Music Man Amps

SWITCH and transformer. Switch just adds some transformer coil for low power so MAYBE either the switch itself is bad or the low power added section of the transformer is bad. Check the voltage going into the transformer from the switch in both ways. That's where I'd start. Mike.

Hi Ian,

if your power transformer is original, the power hi-off-lo switch on the front switches on the primary side of the power transformer. Basically the tubes are running at approx 85% voltage on the low setting (including the filaments). That last part makes the sound difference between high and low more pronounced in the older amps compared to the newer style circuit where the switch is on the secondary side of the power transformer and only affects the B+ feed to the voltage doubler.
Disclaimer: The following information is intended for someone who is capable of and comfortable with working inside a tube amp with its high voltages. Do not attempt if you are not sure how to keep yourself safe when working inside a live amp.
Start by measuring your mains AC voltage. This is your reference. Inside the amp you should be able to find this same voltage between the input of the hi-lo-off switch and the black primary wire of the power transformer. When the switch is set to 'lo' the same voltage should be found between the black primary wire and the terminal on the switch with white wire. Same in 'hi' mode - the voltage should now be between the black and red-yellow (if memory serves). If you get a lower voltage on the 'lo' setting side, you probably have a bad switch.
However, the problem may not be in the primary side. What kind of voltages are you getting on your power tubes (and the 12AX7) in 'lo' mode? Plates should be 725ish in hi and maybe 595 give/take in lo mode.

Sincerely,
Lars Verholt

Depends on WHERE you measure it. Tubes are actually DC creatures. Here, do this. Measure the voltage at the standbye switch. THAT will be AC still. In high, mine measured at 245VAC and on low, 161VAC (To ground, of course). THAT should tell you if the voltage coming OUT of the PT is switching properly. THEN we'll go from there. Mike.

I've got mine apart at the moment if you need any other voltages or measurements. Also have pics. Mike.

Sound like the switch is functioning BUT, you MIGHT still have burned contacts in that the VOLTAGE will pass but the amount of current required is too much for some burned contacts to pass. Just for experiments sake, you might try jumpering the switch with a wire from source to low side and see if the amp performs. If it's still the same, go downstream and check the voltages at the test points. There, you will get voltages out of the choke at 330 high, 221 low and the point h measurement on mine is 448 low, 667 high.

One thing I forgot to mention is that you should also verify that the +and- voltage running the IC's stays constant when you switch the power. It's not SUPPOSED to change but it's one more thing to check. Should be around 15VDC getting to the IC's. Mike.

If you look at the schematics for the 65 series amplifiers you will notice both OLD (1974-76) and NEW (1977-1980) versions.  The two main differences relate to the Phase Splitter driver (12AX7 vs Transistor) AND how the amplifiers switch from LO to HI output power.

On the older amps, the Hi/Lo switch is also the power switch (Center position is Power Off).  The second toggle switch (on the back) is for Standby.  In this configuration setting the amp to LO power is accomplished by selecting a different winding on the primary of the power transformer.  This causes ALL voltages at the secondary to drop including feeds to the preamp power supplies and the heaters which causes the amp to sound noticeably different in LO power than in HI power (as mentioned above).

The later versions made after 1976 or so (newer schematic) separate the Power On/Off function from the Hi/Lo power output selection switch.  The Hi/Lo switch is now on the secondary of the power transformer in the B+ high-voltage plate supply and switches between secondary windings only associated with the high voltage to the output tubes, producing either 700Vdc or 350Vdc plate voltages.  Standby is now the center position of the HI/LO switch and the second toggle switch is now Power On/Off (now both on the front panel).  The low voltage supplies to the preamp are unchanged.  This was done BECAUSE of the sound quality difference between these power modes on the older amps and the complaints that they got from customers.

The power transformers are different between these old and new models (this applies only to the 65 and HD130 series).  Because of the transformer differences, it is not possible to rewire an older model to be like the newer ones because the required LO tap on the B+ secondary winding simply is not there.

I am not saying that there isn't some sort of other problem at work here, but these olders amps HAVE to sound different in Lo power mode than in HI power mode because of the design.  Verify that the voltage at pin 5 (as well as the preamp +/- 16Vdc power supply voltages) doesn't change between LO and HI modes.

I forgot to mention the pilot light thing!  Many people have complained about the Neon pilot light that will not illuminate when the amp is in the LO power mode!  As the Neon bulb ages it is less tolerant of low voltage and will simply fail to fire on Low-line AC!  All of the low-power design issues were solved in the later models so I prefer those.  However, if you run these older models in the HI power mode there should not be any special issues.

Clean high-resolution scans of ANY clean Music Man schematic would be most welcome.  All the ones I have posted here were from the Ernie Ball web site and many DO look like faxes but are actually just low-res scans.  Apparently, even Ernie Ball doesn't have many clean copies of the original schematics and they bought the company!  Some of these scans are VERY difficult to interpret and you have to look at the cleaner schematics of later or other models to help fill in the blanks (as well as looking at the internal circuitry while trying to decode those schematics).

First, your high-voltage (test point "H") should be over 700Vdc.  The fact that it is low will effect many of your other measurements.  My guess is that the amp needs a cap job to fully restore the high voltage to where it should be.  This is assuming that you are operating the amp at the full AC input voltage that the power transformer is designed for.  For example, that 660Vdc might be normal for a 240Vac transformer being run on a 220Vac input.
 
However, operating the amp at low AC line voltage will potentially be another contributing factor for premature distortion when the amp is switched to the LO power position.  Low AC line AND low DC due to old filter caps in combination could be responsible for all the elements (power supply voltages, bias settings, heater voltage, etc.) likely to contribute to your early distortion in the LO power mode.

Choke:  OK for the B+ (high voltage) you are measuring.

Test Points:
A: OK
B: In the ballpark, you need to check the bias at the 10-ohm resistor as indicated on the Factory Service Bulletins.
C: OK
D: No connection here so this looks normal.
E: This is Ground so it better be 0Vdc!

EL34 pin 5:  This is the same as Test Point B but through a few resistors so it is a bit lower than Test Point B due to voltage drop caused by bias current.

12AX7 pin 5:  This is one of the heater connections and has no relevance here.

If you look at the schematic to the left of your Test Points you will see a large + and -.  These are the +16Vdc and -16Vdc supplies for the op amps.  You can measure them from the two zener diodes shown (preferred) or the on the IC's from pin 7 (+16Vdc) and pin 4 (-16Vdc).

The DC voltages (and power output) should normally be measured at 120Vac input (usually set by using a Variac capable of boosting low-line voltages).  At 116Vac into an older model that 660Vdc is probably a good number but it seems a little low.  The LM307 ICs are not socketed at the factory so you would have to identify the right pins and measure the voltage in circuit.  It is probably better just to measure these voltages at the Zener diodes as you have done.

I am assuming that the Zeners do not continue to drop until they are very low in the LO power mode.  Zener diodes are not extremely precise voltage regulators by themselves.  Small deviations like you see can easily be ignored.   The Zeners are there to prevent large deviations from occuring and keep the DC voltage reasonably consistent. I am a little surprised that the Zeners aren't closer to 16Vdc but this by itself is not alarming.  The LM307 spec sheet says that 18Vdc is the max DC supply voltage and 17.3 you measured is approaching that.  Most importantly you do not seem to be suffering from extremely low supply voltages on the op-amps.

Actually, in MY 2-10-65, the IC's ARE socketed. They are in the 130 chassis that I have here too.

None of my 130 or 65 amplifiers have sockets for the ICs, they are all soldered into the PC board.  Weird, maybe you have all very late models?  My newest is 1977.  I have only seen sockets on the later RD/RP and 75/150 models that use DIP 8 IC packages.

Seeing the zeners move around a little isn't too scary BUT if they keep falling that points to an issue of some sort.  I would check the source of the voltage and resistor that feed the zener to see if the source is falling below the zener threshold.  Also, if the zener is supposed to be 16Vdc and you are seeing over 17Vdc then SOMETHING has a problem or is out of spec.  If they hold that voltage there shouldn't be a problem (even if they wander a little) but if they continually lose voltage until it becomes an issue something else is going on.

here's a pic of the inside of my 65 showing the sockets. My 130 has them too. OOPS. said it resized it but have to do it manually.

You know, does YOUR amp have sockets? A little corrosion goes a long way when you have low signal levels and maybe low voltages. Also, we never DID ask the question if it does it on BOTH channels.

I'm beginning to think you're going to have to put a scope on it to find where in the chain that it's fuzzing. With both channels doing it. it pretty much means that it's in the finals so we're talking power amp. Did you ever change the driver tube? Actually, did you ever change the power tubes with another set? Mike.

I'll be honest with you. MINE would break up on 5! They do distort more on low though. I actually use mine and find it to distort on anything past 3.5-4 on the channel with the master on 10. I actually like it that way so I never investigated it further. I ALWAYS was going to but never did as it works out fine for me. I don't need to play any louder than I already do! That being the case, I always wondered if the gain structure of the front end was either too hot OR it was being overdriven too early. If I were to investigate further, I would look at how hot the front end is and MAYBE, try a cooler tube in the PI. I know Lars doesn't like that idea but it might give you some way of determining that the driver is overdriving the power tubes. Maybe use a 12at7 instead of the 12ax7 and see what it does. Always was going to try that in mine but just never got around to it. Mike.

Originally, the amp PROBABLY came with REAL 6CA7's though, even though they later switched to EL34's. EL34's will sound different and break up more than 6CA7's. That's why Marshal used them. Not trying to steer you anywhere but you MIGHT end up trying some KT77's or 6CA7's. I know JJ makes them but have NOT tried them myself. Just fishing here. Like I said, mine breaks up but I kind of like it that way for my use. Mike.

Well, I don't have any suggestions without being able to SEE or HEAR where the distortion is coming from. It's going to be hard to probe and crash a guitar chord at the same time. Are you using a humbuck guitar? If so, does it distort if used in the lower gain(number two) input? If less, maybe the first stage gain structure is wacky. Or bad op amp or shorted resistor IN gain structure, thereby changing the gain structure. I mean, if it distorts in the same way from BOTH channels(reverb channel and straight). that pretty much says it's AFTER the first stage op amps and maybe at the summing amp, OR, they are all crap. I suppose you could reduce the signal going INTO the driver board a little more and see if it's overdriving the back end or not but like I said, that's all a crap shoot unless you know where it's coming from and without being able to see it, It's going to be tough. Personally, I LIKE a little hair in it, unless it's REALLY distorted. Also, just a thought. I seem to remember several years ago, there being a discussion here about some amps having the wrong taper on some pots and them being wide open way too early. You might chack the taper of the volume pots on the channels. I'm just stabbing in the dark here. Mike.

After thinking about this, I have to figure that either the last IC in the chain is distorting or the PI is overdriving the power tubes. You might try changing the last IC with one from earlier in the chain and see if it cleans up ONE channel. After that, look at all the components in the driver circuit, since you said it's doing it with other tubes and even lower gain PI tube. Again, without being able to walk through it, it's tough. Mike.

Did you ever get back to this and get in to it any more? I hate unanswered questions! Maybe I'll go into mine and work on it someday, just for grins. Not like I'm doing anything ELSE with it! Like I've said before, mine works the way I use it but always thought I'd try to clean it up a little. Mike.

Not really unless you have a ESR meter to check them IN CIRCUIT. You may get lucky and pull each cap and measure it with a normal cap reader and find one shorted or wildly out of spec. Or you can shotgun it and just order them all in the path. Mike.

Actually, you should have a clean signal everywhere. I guess clean is a relative term though. Clean in, clean out.