Okay, I'm trying to make sure my Musicman is in the best shape possible. I love this amp, use it almost every weekend (for live shows and practice) and I want it to stay around as long as possible.
I am following the directions found in the MM Sercive Bulletin #1
First of all, the amp sounds really good, although it does have some hum and hiss (could also be from my stupid huge pedal board). I realize that I'm trying to fix something that isn't broken, but I like knowing what is going on with all of my gear, inside and out. The reading I'm getting are confusing me:
When measuring the mVDC on each emitter resistor in the amp here is what I get:
resistor A = 1.2mVDC
resistor B = 9.2 mVDC
This is with the bias pot right in the middle, where someone in the past dabbed ink or nail polish or something.
These are way too low, right?
At these settings, I'm reading 77.9 and 77.5 volts on my output tubes (pin 8).
With the bias pot turned all the way up, I get these readings on the resistors:
resistor A = 2.5mVDC
resistor B = 50.6mVDC
Any clues as to what is going on?
Thanks in advance for the schooling!
Bias
If I had to guess, either the drivers are crap, or the support structure is gone. 150 is the one without the separate driver board, right?
Bias
Hmm. No, it has as separate bias board. I would trash both drivers and just check/replace everything in the driver section. It won't cost much. Make sure you get a couple of matched driver transistors, if you can. Also, tubes themselves can show you a mismatched cathode drain. Are the tubes new, matched, good?
Thanks mm210, I am hoping it
Thanks mm210, I am hoping it is something relatively simple like the driver transistors. Would it make sense that the amp still sounds good, though?
The tubes are relatively new (2 months?) matched JJs.
Any suggestions on where to buy matched driver transistors? Or do I buy a bunch and match them myself?
Thanks again!
transistors
I'd just buy some and match them up as best as possible. Here's a thing I found about an EASY way. Skip down to the part about bottom line.
https://home.comcast.net/~ijfritz/MiscProj/transmat001.pdf
I love it! I've ordered
I love it! I've ordered replacement parts for the entire driver board. Thanks for your help. I'll update when all is done.
More trouble
Well, I ordered all the parts to replace the driver board, but before I had a chance to do it, some more stuff surfaced. At band practice the other night, the amp started to cut in and out as if the cable were bad, then it cut out completely.
The amp is still getting power, but there is no sound on either channel.
I replaced tubes, and no sound.
I was doing several things different at this practice:
1. While I always run a stereo rig, instead of my usual solid state Vox Beatle, I was using my all-tube Silvertone 1484 in stereo with the Music Man.
2. My bass player had plugged in his pedal board to the convenience outlet on the back of my Music Man. I have never used it before. Is it coincidence that it failed while he had his pedal adaptor plugged in?
Thanks for any new advice/knowledge. I still plan on rebuilding the driver board, but I want to have an idea of what else might be wrong. I love my poor 212-75!
Outlet
If it still lights up, they are unrelated. Sounds like you are going to have to find out where the signal is getting lost from front to rear. Do you have a signal generator and scope? Sound probe?