| Author | Message | ||
| Carl Montney |
Let me start by saying that this is a very good site for Music Man owners looking for info on their amps. I recently was given a Music Man 112RP-65. I'm not sure of the year but I remember that my friend had the amp new in the late 70's or early 80's. The only date I can see on the board is 10/18 but no year. Chassis is a 2165-RP and it has 2 6L6's. The amp just dosen't sound right seems dirty at low volumes and breaks up badly at higher volumes. The best way I can describe it is that it kind of farts at high volume it gets worse if you dump more bass on it. It seems to have lots of volume but I know that something is off. The speaker is OK. I put in a new set of Groove Tube G6L6CDTCB coke bottles (rating 5) but it still sounds funny. I've never been a fan of Groove tubes but that was all I could get here on short notice. I checked the bias and get a reading of 9.1 on one resistor and 24.5 on the other. The values pretty much stay the same regardless of how I adjust the bias, one reading is always lower that the other. There is no change even after swapping the tubes back and forth so I figure the tubes are matched and the bias thing is somewhere else in the circuit. I need some ideas as to where to start troubleshooting this thing. I'm decent with a VOM and usually can find a problem and fix it if I know where to look or it's a visual defect but I can't read schematics very well. I'm hoping someone can walk me through this. The chassis on this amp is very clean inside and almost looks new. No rust thanks to the amp living in the southwest desert for most of it's life. The amp has been sitting in storage for the last couple years. Everything looks OK and I can see no dimpled or leaking caps. I took some pics of the amp and posted them up where you can have a look see. Some of the pics are only so so as my digital camera is getting towards the end of it's useful life. You can see the pics here: http://www.carl-amie.com/mmamp TIA for any help with this amp. Carl | ||
| michael kaus |
Usually, fartiness(is that a word?), comes from either bad caps or crappy speakers. I would try another cabinet first but I believe that the filter caps will have to be changed to tighten up the low end. It looks like the rp's use the same driver balance like the 150's so all it does is balance the driver section going in on the CATHODE side. Bias voltage is split and feeds POSITIVE voltage to the grid on pin five. Can you look at the cathode current on both tubes at the same time? If not, try it in the middle and see if they balance better. According to your description, you should have a GP-3A printed on your main circuit board somewhere-that's the onlt rp that I see with 6L6's. The rest use 6ca7's. | ||
| Carl from Aus. |
when my transformer went it was really lifless and would fizzle out. Take it to a reputable amp tech who understands that musicmans aren't your average alve amp, someone on the site can make a recommendation to you, Good Luck. oh no - now there are 3 carls!!!! | ||
| Carl Montney |
Michael <<<< Usually, fartiness(is that a word?), comes from either bad caps or crappy speakers. >>>> The speaker is good, I checked it with another amp <<<< but I believe that the filter caps will have to be changed to tighten up the low end. <<<< Sounds like the right track. I recently looked at a Fender Blues Jr. that sounded similar and it had a loose solder on one end of a cap on the board. As the amp is so clean overall, I'llprobably go ahead and recap the whole amp. I think I'll check those 3.9 ohm resistors as well just to make sure they read in spec. >>>> It looks like the rp's use the same driver balance like the 150's so all it does is balance the driver section going in on the CATHODE side. Bias voltage is split and feeds POSITIVE voltage to the grid on pin five. Can you look at the cathode current on both tubes at the same time? If not, try it in the middle and see if they balance better. <<<< If you can give an idea about exactly what I need to do to check it, I can probably get you that info. If it sounds too tricky I have a longtime friend who lives about 90 miles away and has worked on MM amps in the past so he can help me. I tried the middle position which was where the trim pot was pretty much set when I first checked the bias. I adjusted the trim pot both ways and values pretty much stay the same regardless of how I adjust the pot, one reading across the resistors is always way lower than the other. The tubes look good and no cherry red stuff so I'm assuming the voltage is OK. The amp gets real loud but it just doesn't sound very clean and trys to crap out if you really pick hard. It never totally cuts out though. I remember hearing this amp when it was about a year old and remarked to my friend about loud and clean it was. The amp sat in storage for about 4 or 5 years but is remarkably clean inside. There is a very small amount of fluid in the can that covers the 100mfd 450 volt caps but it looks like it came from the sticky tape that secures the caps from rattling rather than from a leak but you never know. One of the 470m caps on the power tubes has some slight discoloration but other than that the chassis looks almost new inside. If you go to this link http://www.carl-amie.com/mmamp/dsc00002. jpg you can see some dark brown spots by the resistors on the lower right side of the board by where the power xfmer leads go to the board. Maybe it's got a semi cooked trace on the backside of it. There is also some discoloration on the lowermost cap by where the wires come in from the power xfmer. I dont think it's burned though, it looks like it was from when the wires were orginally soldered on. Also can you tell me what the part number of the IC just below the left power tube is supposed to be? In the picture it has a small black marker dot just below it. I can't make out the full # but it looks like it says ERS778. It's different from the others which are all LM1458's so I want to be sure it wasn't changed out with the wrong chip. <<<< According to your description, you should have a GP-3A printed on your main circuit board somewhere-that's the onlt rp that I see with 6L6's. The rest use 6ca7's. >>>> If it's printed on the board it must be on the backside as I've looked everywhere and all I can see is a handwritten date of 10-18 but no year. It had 6L6's in it when I got it but who knows maybe it should have 6CA7's and somebody put in the 6L6's cause they couldn't find the right tubes. Thanks for your help so far, I'm looking forward to getting this thing going. Carl | ||
| michael kaus |
Really sounds like it should go to a doctor. THe driver adjustment is actually supposed to seth the driver to a minimum of a certain voltage on the bias cathode resistors. The procedure is up here somewhere but if you are distorting no matter which way yo u go, it sounds like doctor time. The cathode driven amps are a didfferant animal with positve voltage on the grid. Steve is better with this amp than I am. If it's distorting and lower volumes, it sounds like the driver is either blasting the cathode side or one of your IC's is toast. Let me know if I can help. Mike. | ||
| Carl Montney |
To all Just an update. With the help of my friend he was able to look at the pics of the chassis and inform me that this amp should be running 6CA7/EL34's. I put in a set of Electro Harmonix EL34's and the so called fartiness has gone away. I rechecked the amp with another speaker just to rule that out. Now here's what I have. The amp still is not very clean. Using the high gain input it breaks up at just about any volume, not really bad unless you crank it all the way but it's not clean at lower volumes. It's somewhat clean in the low gain input, but is lacking any type of smooth glassy clean tone. The amp is really loud cranked up so I feel that it's getting full power with the EL34's in it. My voltage/bias readings as far as I can tell are as follows if I measured everything right. Looks like the plate voltage is around 740v. Cathode is 68.6 on one tube and 58.7 on the other. Bias across the two 3.9 resistors reads 8.9 on one and 25.8 on the other. Any ideas where to look next? I'm thinking of replacing all the IC's as they are pretty cheap and doing the caps as well. What is the best source to get the 100MFD 450v caps? Do you think I need to replace the driver transistors as well? Thanks again for any help. Carl | ||
| Carl Montney |
I just wanted to add that the PCB in this amp is a GP-2. I finally got around to lifting the board to have a look. | ||
| Carl Montney |
IT'S ALIVE!!!! With the help of my friend (well actually he did all the long distance trouble shooting) he was able to tell me where and what to check to get the problem resolved. He had it down to either a bad op-amp or bad driver transistor. The problem turned out to be a bad driver transistor which was causing the bias to go south. I ordered up a new set of op-amp's (mouser part #512-LM1458N)and a new set of driver transistors (mouser part #511-2N6488) and the amp is sounding pretty good with the bias set at 24.5 on one side and 23.5 on the other. I put the op-amps in first and then checked it and the bias voltage was still off so I replaced the transistors and the thing worked. I played it for about 20 minutes cranked with and without a master volume patched in through the 3volt front panel jack and rechecked the bias and was still dead on. I'll play it for a few days and see how it is. The brown spots mentioned in my earlier posts on the PCB were OK when I pulled the board and checked it. The traces were good but it was getting hot there at one time. The amp had the wrong tubes (6L6GC) in it when I got it, who knows for how long so maybe that led to the "hot spot" on the PCB and the failure of the transistor. It's a good example of just how bulletproof these amps are that the thing still worked with the wrong tubes and a faulty driver transistor and didn't blow up. Total time to fix it once my friend found the problem was about 1 hour (Camel Light breaks included). Cost in parts was $3.66 plus $16.96 for a can of Blue Shower cleaner degreaser. Shipping / handling was $8.00. Total cost to repair $28.62. Pretty damn cheap! The amp was given to me by an old friend so my total investment in the amp so far is $28.62. I'm planning on using it for my clean sound with an A/B pedal along with a VOX AD30VT for dirty sound with a Fender Standard Strat. The MM will also serve me as a small amp to run through a single 18 to use with my bass when I don't want oe need my big bass rig. Bass is my primary instrument. Thanks Carl  | ||
| michael kaus |
Good fix. The cathode driver has to be balanced and functioning properly. You did the right thing by getting both to start with. The dead giveaway is the differance in cathode current. There really isn't much else to alter it so you hit the nail on the head. Mike. | ||
| Carl Montney |
Just an update. Cost of the amp just went up somewhat as I got a replacement reverb tank, cover bag and set of RCA cables today. Tank installed and working. Not sure exactly what tank was in there but I used an accutronics #8BB2A1B the short 3 spring tank. Sounds good. I really don't crank much reverb on so even if the amp should have the large tank it still works OK for me. I picked up the tank from Antique Electronic Supply www.tubesandmore.com very nice people there. They carry a nice selection of amp restoration parts too. Total cost of tank, cover and cable was $32.00. That makes my total investment in the amp a grand total of $60.62. Still cheap. Later |
Tue, 03/25/2014 - 09:25
#1
