Musicman 2100-130 help!

[Marc]

I'm at a complete loss to what the problem is with this amp. It sounds awful with no head room and distortion at low volume. I'm hoping someone with more experience with the MM amps can shed some light on this.....

Here's what I havefound:
This is the 2100-130 model with the 12AX7 PI.
the B+ is 643V, the screens are at 280V.
The schematic says they should be 725V and 360V respectively. the filter caps have been replaced. None of the resistors in the power supply measure out of tolerance.

Bias voltage at pin 5 is -32V and the tubes are drawing about 3ma. (0.05V across the 10 ohm cathode reisitors which have drifted) even if I turn the bias pot all the way they don't draw much more current.

the preamp board is +/- 16V for the supply voltage.

So the power tubes are biased just about off. They are new TAD EL34B's.
The suppression diodes and caps on the OT primary measure good and the circuit doesn't change with them connected or disconnected.

Is it possible there is a problem in the OT?
Any directions to point me in are much appreciated!

Thanks,
Marc

[smokey]

Hi Marc,
I would check the heaters first, then the negative feedback resistors (r49 r48).

[Amp Kat]

That's .5 not .05 The .05 would be for a 1 ohm resistor these are 10 ohm. Should be around 23 to 25 ma's on those.

[Enzo]

Yes, your tubes are running way too cold.

The first filter is two caps in series, what voltage is across each?

Are both rectifier diodes OK?

Do the B+ voltage ramp right up to where they belong with the power tubes removed? Actually they should then be higher than spec.

[Marc]

That's .5 not .05 The .05 would be for a 1 ohm resistor these are 10 ohm. Should be around 23 to 25 ma's on those.

KB,

I'm aware it's suppoese to be .5-that's what so curious. Even with the bias b]pot cranked all the was I can't get it to budge beyond the 0.05 reading.....

Marc

[Marc]

You da Man!

Yes, your tubes are running way too cold.

The first filter is two caps in series, what voltage is across each?

Are both rectifier diodes OK?

Do the B+ voltage ramp right up to where they belong with the power tubes removed? Actually they should then be higher than spec.

Enzo,
You found it!
Well, at least you pointed me back to the problem. I completed the problem so it would present itself.

I measured at the top of the bottom cap it and it initially read 340V and the top cap read 370V accross it. So when I went back to measure the bottom cap again the + lead popped right out of the end!

Once I replaced the bottom cap, both caps read 370V and the Bias was reading at .49V. I'm not sure why the bias current was reading so low before, but it doesn't really matter now...

It seems the lead had not quite failed until I wiggled it that last time! yea!

Thanks very much

Marc

[Amp Kat]

Ok and nice job ! The main thing is you got it back up and running right. Dang wires.