MusicMan 2475-75 Problem

[Zephyr]

Hi All,

Sorry about the length... this amp circuit design is unlike anything I've encountered.
I understand how it works in general, but it's.. well..unusual and interesting.

A friend asked me to check the Sylvania STR387 tubes from a Musicman 212-75
(2475-75) amp. He said it was 'shutting down' after about 20 minutes.
My tester is an old Mercury 1100A tester. Just a go/no-go emissions tester.
Tubes checked weak but still functioning. No shorts etc.

So I put then back in and we plugged in switched to High Power and let it idle,
strummed occasionally to check it.
Sure enough, after about 20 minutes, no output. No blown fuses, red plating, pops,
or crackles. No Power supply humming either. Just silence. Power indicator still lit.
Switched off and when powered up 5 minutes later it worked normally.
Tried it at idle and played it switched to Low Power. Didn't quit.
Switched back to High and repeated the loss approx 20 minutes later.
Didn't shut it down. Switched to low power and started running volume
pots min to max, and dimed on the #1 non reverb pot the signal came blasting out
at full volume.

I brought it home to check it out.
Pulled the chassis, and on the bench, could not get it to repeat the problem.
Ended up spraying the pots with Caig's DeOxit and cleaning the input jack switches.
That was 3 days ago and after, many test runs at idle and with guitar plugged in.
No more signal loss. Still not certain that the problem won't come back later.

Measured (Hi/Lo)740v/500v on Plates, 380v/245v on screens, and +22V on Grid.
No evidence of any mods or any previous repair work except for the power switch that had been replaced. Everything else looked good - like new.
Circuit board had FP 2-26-81 written on it. Guessing that was the date of assembly
and initials of assembler or board model.
Driver emitter resistors(3.9ohm) measure 4.3ohms in circuit.
Those resistors seem to be a little over the 5% tolerance.
Driver emitter voltages are 76mv and 80mv.
I've read biasing instructions online that state the lowest should be set at 25mv and
the other should be no higher than 55mv.

Should I go ahead and replace the emitter resistors before retubing and rebiasing?

And, with 740v on the plates, which brands of 6L6GCs could stand the punishment?
Should I tell him to find NOS replacements (expensive) or maybe get a a 'replica'
TAD GE 6L6GC STR Fat Bottle - Tall Black Plate here:Tubes
or Ruby Tubes 6L6GCMSTR Pair Matched from Parts Express (much closer to me)

[Enzo]

Measure those resistors with your ohm meter. Now reverse the leads. Does it measure the same both ways? If it does, good. If there is a difference one way from the other, like 4.2 and 3.6, then there is some small residual voltage in the circuit, and it is confusing your meter.

If both ways measure the same, then touch both probes to the same end of the resistor. Does it read ZERO ohms? Or does it measure something like 0.3 ohms?

When measuring low resistances, ALWAYS check the resistance of the meter probes first.

[Zephyr]

Guess I should have been more clear but that post was already too long...

The probes add 1.4ohms to reading which I subtracted from the actual 5.7ohms reading for the 4.3ohms. Checked numerous times both ways to be sure.
Same result every time. Double, triple, quadruple checked probes and measurements.

But you got me thinking... I just hooked up the 25 year old Radio Shack
Micronta 22-210 I used in Tech School, zeroed the probes, set it to the 1ohm
scale and measured 3.85 or maybe 3.9 ohms ohms on both resistors with it!
Thanks...
...think this is a case where analog beats digital?

[Enzo]

Nah, just a case of one meter yields results we like better.

[Zephyr]

OK, good point.
But there's another thread I read here recently about someone who was trying
to measure the resistance of a speaker or OT with a DVM and was told that at those
low resistances many DVMs just aren't accurate. Guess I need to check with a precision
resistor.
Any thoughts on replacement tube?

[jrfrond]

Did you actually TRY a new set of 6L6's? That would be my first line of attack.

No to worry about the 740V on the plates. This particular amp design is similar to the Williamson amp design, which uses grounded grid/cathode drive to maintain very low interelectrode capacitance, and was often used in RF amps as such. The power tubes may appear to be taxed by the high plate voltages, but the reality is that they are hardly breaking a sweat with this design, and tubes tend to last a VERY long time. Not the warmest-sounding power amp design on the market. I have seen occasional driver problems on these though.

[Zephyr]

Thanks John and Enzo. Advice much appreciated.
Didn't have new 6L6s and wasn't quite sure about replacing those STR387 with 6L6s.
That 740v B+ made me wonder if the 6L6s would handle it. Ordered a pair yesterday.
Also ordered some 1 ohm 1% resistors to see which of my meters is better at low resistances.

[Zephyr]

Think I've got it fixed...

Received the new 6L6s and found out they tested the same as the
STR-387s. My tube tester was reading low... think I've
got that fixed also, but have a few questions, but that's for another post.
Also got the 1 ohm resistors and it turns out my DVM was accurate.
(bet Enzo smiles when he reads that)

Hooked up the DVM and attempted to set the bias at 25mV on the 3.9ohm
resistors. Had to turn the trim pot nearly to one limit and after
playing the amp for a while, it started drifting up...over 120-130mV
and still climbing gradually.

Started checking and found one the diodes in the bias circuit under the
bias trimmer looked damaged. Half the shell was missing and it looked
like someone had painted the lead on the damaged side black. But it
didn't look burnt. Not sure if this was done during assembly or
something caused it to pop open. It fell apart at the junction when I
desoldered that lead.

Replaced both the diodes and both the 3.9ohm emitter resistors, set the
bias and checked it for a couple of days before returning it to my friend.
Called him a few days ago and it was still working fine.
So far, so good.

Thanks again all.