD-50 & 112RD-50 models) appears to be a self-biasing design so all you should have to pay attention to is to use matched pairs of output tubes to keep the amp happy.


Lewis Langley
Posted on Sunday, May 26, 2002 - 07:50 pm:   

Re: 210-65, chassis: 2275-65 Ser# BN09734.
Transistorized driver board. Bias.

A prior posting provides a driver transistor calibration proceedure valid for circuits ---
"DB-2, DB-3, DB-4, GP-1, GP-2, GP-3, GP-3A,
GD-1,GD-2 AND GD-2A.".

There are no markings on any of the boards in my 210-65 (except for some penciled initials).
The schematics (my amp came with the wrong one, back in '79) lead me to believe that my amp's
pre-amp board may be a "G B-1" - but this documentation only appears on the schematic for the 12AX7 equiped driver boards. GB-1 isn't on the above 'bias calibration' list, anyway.

Do you know what pre-amp circuit board designation my amp actually has?

Since "G B-1"'s not on your list (as shown above),
do you know what the correct transistor bias
calibration procedure and nominal values are?

Lewis Langley.

BTW - Solid state driver schematics indicate
a 20 - 25 volt drop (if I'm reading
this right) at C43 (off the center tap
ot TR-1). Is this my bias range?
(AC or DC?)
Steve Kennedy
Posted on Tuesday, May 28, 2002 - 10:22 am:   

Anyone who knows better can feel free to correct me, but here is my "interpretation" of the Music Man preamp board type nomenclature:

The models using the 12AX7 phase splitter/driver circuit use a preamp board with a "GB" prefix, which stands for "Grid Bias". This type falls into section 2 of the Bias document above.

GP & GD prefixes stand for "Guitar Phaser" and "Guitar Distortion" and are used only on the RP & RD series of amplifiers.

I think the "DB" prefix stands for "Driver Bias" and indicates a solid-state phase splitter/driver circuit is used. I believe your amp falls into this category.

The output stages of most Music Man amplifiers fall into these two camps, distinguished primarily by the type of output driver circuit they use... tube or solid state. Features of the actual preamp circuits have nothing to do with the output stage and bias requirements.

Unfortunately, Music Man didn't label all their schematics with this info but the above "guess-timates" are based primarily on the ones that they DID label and working backwards.

The schematic at C43 indicates "20uF, 25V" (20, 25V is shorthand) for the C43 value, not "20-25V" as you had indicated.


(Message edited by admin on May 28, 2002)
Lewis Langley
Posted on Tuesday, May 28, 2002 - 12:04 pm:   

Thanks Steve.

Thanks for your clarification regarding the
210-65's circuit board designations.

I adjusted transistorized bias trimpot to 25mv DC across the lowest voltage 3.9 OHM emitter resistor. There is a difference in voltage between the two. The higher voltage drop being about 39mvdc. The previous low voltage drop was around 15mvdc. (the amp was biased by a tech
during repairs many years back.)

I assume that 25mvdc is a "safe" and desirable low setting, and that the previous 15mvdc low setting was some kind of attempt at "balance" between the low/high settings.
Does this make sense?

The amp sounds a little richer, and has and a more creamy distortion at a gain of 7, than it did before. The EL-34's (Groove Tubes EL34S # 7)aren't "glowing red".

And, as you could probably deduce from my misreading of some of the schematic shorthand, I've not an EE hanging in my den. But I do
love to tinker! Unfortunately for me, a little knowledge can be a dangerous thing. Thats why I've come here looking for clarification.
I do apreciate it. I haven't found any other
MM web sites or Techs for that matter, that have
the MM data that I've found here.

Once again, thanks. Your knowledge and a drop of my wifes red nail polish (locks the trim pot) has made my 210-65 sound better.

ben foster
Posted on Monday, August 12, 2002 - 09:43 pm:   

when you bias a 212-65 do you only have to read the .5 volts across the one tube with the 10ohm resistor. not both tubes like the other models that have to bias 25mv measured at each tube.b
Posted on Thursday, August 15, 2002 - 03:40 pm:   

Thanks Ben.

But ".5 at point Y" is only true for the '65's with the 12AX7 driver.

When I bought my'65 back in '79, the amp came with the schematic for the 12AX7 driver -
But the amp has the transistorized driver
and a different proceedure.
Confusing, eh?

Thank G-d that the amp's quality control was
light years better than MusicMan's document control.
Posted on Saturday, April 26, 2003 - 06:22 pm:   

On this picture I guess the orange white gold
gold resistors are where you check bias. There
was about 21mv across both. Is this good or should
they be brought up to 25ma. Thanks.
Posted on Sunday, April 27, 2003 - 10:38 pm:   

Yes those are the 3.9 OHM resistor where you check the bias. 21MV is not real bad ,try 25MV, see if you can tell any difference. Your tubes may like the 21MV better. 25MV is where they suggest setting it with the old big bottle 6L6 tubes. How does the amp sound to you? Is your amp a RP-Reverb Phaser?
Posted on Monday, April 28, 2003 - 03:12 pm:   

Yes it's a 112-RP-65. It sounds pretty good to me.
It had some old Sylvania 6CA7 EL34 tubes that I
switched for new JJ E34L's just to see how they
sound and maybe save the old ones. I have tube audio gear but never a guitar amp before and am
still fooling with it. Thanks.
dave tennyson
Posted on Tuesday, June 17, 2003 - 09:49 am:   

My friend dropped a Music Man 65 head off for me to service and I guess I was thinking it was a little more like a Fender.... I'm getting 688 on the plates!! Anyone have opinions on biasing the EL34's? I'm running it at 22.5 ma cathode current on the hi-power setting and 12 on the low. Is this OK? Also, there are two trimpots. The first is obviously the bias. What is the second for?

Thanks as always!!!

Steve Kennedy
Posted on Saturday, August 09, 2003 - 03:41 am:   

The bias sounds about right. The second pot is to adjust the Tremelo circuit. It is usually adjusted so that at a front-panel setting of 5 for the Tremelo depth, the tremelo circuit starts a "doubled" sounding sweep.

Posted on Monday, October 20, 2003 - 07:04 pm:   

I've just bought myself an 2x12 HD 150 with the original Sylvania 6l6 in them. The reading across the cathode resistors was about 44 mV, I've set them to 25 mV, but there was a loss of volume. Probably just old tubes. They sound ok otherwise. I'm waiting for new ones and wanted to confirm the bias settings for this amp. Unfortunately the board ID is under the board, and I was too lazy to take it apart just enough to read whether it was a GB-2 or a DB-4. Looking at the schematic 2475-150, it seems that these two configuration would be the same bias.


Posted on Wednesday, November 12, 2003 - 08:17 am:   

i have a MM 112-75 RP how do i know if its 8ohm or 4ohm
Steve Kennedy
Posted on Wednesday, January 14, 2004 - 07:11 pm:   

I assume you mean the speaker. If it is the factory-supplied speaker it is most likely 8-ohms.

If you have access to a DVM (Digital Volt Meter) or other multimeter that can read resistances in Ohms, you can measure the speaker resistance.

Disconnect the speaker from the amp, set the meter for its lowest resistance scale (usually 100 or 200 ohm scale) and put the two meter probes on the two speaker connections.

An 8-ohm speaker will measure about 6 ohms in resistance and a 4 ohm speaker will measure about 3 ohms.

Col. Angus
Posted on Monday, August 09, 2004 - 07:24 pm:   

Recently got a two ten 65 with the trans driver.
I found the bias, at 30.2/17 after 10 minuites.
5 mins later it was at 31.2/17.5. Bias creep.

Recapped all eleven electrolytics.
(Caps were so leaky and off spec, both filters
'dimpled' one was 'DC"- surrprised it worked
and sounded good too. Sounds great after recap.)

Checked bias and found 28.2/16.2 after 10 mins
and 29.2/16.7 5 mins later. Bias creep continues.

A lot of the Fender schematics have test points
with their expected voltages. The Music Man
schematic don't.

Could somone let me know what voltages to look for, and points to look for them at, so I can
T-shoot this without just throwing parts at it.

The schematic has 2 types of diode that I dont
recognize, MR510 and MR-250-5. Obsolete? Are there current numbers for them?

Thanks much.

Posted on Tuesday, August 10, 2004 - 08:40 pm:   

About that creeping bias, seems like your instructions missed an instruction.
Turn the Mastr volume UP! Makes a big difference
in the reading. The stable numbers are 21 & 34.
Is the out of balance supposed to mimic an
unbalanced dual triode PI? Would balancing the driver transists accomplish anything, or does the amp get character from it?
Anyway, voltage charts would still be a help.

Steve Kennedy
Posted on Friday, August 20, 2004 - 08:13 pm:   

I'm not real sure how wide a mismatch has to be before it is of concern. If you have matched tubes then the drive circuit itself may be unbalanced.

I have always turned the Master volume control all the way down because DC bias should be set in the presence of SILENCE (no AC signals going through the tubes... this includes hum!).

As soon as hum, hiss or signal passes therough the tube, bias will (and should) change dramatically. You can't properly set bias using a DC meter with a signal or noise present.

You CAN use the "crossover" method and drive the amp into a resistive dummy-load at near clipping while monitoring the output waveform on an oscilloscope but this is different (and more complicated for most people who do not have the required equipment) than measuring what should be quiescent bias current with a DC meter.

The HV diodes on the schematic (MR-250-5) can probably be substituted for by stacking lower-voltage diodes of the same current rating in series (physical size is the clue here).

The MR510 is available as an NTE5809 replacement semiconductor from NTE Electronics (www.nteinc.com) through one of its' dealers (in stock at $2.60 from www.mouser.com).

You can find a PDF spec sheet on this device here:

MR510_NTE5809 Specs (PDF)


Steve Morgan
Posted on Tuesday, September 21, 2004 - 12:19 pm:   

Not being very technically-inclined when it comes to amps, I've got essentially a dumb question.

I have an 112RD-65 circa 1981. The circuit board, per the schematic, is GD-2A. It has a solid state preamp, 6L6 power section. Does this amp require a bias adjustment when I replace the tubes, or is it "fixed" (non-adjustable) bias?

If so, do you know what Groove Tubes rating should be used for replacement tubes?

Do you know if Yellow Jackets will work OK in this amp?

Many thanks, and thanks for this forum!

Glenn Shirey
Posted on Wednesday, September 22, 2004 - 09:56 am:   

What actually happens if don't have a matching set of tubes in an HD-130. I have one (off brand Chezch Republic built)6l6 that someone bought for me as a replacement for when they broke one of the groove tubes that were in the amp when I bought it. If I put that tube in with the three groove tubes what is it going to do to my amp?
Steve Kennedy
Posted on Monday, September 27, 2004 - 05:35 pm:   

Steve Morgan: The Groove Tubes "Hardness" rating used will depend on whether you want the highest possible clean headroom or softer tubes to overload easier.

The RD series amp you have has a bias setting control internally and the schematic calls for 25mV minimum across the 3.9 ohm emitter resistors.

You should be able to use Yellow Jackets in your amp as long as you STAY IN LOW POWER. If you should switch to High Power, you will probably fry the Yellow Jacket adaptors and/or the tubes in them (and potentially hurt your amp as well).

These devices cannot tolerate the 700Vdc plate voltage present when the amp is in high power mode. I have tried Yellow Jackets in my 410-65, but the potential for frying everything caused me not to leave them in there! If you try it and like it, you could have a technician disconnect the high-voltage line to the Hi/Lo switch and prevent High Power from being externally selectable as an insurance policy!


Steve Kennedy
Posted on Monday, September 27, 2004 - 05:47 pm:   

Glenn Shirey: If tubes aren't matched (and you haven't re-biased them to accomodate the weakest tube) you can have some imbalance problems (especially if the replacement tube is older, weak or gassy).

If the tubes are not that far apart in condition, then it should work OK, but you are taking a risk that the amp is working harder than it has to because things are not balanced. I would try to run the amp in low power mode most of the time just in case the replacement isn't as similar to the old tube as you had hoped.

If you are going to run the amp like this, pay special attention to the output tubes looking for anything out of the ordinary and listen for any developing problems. There isn't one answer that covers all the possibilities. I an imperfect world, everything is not perfectly balanced but it is a good goal to have!


(Message edited by admin on September 27, 2004)
scott queen
Posted on Thursday, October 21, 2004 - 10:10 pm:   

Hi, I recently aquired a musicman rd 112 65 . This being my first tube amp I wanted to find out some info regarding tube replacement and bias adjustment.The tubes [2] in the amp now are rca 6ca7 EL34 I noticed playing at low levels with any type of bass added and no gain I get a distortion. The speaker is ok. Does this generally mean the tubes need to be replaced? I'm assuming the tubes have not been replaced in a long time. Could you recommend a good tube that allows clean play at high volumes? Also do I need to set the bias for this model? I noticed alot of the question in this forum are based around this? Do it myself or leave to a pro? Thanks, Scott Queen
Posted on Saturday, December 18, 2004 - 02:21 pm:   

Steve, I know this post is a bit old, but i think some actual photos would really help in performing this adjustment. I guess it would be hard to do...taking pictures unless you have an amp ready for the bias adjustment...
Posted on Tuesday, March 08, 2005 - 10:45 pm:   

Hi, i am sick of paying a tech to bias my amp, i can buy matched quads easy so all i need to do is learn how to bias my amp (hd130 with 12ax7), i presume the bias trim pot is the one furthest to the left if looking at the circuit from the front, the one to the left of that is tremolo?, what i dont know is where to put my multimeter, and i dont understand tech speak to could someone explain it to me, maybe with a picture where to put the two pionts of the multimeter and the reading it should give when the 4 valves are plugged in. BTW i also know to adjust bias on the HI setting.
Thanks everyone
michael kaus
Posted on Wednesday, March 09, 2005 - 06:35 am:   

Well, I have a 130 on the bench right now but it's toast. Maybe I will see about taking some pictures though. I could at least poit and shoot! Mike.
michael kaus
Posted on Thursday, March 10, 2005 - 10:28 am:   

My digital took a dump so that idea's out for a while. I will say, however, that I don't like the procedure of adjusting the bias at the cathode resistor because it gives you a total bias current reading and doesn't show if your tubes are balanced or even close. I prefer to measure bias current PER TUBE. Mike.
Posted on Thursday, March 10, 2005 - 11:47 pm:   

spewin, so if you measure the valves individually can you bis them individually?
michael kaus
Posted on Friday, March 11, 2005 - 11:09 am:   

Well, you can't with the way the amp is wired stock and there really is no need to. My point about biasing tubes individually is so that you can verify that the tubes are balanced within reason. With the measuring of a total bias current of two tubes, it is possible to have a very high bias reading on one tube and a lower current draw on the other and have a total of .5 volts on the ballast resistor. This could give you (theoretically) 40 mA on one tube and 10mA on the other(extremes) and you would have a total current draw of 50mA-or .5 volts across the resistor. These are extremes, to show the possibilities. Mike.
Posted on Friday, March 11, 2005 - 08:02 pm:   

Oh ok, so you can you bias yours individually ie has it had a bias mod, or do you just check that they are all the correct voltage just to be certain. And then you know that the .5V reading is because both the valves are at 25mA and one isnt hotter/cooler than the other. Doesnt matched valves eliminate this possibility? (BTW i have 4 valves = 130W). thanks heaps for your time and informative responses.
Steve Kennedy (admin)
Posted on Friday, March 11, 2005 - 08:21 pm:   

Yeah, I had planned to take photos for internal "how to" sections for this web site, but my current job (in its' fifth year) is quite demanding so I haven't gotten around to the Music Man projects that I have in the wings (Recapping an HD130 head, restoring a 210RD-100-EVM, etc.).

One of these days...

If you NEED photos and step by step, then maybe it is a good idea NOT to get in there and do it yourself! You can kill yourself doing the most innocent thing when inside a live amp on high power (or low power too)!


Posted on Saturday, March 12, 2005 - 05:11 am:   

Well i'm half way there, i have the chassis out i can identify all the components except their values, i have matched valves, i know its the trim pot to the right, i have a multimeter etc etc, i just need to know what the bloody hell the 10 OHM cathode resistor is situated then i can make sure im getting the right reading (.5V? for 4 valves).
You have to pay a tech $200 to do this and 4 valves will cost me more like $120. Plus i have been told to check your bias every 6 months even when not in need of new valves.
Thanks everyone.
PS and i guess it would be good to test the bias of each valve also as michael suggests.
michael kaus
Posted on Saturday, March 12, 2005 - 09:26 am:   

OK, I'll try to give you the info you need. The 10 ohm 5 watt resistor is the big square white cement block looking thing on the bottom side of the tube sockets. There should be two of them. One for each pair of tubes. These are NOT the brown ones that are placed BETWEEN the two center tubes. Those are 1.5k. The white cement block resistors are the ones you want. If you insist on that old .5 volt at point Y thing this is wher you will measure it. Measure from pin 8 to ground. I/m not sure if you have cement resistors or not, now that I think of it.I replaced mine with cement for the spoink of it. You will probably have 10 ohm 1 watt resistors, They just look like big resistors. They will have the band code of brown, black, black. Still you willl measure from pin 8 to ground to get your bias number.
Posted on Saturday, March 12, 2005 - 08:06 pm:   

ok thanks ill keep you updated.
david manzullo
Posted on Wednesday, March 30, 2005 - 07:18 pm:   

I have a 1974-75 212 sixty five....12ax7 PI...and 2 6ca7eh. I have a bias probe which I check the tubes. 1 tube was at 29 ma and the other tube was at 34ma..(the tubes are not matched I guess)do I need to bias the amp and where on the amp is the bias trim pot to adjust the bias....I need your help thank-you...
michael kaus
Posted on Thursday, March 31, 2005 - 06:44 am:   

The bias pot is the one farthest to the right(closest to the power trans. 29 and 34 is HOT for these amps. Somebody probably biased it like a marshall without taking into account the higher plate voltage. 21-23mA(no higher than 25 max)is what you what on the HOTTEST tube-your 34 tube.
david manzullo
Posted on Thursday, March 31, 2005 - 02:45 pm:   

Hi Micheal,
Regarding the bias pot for my 212 65 mm amp...do I need to bias each tube separately or the bias pot will bias both tubes....thx again for your quick response...this site is great!!!!!
michael kaus
Posted on Friday, April 01, 2005 - 06:35 am:   

The bias pot sets the control grid voltage for all four(or two, in your case) tubes at once. What I meant was that you need to check the idle current for each tube first, find the hottest one(if there is one), and set your max bias current to that one. You will be adjusting both tubes at once though. MIke.
david manzullo
Posted on Monday, April 04, 2005 - 02:13 pm:   

Hi Michael,
thx for the info on my 212 65 mm amp. I lowered the bias to 25ma on the hottest tube, now the amp sounds great. I also have a 1980ish(I'm not sure which year)150HD head...I opened it up and noticed that the trim pot stands straight up. I must have touch it accidently because my biasprobe is now reading 9ma on the hottest tube....what is the correct setting for the amp..thx david
david manzullo
Posted on Monday, April 04, 2005 - 03:42 pm:   

Hi Michael...I tried to bias the HD150, the amp has 4 6L6GC and I'm using a biasprobe w/meter....the two on the left side I bias to 25ma...but the two tubes on the right side (looking from the back of the amp)will not go over 21ma on the max setting of the trim pot so I left it at that setting, but when I go back to the left-side, the bias is over 32ma. I believe I need a good set of match quads....what do I do?
thx david
michael kaus
Posted on Monday, April 04, 2005 - 03:59 pm:   

Just looked ata 150 schematic and it pretty much a plug and play but I think, judging from the schem that the pot is a balance control that balances the bias drive. I think that if you set the trim pot in the middle somewhere, you will see the tubes are closer to being balanced. It looks like they use a SS driver section and the pot only balances the two halves. The amp is fed THROUGH the cathodes instead of the other way around so try it in the middle and measure the cathode current on all four tubes.You are actually reading the driver current that way. Just balance it. Let me know. By the way, that's why the bias voltage is POSITIVE, not neg.
Posted on Saturday, June 18, 2005 - 01:28 am:   

I have a HD-130 CLONE!! yes a clone.
GB-1 Board and Exact same size and components (not the caps obviously). bought this ultralowpriced (less than 150 US dollars).

Problems. The Clone doesn't have LO mode, all the other stuff seems to be exact as any MUSICMAN tremolo & reverb 2 channels fixed mid control in "channel 1". This one runs with a 12ax7 Drivers with four EL34. i have only 3 working. how can i run this amp with only 2 power tubes? (just a question). The tubes aren't matchend and also different brands. Going to put new Svetlana's EL34 and a Sovtek 12ax7 for the driver section but i have a big question.

The Resistor Between the 22uf 600 Volt caps on the original schematic is 10K , this amp has something like 3.3K. The REST IS EXACTLY AS SHOWN IN THE SCHEMATIC, but the diodes going to ground in the power amp section are not connected.

I have the skills to setup and repair it. I think i'm gonna desing a new board with another opamps (TL081 or JRC4558) and regulated +/- 15 Volts.

Thanks for your Help. This is the best amp i've ever used. I've repaired a 212HD before but the owner had the "incredible" idea to leave the power section "ALL SOLID STATE" no tubes. i have to re-wire and mod it to original state soon.

Steve Kennedy (admin)
Posted on Saturday, June 18, 2005 - 05:22 pm:   

I have never heard of or seen a Music Man clone! I have seen Music Man "forgeries" (where a cheap amp is dressed up with phony badges to be sold as a Music Man to the uninformed) but haven't seen an honest to goodness "copy"!

Does it have a brand name or manufacturer anywhere on it? Was it at custom one-off or an actual production amp?


Posted on Friday, June 24, 2005 - 06:38 pm:   

Actual production amp, very few made. a local technician that made clones in the late 70's of famous amps (fender, marshall, etc.).

The brand "supreme". Known as "el-cheapo" amp BIGGER TRANSFORMERS EVER! i like that the OT it's rusted as fuck B/C i'm not a clean player.

Changes from the original version.

100K resistors in the feedback loop changed to 220K for more gain, 50pf caps in feedback loop to supress RF interferences are not present, 10K resistor in the filter cap section changed to 8K 1/2 Watts. Caps "ARGON" looks ceramic but seems to be poly film or electrolitic non-polar.
Diodes to ground in the tubes section are not present in this clone.

Tremolo Section

LFO transistor : 2SC945,
FET transistor : 2SK30ATM (sure it's doesn't sounds like the original in modulation).

Al pots are ALPHA, Bias Trim CTS and Trem Trim Too. values are EXACTLY THE SAME, toggle switches "ARROW". This is a great amp, really. looks like it has survived toons of gigs but remains "healthy", now i have to change the power tubes to svetlanas EL34. i like the diying sound of the phase splitter (or tube driver i don't know exactly), i need more info on this amp cause it's looks exactly like original, the chassis is from a musicman amp (I've repaired a 212HD combo 2 months ago). the Rocker switches for Bright and Deep are toogle Switches, the Front Panels Looks the same without the musicman logo and letters, back panel the same without Footswitch jacks for Trem and Reverb.

OP-AMPS= LM307 and 2 741 DIP 8 Format.

I'll try to take pictures if i can.

really a great amp, i've used bassman , fender twin, heared a bandmaster live.. and i really prefer this one. i already have a triangle knob bigmuff so i just want to hear it LOUDER not more distorted from the pre-amp tubes. (like in a marshall plexi).

Thanks for the reply!

Posted on Saturday, June 25, 2005 - 02:42 am:   

Correction All values are ok except..

Rectifier Board..
330K instead of 150K
100Uf instead of 150uf

Filter Board
8.2K instead of 10K.

I can use safely some EL34 Svetlanas? with this bias circuit? can i decrease the 725 volt reference to 700 to make it safer? (and more tube life?=)

Steve Kennedy (admin)
Posted on Saturday, July 23, 2005 - 07:34 pm:   

To reduce the plate voltage, you would have to put in a large series resistor to drop it to the level you want. This would change the bias requirements as well as reduce the plate voltage AND it might change the dynamics of the amp since this voltage would go up and down in relation to how hard you drive the amp...kind of like a "spring".

I have heard tha JJ EL34 tubes seem to be more tolerant of the Music Man high voltage but I have used both Svetlana (Winged C) and Sovtek tubes in MM amplifiers with no real problems (at least initially).


michael kaus
Posted on Sunday, July 24, 2005 - 06:19 am:   

My 2-10 65 has 700v on the plate just the way it is. Seams like some amps are a little lower than others. I use the JJ tubes in mine but th eones you want are the E34L's, not the regular EL34's. They are a little more stoutly built from waht I am finding out. I've been getting 3 yrs or more before they start rattling. This is pretty damned good considering waht I was putting them through. I usually bias them at around 23-25mA@ idle PER TUBE measured separately. Mike.
Randall Morabito
Posted on Tuesday, August 30, 2005 - 02:31 pm:   

Can someone explain why the EL34's want to be biased at 25mA or less in these amps? I have a 210 SixtyFive with a 12AX7A driver. I get conflicting advice from a custom amp builder I know. He insists that 25mA is on the low side for these tubes, and the amp would run better at about 65mA for the pair. What are we missing here? How does the 700v plate voltage figure into this? New Svetlana matched pair EL34s. Thanks.
Karsten Gorkow
Posted on Thursday, October 06, 2005 - 07:44 am:   

Yesterday me and my pal replaced the tubes in my 210-65 (op amp driven) with JJEL34Ls. When I'm reading the postings here, I'm in doubt if my amp is ok: We measured 275mV and 330mV across each of the (big) 3.9 Ohm emitter resistors (same as before we changed the tubes)! When we played with TR1 the voltage was reduced on one side and climbed accordingly on the other but we woulnd't have been able to reduce to 25mV. We finally adjusted TR1 with the aid of an oscilloscope, checked for no crossover distortions and symmetrical clipping. We finally ended up at the factory setting (visible by a red dot). The sound and power of the amp is good. What's wrong with my amp?
Posted on Friday, October 07, 2005 - 01:52 pm:   

If your readings are correct your tubes and
transformer would melt.
Karsten Gorkow
Posted on Friday, October 14, 2005 - 08:15 am:   

The readings should be correct. I asked my buddy to check if maybe the decimal dot on the LED display of his voltmeter is dead ... it isn't. With our readings the current through Re is around 70mA then, according to what my buddy says that's more or less standard for an amp like that. Fact is: nothing melts - except the listeners due to the good sounding amp ...
As I wrote before: the range of TR1 is even not wide enough to adjust the voltage drop on Re to 25mV.
Where's the fault?
michael kaus
Posted on Saturday, October 15, 2005 - 09:03 am:   

Don't really understand why you have that much emmiter voltage but If I read your post properly, you have 70Ma of cathode idle current and that is way too much for a MM if you have 700v on the plates. Don't have a schem for your amp in front of me but I'm pretty sure the SS driver versions used the same power supply as the others. You should have only 50ma of idle current PER PAIR which is 25 per tube. I would say yeah, the amp will sound amazing for a real short time at that rate. Mike.
Karsten Gorkow
Posted on Monday, October 17, 2005 - 07:32 am:   

Thanks for your response. The 70mA idle current is just a calculated value we got from the voltage we measured across the 3.9Ohm Re's. We measured 275mV across one of the them and around 330mV across the other *before* we changed the tubes and *similar values afterwards*. This makes 70mA and 85mA idle current per tube, right? The amp works for years with these settings.
Here's the link to the schematics:
http://www.ernieball.com/mmonline/techin fo/old_amps/2475-65_&_2275-65.pdf
When we adjusted the bias via TR1 we watched the output of the amp with an osci and saw crossover distortions when we went to far in one direction. BTW, as I stated before: We would have not even been able to adjust 25mV! The range of TR1 was not big enough and if we lowered the voltage drop on one side, it climbed on the other.
Karsten Gorkow
Posted on Monday, October 17, 2005 - 07:38 am:   

One more thing:
Opposite to the above picture, the 3.9Ohm (orange-white-gold-gold) Re's in my amp are big 1W resistors (see the schematics), not the small one's that are visible on the picture. On the same board are the driver transistors mounted on a cooling "metal" (don't know the correct translation!). Regarding the 700V plate voltage: We measured around 350V on the positive tip of each of the big 100?F capacitors (we changed them too).
mike kaus
Posted on Wednesday, October 19, 2005 - 06:49 am:   

Your plate voltage will be measured at the plate on the tube but like I said, the ss driver amps may be SLIGHTLY dif but I don't think so. The best way to check your current draw is to install temp 1 ohm resistors from pin 8 to ground through the existing ballast resistors if yuo have them. There is ususally a big ceramic or cement resistor tying pins 8 together to ground. Measure the voltage drop across the 1 ohm resistor and that will give you a direct reading of current(yes, I know it's voltage) though that resistor which is your idle current. Remember, master on zero, channel zero, no signal.
Posted on Tuesday, November 29, 2005 - 01:21 am:   

"Fact is: nothing melts - except the listeners due to the good sounding amp ..."

you might wanna listen to reason here, these people are trying to help you. something is definately incorrect! you can joke around if you want to, but it's YOUR amp that's at stake!
Posted on Wednesday, December 07, 2005 - 08:35 pm:   

TR2, the one in DB-2 board should be a 100 Ohm trimpot, right?
I have a 2100-65 transistor driven version amp for repair and it has a huge 100 KOhm potentiometer pushed in there!
Please help!
Phil G
Posted on Thursday, March 02, 2006 - 09:50 pm:   

I have a question on replacing Je1692 transistors.
I ordered replacement 2n488 transistors. The new transistors are 75w 80v and are much larger (will not fit in the sockets) the old transistors are part # sf 61176 604 (not sure what type). This is for a 112 65 combo. Are these the correct transistors? if so, I'll trim them down to fit.

Thanks in advance
Posted on Sunday, March 05, 2006 - 11:27 pm:   

hi steve n guys,
any chance of someone posting a pictured step by step biasing doco? (prefer hd130:-)

I am not new to HV elec, just to biasing amps! and as they say a picture is worth a thousand words...i am just hoping to have the procedure clear in my head before pulling apart my amp.

Steve Kennedy (admin)
Username: admin

Registered: 03-2002
Posted on Thursday, April 06, 2006 - 09:22 pm:   

I am of two minds on photo-essays showing step-by-step how to do ANYTHING inside a Music Man amplifier. The potentially lethal voltages inside are minimized by "easy to follow" instructions that may entice people who have no business being inside a tube guitar amp to try something they may not be prepared or equipped to safely accomplish.

Also, there are many variations on this technique over many different models and generations of Music Man amplifiers and the user would have to pick the correct one for his amp! Picking the wrong one can lead to disaster!

My take on it is if you cannot interpret the correct schematic for your amp and apply the appropriate factory instructions posted at the top of this topic, then you probably shouldn't be inside your amp trying to do a procedure. Either this, or you simply haven't done enough homework to pass the entrance exam!

If you have enough experience to safely do this sort of thing you should be able to figure it out from the postings and materials here. If not, it would be safer to have it done by a professional tech.

Chris Metcalfe (chris_m)
Username: chris_m

Registered: 10-2006
Posted on Thursday, November 02, 2006 - 08:15 am:   

I have posted something today in the 'circuit modifications' section about adjusting the self-biasing point in RD50 series amps,using fixed resistors and proceeding very cautiously, if anyone's interested.

Dirk Wacker (doc_holiday)
Username: doc_holiday

Registered: 02-2007
Posted on Tuesday, February 13, 2007 - 03:57 am:   

From what I can see, my 112RD50 is a self-biasing amp design, is this right ? If so, does this really mean that I only have to put in a pair of matched power tubes without any re-biasing operations ?
Larry Marseglia (lrmars)
Username: lrmars

Registered: 07-2006
Posted on Thursday, February 14, 2008 - 01:58 pm:   

How can I identify the bias trim pot on my HD150R chassis? The inside of the chassis doesn't look like the photos up near the top of this thread. I've downloaded the schematic but does anyone know what the bias pot looks like?
Many thanks.
Thomas Leong (tyguitar)
Username: tyguitar

Registered: 03-2009
Posted on Tuesday, March 24, 2009 - 12:22 am:   

Greetings from Singapore! Pls help me! MM service Bulletin says 10 ohms resistor at power tube pin 8, but my MM ( 12 AX7 PI ) amp has 100 ohms resistor and not 10 ohms, at both power tubes. They are greyish cememt color and is original! Bias trim to .5v does not work here, amp gets distorted and sounds thin, especially on LO channel. I bias to .100v and it is fine, but not sure is this correct. Before I restore amp, bias measures to .160v!!
Also many in this forum, mention swapping op amps to get Mullard sounds, etc,etc. I tried, does change the overall MM tones, but, other than the original LM07H, other op amps affect the tremolo effect? The tremolo gets weaker in intensity and speed..sucks! Whay is this?
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Wednesday, March 25, 2009 - 08:21 pm:   

Need to know exactly WHAT amp you have. 65? 130? Rd? RP/ 50? They all have different bias schemes.
Thomas Leong (tyguitar)
Username: tyguitar

Registered: 03-2009
Posted on Sunday, March 29, 2009 - 03:16 am:   

Kaus Sir, The amp I have is MM HD410 One Thirty Combo. Serial No B004597 2275-130EX at the back plate.
Appreciate if you can share anything, as I said it has 100 ohms resistor instead of 10 ohm at pin 8 of power tubes.
And to top it, all the recommended op amp substitues works like described in the Forum, except no one mentioned that it weakens the tremolo intensity and speed. Only the LM307H does not affect the quality of the tremolo. Yes, I did try the tremolo trimpot as well!
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Monday, March 30, 2009 - 06:02 am:   

If you indeed DO have 100 ohm instead of 10 ohm,(and I have no idea why they would be in there) you should be reading 5v across that. I'm sure that all the IC's will change SOMETHING, you just have to find a balance between what sounds right in there and what will work.
Thomas Leong (tyguitar)
Username: tyguitar

Registered: 03-2009
Posted on Monday, March 30, 2009 - 09:19 pm:   

Kaus Sir,
Thanks, please explain how you derive the calculation for 5V across for 100 ohms, from the .5v for the 10 ohms resistor bias. I will try it anyway. Do you recommend I change the resistor to 10 ohms, though I swear mine looks original! Anyone has the 100 ohms at pin 8 pls shout, we may get to know more or it is just my amp!
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Tuesday, March 31, 2009 - 05:52 am:   

If it was a 1 ohm resistor, it would read .050v or 50mV. .5v or 500mV at 10ohm, 5.0v at 100. It is a factor of ten.
Thomas Leong (tyguitar)
Username: tyguitar

Registered: 03-2009
Posted on Tuesday, March 31, 2009 - 10:49 am:   

Kaus Sir,
Thanks for your prompt reply and assistance. I will bias to 5.0v and revert. Do you recommend I change the resistors back to 10 ohms, thought the 100 ohms at .100v did not have any ill effects. Wonder why the 100 ohms, but the amp is running nicely, EL34 tubes are stable and not running hot, which is also good.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Wednesday, April 01, 2009 - 06:33 am:   

If it runs, leave it.
Crispin Gill (cris)
Username: cris

Registered: 06-2010
Posted on Wednesday, June 30, 2010 - 08:13 pm:   

Greetings all
I have a 112RP-65, re: Bias setting, I have followed the procedure at the top this section, setting the lower of the two at 25mv. If I then check the reading from a probe placed between the power tube and socket, I get a very high reading in milliamps - is this to be expected?
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Thursday, July 01, 2010 - 11:24 am:   

How high of readings are you getting? If you back the setting back even more, how does it affect the readings on the bias probe? Also, which probe are you using-one that measures current directly or does is have the 1 ohm resistor in it?
Crispin Gill (cris)
Username: cris

Registered: 06-2010
Posted on Sunday, July 04, 2010 - 04:57 pm:   

The readings on the Bias Probe (no 1 ohm resistor) are at 50 milliamps when I read 25mv DC across the 3.9
OHM emitter resistor. There is a difference between the two power tube probe readings and the emitter resistors readings - with matched tubes (deliberate ? - to allow for unmatched tubes back in pre digital 1979 ?).
I have set the lower tube reading on the probe to 25ma and get a reading of 12.5mv from the emitter resistor. The amp sounds good, I will fine tune the setting and try unmatched tubes to see if it makes a difference
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Monday, July 05, 2010 - 05:39 pm:   

HMMMMMM. Sounds to ME like I'd try some new drivers in there. I can't see any other reason for the big difference. The tubes themselves should be conducting about the same, especially if they are matched. I didn't look to see what drivers the rp's used but I think I'd try replacing the drivers and checking all the components on the driver board and then re-balancing.
Crispin Gill (cris)
Username: cris

Registered: 06-2010
Posted on Tuesday, August 03, 2010 - 06:17 am:   

Thanks, I have tried swapping the power tubes round in the sockets to see if the readings (low output on stnby) differ and they do.
Using Bias Probe readings (milliamps) I get this - socket 1 & tuba A = 39ma with socket 2 & tube B = 21ma. With tubes swapped round I get this -
soc 1 & tube B = 36ma with soc 2 & tube A = 21ma.
When I adjust the bias pot now (soc1 & tube B etc.)
I get 1B = 37ma then 2A = 25ma,
when 1B = 38ma then 2A = 30ma.
I've left the setting at the last one & it sounds OK.
Would unmatched tubes help? or is there a problem?
I'm in North West Ireland & there are no technicians around, I can check components up to a point but rarely find the time, so replacing the drivers as you mentioned might not be an option unless I find someone I can trust to send to send the chassis to. Cheers.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Tuesday, August 03, 2010 - 10:48 am:   

If those readings stay consistant, I'd probably leave it alone. I still think it has to do with the driver transistors that are driving the tubes. They are not THAT far apart that I'd get too concerned. If it were humming bad or distorting, THEN I'd look furrther. Mike.
Crispin Gill (cris)
Username: cris

Registered: 06-2010
Posted on Monday, August 09, 2010 - 04:52 am:   

Thanks again, there's no humming & it's not distorting. I'll check the readings to see if they change.
I got the amp secondhand in the mid 1980's & it had a Sylvania 6CA7/EL34 and a Zaerix 6L6GT installed. The chassis has a hand written date of 1979 so the 6L6 must have been a replacement at some point. Could this have caused problems?
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Monday, August 09, 2010 - 12:34 pm:   

Oh hell yes! Those two tubes are going to draw two totally different current draws. I don't remember which one it was supposed to have but the early versions used 6CA7's and 79 was about the breakover period so I'm going to assume that YOUR amp should have had 6CA7's. You can use EL34's or the heavier E34l in it and it should set up nicely. I am shocked that you could get two different types of tubee THAT balanced. I do know that Bob at Eurotubes was marketing MATCHED EL34/6L6 combinations that were geared to give a little of each but you just got lucky enough to have one that worked. The 6CA7 type tubes drew a lot more filament current though so it should be safe to go 6L6's completely, if you wish.
spudmurphy (spudmurphy)
Username: spudmurphy

Registered: 06-2007
Posted on Monday, November 12, 2012 - 06:51 am:   

I have the 112-RP-65 and I will be changing the output valves using matched Sovateks. I will be taking it to an amp tech friend who has all the gear - ocilliscopes, signal generators etc.

Can anybody point me in the right direction for a definitive description on checking the bias on these amps please? I'd like to get it done ready for this week ends gig.

Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Tuesday, November 13, 2012 - 07:44 am:   

Look at the TOP of this thread. Check the one for SS drivers-that should be yours. Adjust the voltage across the 3.9 ohm resistors. Mike.
spudmurphy (spudmurphy)
Username: spudmurphy

Registered: 06-2007
Posted on Wednesday, November 14, 2012 - 04:44 am:   

Thanks for the answer Mike.
Having limited electronic experiemce I was getting confused, but you have pointed me to the relevant info.

Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Wednesday, November 14, 2012 - 10:35 am:   

Just be CAREFUL. THere will be around 700v bouncing around in there. If you're not comfortable, have a tech do it. It's not rocket science but it IS dangerous. Mike.
spudmurphy (spudmurphy)
Username: spudmurphy

Registered: 06-2007
Posted on Thursday, November 15, 2012 - 02:57 am:   

Hi Mike.

No way am I doing it - I'll keep to guitar maintenance. A good friend who I've known and played in bands with is a pro amp tech and he will be doing it. He stocks matched sovatek valves and has worked on old Marshall and WEM amps of mine. I'll take the amp out of the case and then watch him as he biases the amp. I just wanted to know the requirements before hand. I'll be keeping my fingers well away from it when he's prodding around.
Ironically enough, he's a drummer!!!
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Thursday, November 15, 2012 - 09:40 am:   

Well, we won't hold THAT against him. My experience with Sovtek tubes is that they sound a little brittle in MM amps. Try them and let us know. I have installed them in the 12ax7 and SSdriver versions and they always sounded a little rattly to me but hey, what the hell do I know! I like either the winged C or JJ's in a MM. The winged c's are just getting REAL expensive right now. Mike.
spudmurphy (spudmurphy)
Username: spudmurphy

Registered: 06-2007
Posted on Friday, November 16, 2012 - 04:06 am:   

Hee hee - nah he's a good 'un. Always useful to know a good amp man I say. he's a great repairer of keyboards too!!
I'm not one of these people who have the ears of a bat lol and have played a Marshall with Sovteks for a while now - sounded pretty much the same as the stock valves as was previously in the Marshall.
I know he had a large consignment of sovteks delivered a few months back - all "measured" and paired up. I'll be sure to let you know what it sounds like.

Now if only I could find a RD 112 in the UK in good condition and at a good price, I would be a happy man!!
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Saturday, November 17, 2012 - 07:28 am:   

Ah-you're in the UK. That makes finding a MM a little tougher. From what I've heard, they are kind of scarce over there. Mike.
spudmurphy (spudmurphy)
Username: spudmurphy

Registered: 06-2007
Posted on Tuesday, November 20, 2012 - 03:30 am:   

Yes Mike they are - but they do crop up now and then - my recent aquisition being an example.
I don't really want a US spec one and then having to run a transformer.
A guy who works part time with my amp tech mate, has a habit of "sniffing" amps out. He has a fantastic stock of old Vox's, Selmers, some old Marshalls, Laneys which he always intended to sell on in his twilight years. He reached that stage a little while back and has been selling a few to the US and Canada - asking for and getting stupid money for them. I was lucky enough to be around his house and went through a late 50's Champ, a 60's JTM45, a 72 Marshall 50 an top boost AC30. All nice amps - but I wouldn't trust them to last on the road in "their" twilight years!!!
He was the guy who tracked down the RP112. I'm hoping one will crop up one day, but I already have some nice amps that cover all the bases when I pplay in my (covers) band. I have a Marshall 50 Silver Jubilee combo, a Mesa F50 (not the gain monster of it's compatriots)a Fender Blues Junior (awesome little amp) and a WEM 5 watt combo.I've been using the RP112 live, since I bought it. The RD112 would be the icing on the cake.
jnewmark (jnewmark)
Username: jnewmark

Registered: 11-2012
Posted on Sunday, November 25, 2012 - 03:48 pm:   

Recently was told this about my HD150: "This amp is biased by a pair of transistors Q3 and Q4. If this bias circuit is not operating properly, you will promptly cook your tubes. Probably the service manual, if you can find one, will show the voltage that should be present on the resistors R39 and R40 that tie Q3 and Q4 to ground--unfortunately the schematic does not show that voltage. " Sound right ?
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Monday, November 26, 2012 - 10:01 am:   

No-amp is "biased" by q5 and 6. r39 is in the splitter at the reverb circuit.
jnewmark (jnewmark)
Username: jnewmark

Registered: 11-2012
Posted on Monday, November 26, 2012 - 06:17 pm:   

According to him, the schematic shown here, for 2100-150, shows Q3 and 4.
Lars Verholt (lmv)
Username: lmv

Registered: 11-2009
Posted on Monday, November 26, 2012 - 09:45 pm:   

Correct, the cathode driver transistors in the 2100-150 schematic are marked Q3 and Q4. The BB-3 is biased the same way as the DB-3 board: Make sure you have just over 25mVDC to ground on the emitters. If you get two different numbers, the lowest should be 25mV. Fine-tuning with a 'scope and a tone generator is of course possible too.

Lars Verholt
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Tuesday, November 27, 2012 - 08:25 am:   

Sorry-looked at the schem for the 2475-150. You said HD150 and I took it to mean the big headed reverb model. Mike.
Lars Verholt (lmv)
Username: lmv

Registered: 11-2009
Posted on Tuesday, November 27, 2012 - 01:29 pm:   

Hey, I'm not big headed!
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Wednesday, November 28, 2012 - 09:03 am:   

No you're not! You're one of the nice guys that tries to help people and I applaud that. Mike.
Barry Banks (banksy76)
Username: banksy76

Registered: 06-2013
Posted on Friday, June 14, 2013 - 09:55 am:   

I own a 212 HD 130 (later version without the 12AX7 pre amp tube for the phase inverter section) which I need to check the bias of using a multimeter, and I am hoping that someone here could kindly provide me with some clarification.

Having read the above posts I understand that I need to take a reading from the 3.9 Ohm emitter resister which I can only detect 2 of and I?m therefore assuming that these are the 2 shown in the image below that are colour coded Gold, Gold, White, Orange. Is this correct?

Text description

Also, is the bias adjusted via the pot shown on the far right of the image below?

Text description

Many thanks in advance.
Barry Banks (banksy76)
Username: banksy76

Registered: 06-2013
Posted on Friday, June 14, 2013 - 09:59 am:   

I own a 212 HD 130 (version without the 12AX7 pre amp tube for the phase inverter section) which I need to check the bias of using a multimeter, and I am hoping that someone here could kindly provide me with some clarification.

Having read the above posts I understand that I need to take a reading from the 3.9 Ohm emitter resister which I can only detect 2 of and I?m therefore assuming that these are the 2 shown in the image below that are colour coded Gold, Gold, White, Orange. Is this correct?

Text description

Also, is the bias adjusted via the pot shown on the far right of the image below?

Text description

Many thanks in advance.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Friday, June 14, 2013 - 10:42 am:   

Actually, that would be orange white gold. 3, then 9, then the multiplier which is gold. Yes, those are the ones. Voltage measured across those resistors. If you have the separate driver board right smack in the middle on the back, THAT is the one.

Text description
Barry Banks (banksy76)
Username: banksy76

Registered: 06-2013
Posted on Friday, June 14, 2013 - 11:39 am:   

Thanks so much Mike, much appreciated. Sorry about the double post, please feel free to delete as appropriate.

So using a basic multimeter I connect the probe to the 3.9 Ohm resistor and the black probe to ground/the chassis of the amp, then adjust the bias pot which is the blue potentiometer furthest to the right on the board when viewing from the front of the amp, Yes?
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Friday, June 14, 2013 - 02:10 pm:   

Best way is across both legs of the resistor so that you don't accidentally read nothing as one of those legs is connected to ground and without taking the board out and LOOKING which one is which, it's easier to just measure both legs themselves. Does your driver board look like the one above or is everything ALL on one BIG board?
Dick Billups (dbillups)
Username: dbillups

Registered: 06-2013
Posted on Sunday, June 16, 2013 - 02:20 pm:   

I have a 2275-65 with 12ax7, I can't get more than .38 volts across the 10 ohm bias resistor. The output waveform clips off the top and is low in power, sounds bad. Seems like it's in the phase splitter. 2 problems? Any similar experiences?
Glad the site is still here! It's been a decade since my last post.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Sunday, June 16, 2013 - 02:58 pm:   

Sounds like you are getting too much control grid voltage OR the tubes themselves are just shot and won't conduct any more. Do you have any other valves to try? It kind of sounds like to me that maybe ONE tube might have taken a crap. Mike.
Barry Banks (banksy76)
Username: banksy76

Registered: 06-2013
Posted on Sunday, June 16, 2013 - 09:05 pm:   

Hi Mike.
Yes, the board in my amp look exactly like the one in the image that you have posted. I have taken a reading from the 3.9 Ohm resistors and I am getting a reading of 10.5 on one and 13.5 from the other. However, when I adjust the bias trimpot it doesn't appear to be affecting the readings. Any suggestions/input would greatly appreciated.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Monday, June 17, 2013 - 06:10 am:   

I am sticking with my guns on this one. Tubes, then adjust and see. The driver transitors could be shot but tubes is more likely.
Christophe (vignau)
Username: vignau

Registered: 06-2011
Posted on Friday, June 21, 2013 - 08:20 am:   

I have a 210HD130 no 12AX7, exactly same board and I have no problem in adjusting bias with trim pot within spec (one 26 mV, the other 34V)
Barry Banks (banksy76)
Username: banksy76

Registered: 06-2013
Posted on Wednesday, June 26, 2013 - 02:30 pm:   

Mike, are you saying that a bad tube/s would result in adjustment to the bias trim pot having no affect at all on the reading on the multimeter?

If so, is there anyway of verifying that it is indeed a bad tube/s without purchasing a complete new set? I understand that the amp can function with just 2 tubes installed so surely if I power the amp up with just 2 of the 4 tubes installed and alternate them, at some point I should have 2 good tubes that will allow the bias trim pot to be operational; unless of course all 4 tubes are faulty which I think would be unlikely.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Wednesday, June 26, 2013 - 02:55 pm:   

You say you get NO change AT ALL? Nothing? Testing the tubes on a tube tester would show you relative strength but finding one locally might be tough. Don't know where you are but MAYBE an old TV shop would still have one. My old Hickok is still working after all these years so they DO survive.
Barry Banks (banksy76)
Username: banksy76

Registered: 06-2013
Posted on Wednesday, June 26, 2013 - 08:21 pm:   

No, no change at all. Like I say, If I take a reading from the 3.9 Ohm resistors I am getting 10.5 on one and 13.5 from the other. However, when I adjust the bias trimpot it doesn't affecting the readings at all. Am I overlooking something simple here and making a schoolboy error?
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Thursday, June 27, 2013 - 08:09 am:   

I think your on;y alternative is to get a set of tubes unless you can get somebody to TEST those. Without the tools to test them, you're guessing. I'm just sayin.... You'll have to start somewhere and my bet is on the tubes themselves. You can remove the driver transistors and check them for shorts and check the surrounding components in the driver circuit. Also, you might check the pot itself and make sure IT'S good but I'm still betting on tubes. Mike.
Tom Bruyn (tommy64)
Username: tommy64

Registered: 07-2013
Posted on Wednesday, July 03, 2013 - 10:14 am:   

My first post on this forum.
Just tuned bias in my 212/2475-130 with new 6CA7 tubes. It's a '76 with 12AX7.
I guess the first series of circuit boards.

This is my first time tuning bias.
The amp was sounding very well until now, and there has not been done anything on this amp for 20 years. Until now there were EL34's in.
But with the new tubes I thought, let's do it.

Before the tuning, I replaced the 10ohm grid resistors (R56/61) for 5W types because there were small 1/4W in there...
When I started, I measured 0.1V on pin 8 over the resistor, where it should be 0.5V. Never was a problem :-)
I hestitated a bit turning it up so much in once now. So I did this in small steps and kept it on 0.35V and let's wait what happens.
Trimmer is now at 2/3 turn.
Is this 0.1V measured voltage, because of aging of the components or could there be a specific reason why this was so low tuned ?

Another thing around this bias circuit:
The big 2W resistors and diodes around the bias trimmer get very hot. Is this normal, or is there a reason for so much heat there ?
Voltage drop over this bigR is 48,8 to 17,8 volt, so I am not very surprised.
Elektrolyt caps are original: 50V 150MF Mexico, silver coloured.
The pcb is bit brown coloured around the legs, but this has been there for a very long time.
If this heat is normal I will not bother any further, because this 37 year old amp is performing perfect in sound. Or does this tell me it needs a cap job ?
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Wednesday, July 03, 2013 - 02:15 pm:   

If it hasn't been capped, now is the time before it goes boom. New tubes will pull different current so the adjustment is normal. Just means the tubes are different. If you CAN'T get the voltage up on the drain resistor, then you might have to juggle the bias supply. Getting hot CAN be normal but but they might be old too. I usually replace that stuff weth metal oxide resistors so they will take the heat. I would suggest capping it now before one of those electrolytics takes a dumps, shorts out and take a transformer with it. Mike.
Tom Bruyn (tommy64)
Username: tommy64

Registered: 07-2013
Posted on Monday, July 29, 2013 - 08:08 am:   

Ok Mike, I started to recap my Music Man 212 after my last post.
I recently noticed there was something not well with my old '76 amp, especially drasticly reduction of low frequencies.
I put on a signal generator and found that with frequencies under 1 kHz the sound from the speakers started heavily clipping. This could point somewhere, I guessed.
I thought, NOW really is the time to do something with the amp.

As you see on the photo, I found out the old filter caps really needed to get replaced by new ones....
A mushroom growing from the top.
They exploded and obviously were doing nothing there anymore.
Either you get a shortcut which ruins your transformer, or they will get endless high. So I was lucky.
Replaced all nine caps by new ones, and tuned bias again on the new JJ 6CA7's.
Also changed the big bias resistors and the two +-16V zenerdioides.

Also found out that the little static cracking I sometimes had, came from a bad soldering on the driver board.
If I ticked against one of the brown Mallory .047's I could reproduce the sound. So I resoldered this whole paper board.
This 212 amp is now ok, and I know for sure I saved her live.
I have lot of electronic and soldering experience, and only recently started on my tube amps.
This amp was already heavily used when I got it in 1992, and obviously had some drink poured over it.
I would not suggest this job to everybody unless you are very sure and take your time. You certainly need to have an eye for bad spots in soldering and components to start with.
I decided to do NOW my other two MM amps too...
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Monday, July 29, 2013 - 12:42 pm:   

Yuck! Sometimes that crap stinks too. Glad you caught it before it took out the PT. Mike.
Paul Huish (oweyes)
Username: oweyes

Registered: 08-2013
Posted on Friday, August 23, 2013 - 10:45 am:   

It is rare I have to resort to the global force of technical electricaries to solve a valve/tube amp problem, but this has got me pulling my hair out and I am at the end of the line in terms of solving it. So here goes and any suggestions (apart from binning it) I'll take on board:

A client gave me his Musicman RP-65 for repair (Chassis GP-2 / 3) - He'd not used it for sometime and when switched on a terribly load hum was heard - I verified this to be the case on a Variac, with hardly any musical output sound coming out.

So, as a first step I did the following:

Checked both EL34s - OK
All electrolytics replaced
Both 3.9K resistors replaced
Generally checked values of other related output components.

Hooray I thought as I wound up the Variac again and set the 25mV at the 3.9K resistors. the loud hum was no more. However the sound was still literally none existent and when wound up to around the 8 mark on both Gain and Volume, distortion set in.

So I tried the following:

Two different sets of EL34 valves - No difference (checking the Bias each time).
Cloned an O.T. with no difference
Changed IC7 a number of times
Replaced the 16V D10 & D11 Zenors
Disconnected the Diodes from Pin3 to Earth on both EL34s and the cap there also.
Replaced the 50pf Pin4 to Earth cap on one of the EL34 valves.

No changed, still far too quiet and distortion up at 8.

Injected a 3v sine wave in the 3v Line In (using a stereo jack of course) and got a nice clean healthy hitting the Base of both JE1692s. However the signal out of the Collector was not to my liking, being around 40% higher on one collector than the other. So I replaced both drive transistors with 2N6488 as recommended in the MM Service Bulletin.

Still no change!!!!!!

I had during the above been measuring the various voltages and I again measured them and this is what I got (they were so close only one list is shown).

Pin1&2 = Earthed
Pin3 = 680v (H) / 450v (L)
Pin4= 336v (H) / 223v (L)
Pin5= 22.16v
Pin7= 5.9vac
Pin8= 66v (H) / 55v (L) => Collector Voltage
Off of the Drive Transistors
E= 26mV
B= 350mV
C= 66V

IC7 (LM1458)
Pin4&8= -15.6V & +15.6V respectively

Between D7 & D8 = 0.65v and before D7 = 1.3v

I only have the valve bases left to change now, since I don't think there is anything else on the output side that I have not verified as functional. The 3v Line In test proves it is not on the preamp side and the Volume pot performs without any issue at all.

So there you have it - Any magical solutions gratefully accepted.
Paul Huish (oweyes)
Username: oweyes

Registered: 08-2013
Posted on Saturday, August 24, 2013 - 03:28 am:   

A lesson learnt - Never hold stock of old transformers UNLESS YOU MARK THEM GOOD or BAD! Tried another O.T. and found problem solved.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Saturday, August 24, 2013 - 07:52 am:   

Glad you found it. Unusual for a MM OT to go south but anything's possible. That's why somebody makes them. I would be kind of interested to hear if you measured the windings of a good one and the bad one to compare. Mike.
Paul Huish (oweyes)
Username: oweyes

Registered: 08-2013
Posted on Thursday, August 29, 2013 - 04:28 am:   

Sorry Mike, that was something I never bothered to do - I mainly use the component tester side of my scope to pick up faulty pieces.
Gaëtan Vincent

Problem bias

Hello, i'm a french without google translate! sorry for my english...
SO, i have un problem with the BIAS, on my 212 HD 130 the value is 9mV and it's the maximum... do you know why ?
Of course, i've replace all the cap.
the transistor is BD911X, the problem can be here ?
Thanks for your help.


112HD 150 EVM

Hello, are any of the tech's who wrote this blog still available to answer a question?


That was one question

The answer is yes, now you have two questions left.

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