Ampeg VT-40 Red Plating

[g1]

Then you were correct that it is the imbalanced drive level (you had tried swapping coupling caps which made no difference).
If you can find a way to balance the PI, I guess they would both redplate at the same time.
You mentioned you had to push it fairly hard to get it to do this, will the user be running it like this?
As it is biased fairly cold, I don't think you want to play with that, how about increasing value of screen resistors, or decreasing screen supply voltage level?

[Joe L]

That sounds like the duty cycle imbalance of the PI which causes so many Marshalls to redplate. According to Randle Aiken, the best compromise is to switch the impedance on the amp down one notch. That is, amp on 8 ohms, speakers are 16 ohm rated.

This helps to keep the tube operation under max ratings. For more info - Marshall Redplating. Pay attention to the posts by RAIKEN.

[rf7]

That sounds like the duty cycle imbalance of the PI which causes so many Marshalls to redplate.

That is really interesting. This seems to mainly be an issue with the VT-40 amps and not the V-4 series, as the VT-40 has higher plate voltages, which Randall Aiken says makes the issue worse. I had previously noticed that the PI stage clips before the power amp stage does. I'm going to check this out.

[Enzo]

Remember, idle bias is a DC condition. measure the resistance of the OT primary, noting if one side has a lower resistance from the other.

[rf7]

The OT is ok. I checked it out and even tried a known good one, and there was no difference.

[rf7]

Well, I jumped through a bunch of hoops and got the amp to stop red-plating the one power tube. Everything that I did came from Randall Aiken's comments referenced above on repairing a red-plating 1959 Marshall.

I tried using the 4 ohm tap on the OT into an 8 ohm load and that did not stop it from red-plating.

I increased the value of the PI cathode resistor from 1.5K to 10K and then to 4.7K and that ended up reducing the output power of the amp by quite a bit, which is interesting, but not what I'm trying to do.

I dropped the value of the plate resistor of the top half of the PI from 47K down to 22K, then to 15K, and eventually settled on 10K to get that one tube to stop red-plating. The lower half of the waveform clips before the top half of the waveform at this point. Using 15K, the clipping was more symmetrical, but the tube started to red-plate then too.

[antieatingactivist]

I think I was having the same issue with my V2. I never figured it out. The signal was hitting the PI way too hard making the output super imbalanced. I dumped a lot f the signal to ground to see if helped and it worked. I shouldn't have had to re-engineer the amp, but its all I could think of. I never had this problem with a v4. Unfortunately I don't see to many V2/VT40's.