Red-plating Champ amp...

[Bob M.]

Hi all, I have a red-plating Champ amp that's giving me some trouble. Now, normally I just swap in various (new) 6V6 tubes until I find one that won't red plate but after 5 or 6 tubes, all of which are red plating I decided to change out the cathode resistor. I started with 510 ohm, 5 watt then 560 ohm, 5 watt and finally 600 ohm, 5 watt (the stock resistor is usually a 470 ohm, 1 or 2 watt). Now with the 600 ohm resistor in the bias (cathode) circuit, the 6V6 tube still red plates but far less than before (but in a darkened room, you can see that vertical red line on the plate of any 6V6 I might put in the amp). However, I think I should eliminate any sign of red plating to make this amp long lived. Now, the plate, B+ and cathode voltages are all completely in line with most every SF Champ (or VibroChamp) I've seen. The tubes I've subbed in are pulling about 39 mAs to about 45 mAs, depending on the tube I chosen. The amp sounds perfectly fine, just like a Champ should. I own about 5 or 6 Champs/VibroChamp and have probably worked on 40 or 50 of these amps over the years. The cathode cap, the screen grid resistor (which I added) are all brand new, in fact, all the parts in and around the power tube are new. All voltages in this amp are quite typical, nothing sticks out as abnormal. Any ideas what else to check that could be causing this amp to continue to red plate? I'm feeling a little stuck on this one.
Thanks in adv,
Bob M.

[Helmholtz]

What are 6V6 plate, screen and cathode voltages (depending on cathode resistor)?

[tboy]

Any ideas what else to check...

You didn't mention checking the control grid for any positive DC voltage. Can we rule that out?

[Helmholtz]

Positive grid voltage would increase cathode voltage. That's why I'm asking for the voltage values.

https://www.ampbooks.com/mobile/clas...der-champ-5e1/

[g1]

57 re-issue runs about 13W idle dissipation. 340VDC plate, 305V screen, 18V on 470R cathode resistor. To be red-plating it seems your plate voltage must be quite a bit higher than that.

[drewl]

What tboy and Helmholtz are saying is, if the coupling cap connected to pin 5 of the 6V6 output tube is bad/leaking positive dc voltage onto the grid, it will cause red plating.

[Bob M.]

Hi All,
Thanks for the on-point replies.
I did check the grid voltage and it seemed too high (1 Vdc) so I swapped out the (brand new) coupling cap into the grid for another and the voltage dropped to about 8~10 mV. The amp still red-plated but only a very little bit now, and seemed to be running and performing fine but I'm keeping a close eye on it in future.

Here's the other voltages:
B+ 409 Vdc
Plate 399 Vdc
Screen 397 Vdc
Grid 8~10 mV (now)
Cathode 27.43 Vdc

It's a silverface variety amp and uses the ubiquitous power transformer L010020 (also used in the VibroChamp, Princeton and Princeton Reverb of the same era). These voltages are a little higher than BF Champs or tweed Champs. The only mods in this amp (besides the increased values for the cathode resistor and cap) are a 510 ohm/1 watt screen resistor and a 270 ohm grid stopper. It would be great if I could eliminate all signs of red-plating but I have lessened the problem alot.

Bob M.

[g1]

What is your line voltage? Heater voltage?

[Dave H]

It would be great if I could eliminate all signs of red-plating but I have lessened the problem alot.

You could reduce the B+ voltage to around 350V by adding a resistor in series with the center tap of the B+ winding and ground.

[Helmholtz]

Please find and post appropriate schematic.

1V more or less at the grid has little effect with a cathode biased class A stage.

Assuming a cathode resistor of 600R, your cathode voltage means a cathode current of 46mA, resulting in a total dissipation of 17W. This is above the 16,2W limit for a 6V6.

A grid stopper has no influence on dissipation or redplating. It is used to prevent parasitic HF oscillation. But 270R is too little to have any significant effect.

You could bring down dissipation with a considerably higher value screen resistor, say 10k or more.

[Bob M.]

Thanks for the additional replies.
My line voltage was 122.0 VAC when I took the above measurements. It is normally pretty high in my shop, usually between 119.0 and 122.0 VAC. The heater voltage was 3.34 /3.33 VAC. I use ground reference heater resistors and have a twisted pair heater string, as I like quiet amps.

The addition of screen grid resistors and grid stopper resistors just sort of evolved over time with me. I'm just trying to make these little practice amps as good as they can be and I've borrowed some practices from the Fender big brother amps. Generally, I've been pretty successful at getting some more performance and fidelity from these under appreciated amps but this particular one has resisted my coaxing. I'll consider a higher value screen grid resistor, as long as it doesn't have an ill effect on the tone. Thanks for the suggestion.

I doubt I'll drop the B+ much, as I like bright amps and I'm not much of a fan of that 'browned out' sound. If I wanted that whole thing you get with 350 Vdc (B+), I think I'd just go buy a tweed champ.

Bob M.