Musicman 212-75 Driver Tansistor Bias Calibration

[hossymandias]

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'm following the directions found here:



http://pacair.com/mmamps3/sites/default ... %20All.pdf

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 .

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!

[cxx]

Is that 77 volts on pin 8 ?

I had one of these in the past and they provided a loud clean but somewhat "hard" sound. These MM designs are pretty unique, with the transistor controlling the current through the tube. The grid of the tube is set at + 22 volts so the tube is always conducting. edit: maybe not, not sure what voltage you get on the cathode as the transistor approaches cut off.

I would suspect the tube before anything else and would try some new ones to see. I'm not sure which brands would be up to the rigors of this application. I can't help much more than that other than to speculate that the resistor and then the transistor would be next to check. Maybe someone has worked on them and could provide more assistance. I don't think I'd do anything until there is an audible problem.

From a practical standpoint, I'd invest in another. Can't beat a spare.

[Stevem]

Since the output stage is biased class B, the match on the output tubes means even more to low 60 and 120 hz hum levels in these amps, as does good first node filters for low ac ripple on the output tube plates, screens and even bias on pin 5!

[gingertube]

This is teh schematic I looked at:
https://docs.google.com/viewer?url=http ... 100-75.pdf

Hope that is the right circuit for your amp.

According to the Srevice Bulletin you shoud adjust the bias pot for 25mV as the lowest reading across the across the two 3.9R (6.4mA) but not have either exceed 55mV (14mA).

Make sure you are messing with the right adjustment pot FIRST.
The if you are still having problems then I would check the following:

1) The + and - 16 Volt supplies, check these by measuring directly across the zener diodes.
2) Check D3 and D4, make sure they are not open or shorted. In operation you should measure about 0.7 to 1V across each of them. You can use the multimeter diode tester but watch that one diode has the 100R bias pot across it so won't show proper reverse bias value.
3) If they are OK then I would suspect leaky C26 and/or C27, replace them.

Cheers,
Ian

[Stevem]

Note that since this amp runs its semi conductors from a positive and negative 16 volt power supply rails, do not short out or disconnect either one with the amp powered up or you stand a good chance of taking out many of the transistors and IC chips!

[hossymandias]

Thanks for all the replies, everyone.

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!

[Stevem]

The failure had nothing to do with the ac outlet on the rear of the amp being used!
Time do dive in and see whats up, do the output tubes have over 400 volts on pin 3 and pin 4, if so than the high voltage section is up and running good enough to pass audio, so than its off to the SS section to make voltage checks right!