| Author | Message | ||
     Eddie Morin (loopyeddie) Username: loopyeddie Registered: 06-2012 |
I'm pretty new to servicing amps, and very new to Music Mans. I just got a new to me, but very dirty 212-HD 130. It had no sound. Cleaned the pots, it came to life. I replaced all the filter caps and some leaky electrolytics inside. Tested it out, for a while, everything was fine. I replaced the tubes with 4 new matched JJ 6CA7s and 1 12ax7. Still sounded ok. This is where I got stupid. I started adjusting the tube bias. I've never tried really messing with bias. I've always just slightly adjust it to get it right. I wanted to hear what it actually sounded like in the full range. So I turned the bias trim pot all the way clockwise, took note of the sound. Then turned it all the way counterclockwise. Just a second after I turned it all the way counterclockwise, I heard a very slight pop. After that the volume is very weak and it sounds pretty distorted all the time. Any ideas of what I might have blown up? I swapped the tubes back and it changed nothing, so they are ok. I checked all the diodes, I'm pretty sure they are ok. My next guess is the transistors, but I never messed with the transistor bias. I'm going to keep reading as much as I can find, but thought I'd post this incase anybody knew exactly what I blew up. Thank you in advance for any help! | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
No help exactly but you obviously had some form of malfunction. First thing though, verify that you didn't smoke the speakers. Try it with another cabinet. Probably NOT the problem, but it's diagnostic 101-start at the end and work backwards. You say you put the other tubes back in and it's the same. Did you look at the tube sockets? Being THAT old and dirty, corroded tube sockets that are loose can be a mess. Tighten, clean and re-try. THEN we'll get in to the other stuff. The fact that you were adjusting the bias driver bias control is suspicious so right off the bat, you should check the driver transistors and their associated compnents. Barring that, it's going to take some investigation with more time and equipment. Do you have a scope? VTVM? DMM? Let us know what you have and your experience. We don't want to get you killed! | ||
| Bryan Knox (bamaauctioneer) Username: bamaauctioneer Registered: 01-2011 |
Eddie, I would also like to recommend you also visit another forum I like to read... http://ampworkshop.yuku.com/directory They have a lot of guys on there that have been very helpful to me. | ||
| Eddie Morin (loopyeddie) Username: loopyeddie Registered: 06-2012 |
Thanks so much for your help so far! I don't have anywhere else to go (sob sob!) I tested it with another speaker, same thing. I already cleaned and tightened the tube sockets, but I'll try it again. I don't know how to check the transistors (other than replacing them) or what their associated components are. I've never worked with solid state. I do not have a scope or a vtvm, but I do have a good DMM. I have fixed a few tubes amps up so far, but I'm pretty inexperienced. I've read about a half-dozen tube amp service books already. I'm really aware of what the dangerous parts of the amp are as well as how not to get killed. Oh, I did read the "how to bias a music man amp" on this forum. One thing that confuses me is that it says to read .5v across the 10ohm tube resistor. Even if I crank the pot all the way up I can only get to about .35v. Is that a problem or is it .25v per tube pair (totaling .5v for the 2 pairs)? Thanks again, this forum has been such a great resource for the few days that I've been on it. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Oh wait a minute. I don't know why but when you said transistors, I thought you had a SS driver amp. That point Y crap is .5 volts per pair meaning 500mV and divided by ten = 50mv or 25 per tube. I would start checking the driver board components. Cap shorted, resistor burned, etc. Then let us know. Also, check the 1ohm resistors themselves, though I doubt it would be bad. Just narrowing things down. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Actually, the more I thought about it, check the bias supply board. MAYBE it's generating WAY too much grid control voltage and squeezing the power tubes. It's hard to tell without the amp though. It needs to be checked from back to front. Hell, it COULD just have a driver tube that took a crap. It's the voltage reading that you get across the 10 ohm though that makes me think the bias has gone wacko./ Mike. | ||
| Eddie Morin (loopyeddie) Username: loopyeddie Registered: 06-2012 |
DING DING DING! You got it mike! As big of an idiot as a feel for dinking with the amp before reading everything, I feel like a bigger idiot for not seeing the problem sooner. It was in fact the driver tube that was bad or I blew up. I guess I didn't check it earlier because I figured it had something to do with the power tubes. I have learned a lot from this forum already, I plan on reading a lot more and probably picking up some more music man amps. i bought this one on a whim, not really sure what to expect, but I am falling it love with it more everyday I have it. Thanks again for working so hard with me to diagnose this problem! |