I fixed my own tube amp

[Mr.Grumpy]

...and I didn't get shocked, didn't get burned, didn't blow any fuses, and didn't let out any magic smoke.

 

My amp had sat unused for a couple of months, turned it on and nothing. Well, not quite nothing, you could hear audio trying to 'break through' but it was very faint and crackly. It's a Music Man HD-212, a sort of Fender Twin on steroids, but it has its own quirks too. It's a backwards hybrid, with a solid state (op amp) preamp section but a tube power section. They managed to make a tube amp as clean and sterile as any solid-state amp.

 

I've owned the amp for about 25 years now, and have repaired it a couple of time previously. About 20 years ago, I replaced the driver transistors, and about 4 years ago I replaced the high-voltage filter caps. This time I went ahead and replaced the low-voltage power supply filter caps - there's 5 of them - and went ahead and replaced all the electrolytic caps in the audio path too. I figured that the 35 year old caps HAD to be the problem, and the amp would work when I powered it back up. NOPE! Time to start probing with the volt meter. The low voltage supplies are spot-on, but the B+ voltage was very low and seemed to jump around, hmmmmm.

 

So I had to turn the chassis over and remove the cover over the 'high-voltage' can. I noticed when switching from standby to "on" there was a small spark on the fiberboard where the filter caps, bleeder resistors and high voltage rectifiers are mounted. OH, it's a crummy solder joint that I made back when I re-capped the amp. I applied some liquid flux, got my soldering iron good'n'hot, and reflowed the joint. It works!

 

I did not need an oscilloscope or signal generator, all troubleshooting and re-biasing after the repair was done with a $50 digital multimeter from radioshack.

[t_e_l_e]

grats, but would you have found it if there were no sparks?

 

i'm really lousy in searching for cold solder joints and if there is not obviously something visual wrong chances are high i will never find the problem

[Mr.Grumpy]

grats, but would you have found it if there were no sparks?

 

I think I would have. I had checked the power supply voltages after I replaced the low-voltage caps, but didn't bother testing the B+ voltage until it was obvious my amp still wasn't working. I was working my way up the circuit...high voltage filter caps and rectifiers diodes are in their own enclosure on top of the amp chassis, so it was time to check under there. It's not a PC board, but a fiberboard with brass grommets in it, and the brass grommets act as the interconnect points.There's only a few connections, so I would have figured it out even without the sparks.

 

I once built a fuzz pedal with a mail-ordered PC board, everything looked fine and dandy, but it would.....not......work. I checked and double checked and triple checked my component placement and made sure everything was correct. Finally, in desperation I started probing with my ohm meter, checking continuity between component legs and the circuit traces. Sure enough, I found one joint that looked OK but obviously wasn't, as the ohm meter showed it to be open. A quick re-solder and it was fine. Just another reminder that you can't troubleshoot electronics visually.

 

And by the way, I am a trained electronics technician although I don't work in that field professionally anymore. I know which wires in my amp are the SCARY B+ voltage, and believe me, I am very wary when I have the amp powered up on the bench. The B+ is 470 volts in 'lo power' mode and just over 700 volts in 'hi power' mode.

 

[WRGKMC]

Its funny because I own a Music Man 65 head. I bought it off a well known guitarist I know who was moving out of state. He asked me to buy in for $50, partially because I did him allot of free repairs in the past, but also because he needed to sell all his gear so he could move back up north.

 

The thing is in absolute mint condition considering its a 1970's head.

 

The amp produced similar problems to yours. It made some nasty low volume distortion. It didn't smoke or blow fuses so The major items like Transformers were good.

 

Upon close inspection I found two bad solder joints. One to the deep switch and one on a grid resistor. Quick solder job and I was cooking with high test. I did go through and test the voltages as well. Found the amp was running EL34 tubes instead of 6CA7 Tubes which were the original power tubes used in Music Man amps. The EL34's would run the extremely high plate voltages but its one of those at your own risk fixes when the Big bottles became obsolete.

 

Within a month or two after getting the amp, Electroharmonix came out with 6CA7 reissues so I bought a matched set. I then realized why the bias wasn't right. The 6CA7 is a Beam Tetrode and the EL34's are Pentodes. The major difference is the 6CA7 can handle the nearly double bias voltages in the Music Man amps of 800V safely and produce some really nice fat yet tight lows. The bias settings made sence according to the Service manual that was originally written using those tubes.

 

Sound wise The 6CA7 is about as close as you can get to melding a 6L6 Fender tube and EL34 dirty Marshall tube together. It has a fat, round low much like an KT77 but has different harmonics and a high end sizzle. The 6CA7 is open mids subdued on the top end.

 

Eddie Van Halen was noted for using 6CA7's in his Marshalls back in the day to get his bigger then life Thump out of his amps.

[t_e_l_e]

 

And by the way, I am a trained electronics technician although I don't work in that field professionally anymore. I know which wires in my amp are the SCARY B+ voltage, and believe me, I am very wary when I have the amp powered up on the bench. The B+ is 470 volts in 'lo power' mode and just over 700 volts in 'hi power' mode.

 

i visited an electronic engineering high school (a special school format only here available). we got a lot of theory, how to calculate circuits (from basic to complicated) how do basic amplifiers work etc to how does a tv work... there was also a practical part, but repairing things and analyzing problems came way to short

i can read schematics, i (at least basically) understand how it works and can walk through a circuit board and maybe be able to find the signal path.

but i hate to search for the one bad solder joint.

i have two byoc fuzz pedals lying around, which do not work as supposed anymore and tried already two or 3 times without success...

 

[guitarcapo]

I had a Magnatone 460 that had too much hiss. I decided to replace the carbon comp plate resistors around V1 with metal film to see if that would help. Now the amp is totally silent! Great. I figured I had shorted something in the process. After an hour I took voltaged and got something like 35v at the cathode of V1. Turns out instead of 480 ohm cathode resistors I used 480MEG. Didn't read the meter right

[Mr.Grumpy]

Took the amp to band practice Friday night, and it worked great. 'Band volume' is about 4 on the master volume, about 5 on the channel volume, and in the 'lo' power setting. Tons of clean headroom, which is what these amps are known for. Thanks to the re-cap, it's dead quiet. No hum at all, just a little bit of white noise. Probably just the old 'confirmation bias' at work, but I would swear the amp sounds better after the replacing the caps, seems less shrill in the treble.

 

I was seriously considering buying a new amplifier, I'm glad I took the time to fix this one.

[onelife]

grats, but would you have found it if there were no sparks?

 

i'm really lousy in searching for cold solder joints and if there is not obviously something visual wrong chances are high i will never find the problem

 

That's the thing about tube amps. They require high voltage to run and that can result in sparks and blown up resistors which help a great dealt with the visual aspect of troubleshooting.