Cathode Bias Red Plating Discussion

[g1]

Can you lift both those cathode resistors and check resistance from pin 8 to ground?
Also power up and see that it is not red-plating (with both resistors lifted) ?
If you still have a problem, then I think there is a board issue. If no more problem, probably plate loading as Chuck mentioned in post #13.

[Mr_bibbles]

Thanks everyone, to clarify there are 2 x 6v6 in the power section. I think another tech was working on this before me, it's possibly that he added the second resistor- it's wired straight from the pin to ground instead of to an appropriate spot on the PCB. Not sure on this though.

It seems like the two resistors are in fact wired in parallel, when I removed both of them from the circuit and powered it on I still get red plating. Resistance from cathode pin 1 and 8 to ground with both removed became OL. (I measured it a few times, sometimes it showed OL, sometimes it shows 7Mohms, sometimes it shows 2MOhms..)

So in summation-

There are two Resistors wired in parallel, Possibly a modification, possibly intentional. There is no short from cathode to ground, but the tubes are still red-plating even with both resistors removed from the circuit.

Possible next steps- Something is pulling B+? Can we rule out a bypass capacitor short with these measurements?

[Enzo]

Red plating tubes means there is a current path through them. If you measure open to ground at the cathodes (2M or 7M or any other really high reading doesn't matter - that is open for us), then look to see if a tube is shorted internally. ANything shorting to heater?

[eschertron]

How about a low resistance to ground from grid 2? Seems unlikely, but with 'surprise' resistors, anything else is possible. Is there something special about the screen circuit in this amp? The red-plating is on both tubes, correct?

edit: I guess this is really a question: can grounded screens cause the plates to glow? Or do the screens melt first?

[eschertron]

Tried with (several) known working tubes, they all red plated. (These were crummy test tubes FYI).

Did you check the test devices to see if they still worked in other amps after they red-plated? Just wondering if the screens could have been damaged, given that now we know the cathode isn't the source of low potential.
Also, have you checked with different tubes, after removing both resistors and verifying no continuity to ground from the cathode pin?

[Enzo]

I think grounding the screens is a good way to shut off current in the tube.

[eschertron]

Which is why I asked, not sure if the cathodes floating makes a difference. Now there's not a ground-potential 'target' the size of the side of a barn, and the screens could be exposed. I'd expect screen dissipation could work both ways

[g1]

I'm wondering if there may be a pc board leakage resulting in cathode short to ground only when powered up.
If I read correctly, the voltage measured at the cathode was always close to 0 VDC.

[Chuck H]

B+ is dragged down hard
Red plating with an OL on the cathode
When cathode resistor is in there is less than 2V standing
Amp does this with several subbed tubes

This one is a corker. But I'me going out on a limb and saying the answer is in the measurements, and likely how those measurements were taken. I'll guess that your cathode bypass cap is shorted, but only when the amp is on. Your meters test conditions may not be sufficient to induce the short, which may be internal arcing. The very next thing I would try is lifting the cathode bypass cap.

[Chuck H]

I'm thinking the cathode bypass cap must be shorting with operating conditions and the DMM test parameters don't induce the short. The next thing I'd try would be to lift the cathode bypass cap OR just replace it if it's a PITA taking the board out and putting things back together. A new cap won't hurt if that saves labor.

[Mr_bibbles]

Red plating tubes means there is a current path through them. If you measure open to ground at the cathodes (2M or 7M or any other really high reading doesn't matter - that is open for us), then look to see if a tube is shorted internally. ANything shorting to heater?

Nope! Tube shows OL to all pins from heater.

I'm wondering if there may be a pc board leakage resulting in cathode short to ground only when powered up.
If I read correctly, the voltage measured at the cathode was always close to 0 VDC.

This sounds like the next step- can you explain a little more what you mean? Im guessing you mean a short somewhere in the PCB, but wouldn't it be limited in this case to the cathode bias circuit, which is only the two resistors? In this case, that would mean that the pins / traces around the resistor are damaged yes? If that's the case I'll poke around with a meter...

Did you check the test devices to see if they still worked in other amps after they red-plated? Just wondering if the screens could have been damaged, given that now we know the cathode isn't the source of low potential.
Also, have you checked with different tubes, after removing both resistors and verifying no continuity to ground from the cathode pin?

I repeated the experiment with another set of crappy,known working from another amp, test tubes (I have a whole box of 6l6's...) They both redplated, but one of them started much faster (I'll call this tube A). When I swapped sockets, they both redplated again, and tube A started first again in the other socket. Guessing that just means they're off by a few MA..

[eschertron]

Have you taken pin voltages with tubes in, under power, since you removed all components in the cathode network?
Did you say you were going to remove the cathode bypass cap? Is that out as of the last reported test?

...I think I mis-spoke when I asserted the cathode pin was not the source of the ground potential. Forgive me if I got ahead of you.

[Chuck H]

Cathode bypass cap.

[Mr_bibbles]

Cathode bypass cap.

That was my thought too, but I snipped both caps that could potentially be the bypass capacitor, one by one. each time I started it up, still redplated. Replaced both with 22uf new caps, still red plating.

BUTTTTTTTT

There are two diodes in the circuit, d3 and d4. I reconnected the cathode resistors, lifted the leg of one of the diodes, taking a guess that it might be a flyback / snubber / whatever you call it. Red plating has stopped, sound is good, a little buzzy. cathode of the Diode is tied to ground, anode tied to both pins 1 and 8. It's a IN4007, so I replaced it with a known good one. The red plating has come back!

So what is the function of that diode, and how could it be tied to the bias? Could it be possible that there is an underboard short to this diode, forcing current through ground from the cathode, effectively shorting the cathode ---> ground

[g1]

What is the other diode, and what is it connected to? I was wondering about maybe a zener somewhere and if it could be shorted.