I need to take the Ampeg VT-22 that I bought last year out of it's case to clean the pots and give it an overall checking out, but just for fun I plugged in a new set of Tesla 6L6's in place of the very old 7027A's. WOW! This is a kick a$$ amp but I have a few questions for those who know more than me.

I plugged in my GT bias tool just to check the idle current on the output tubes and it's way down there at about 19mv and according to the chart that comes with the tool that's low for an amp with 540 volts B+. The bias voltage measures -60 volts.

But, if I drive the amp really hard, put it into a feedback the output tube current goes up to about 260mv and you can make the plates start to go cherry! I wonder if this is normal for one of these amps? If that's the case I would never want to run the 6L6 as high as the recommended 32mv for this B+.

I wonder how the amp would act with a set of 6550's or KT88's with their higher rated dissipation. On the schematic that I have Ampeg recommended reducing the bias voltage if you use the 6550, did people really do that or just plug them in?

I guess I need to take it to the shop and scope it to see what the waveform looks like, hell of an amp but I'll bet this thing will be tough on a 6L6, no wonder the old 7027's sounded sort of tired:^)

My Hiwatt DR504 is sort of this way with an E34L, it tends to wear them out pretty quickly, I guess that's the price to pay for an a$$ kicker!

They rawk. Nice, solid, good bottom and break up sweetly. I love Teslas.

I did not check the bias voltage or the idle current on Buddy's amp. I set the bias with the scope and it looked good. Would have been helpful now if I had just jotted down the voltage, huh? I did rearrange the power supply including the bias and I added a bias pot so things are a bit different in his amp now.

I suggest that you monitor the bias voltage. I wonder if bias voltage is not drifting towards 0 as current increases. Glowing plates is a sign of danger, as you know. Perhaps the bias circuit filter has aged to the point that it is drawing current, or the diode is a bit faulty. Maybe the output tube coupling caps are leaking DC.

I'll add that to my threat collection.
-- Harry Callahan (from "Dirty Harry")

I'll check out the bias circuit when I get the amp on the bench. As you suggested, I'll see what it's actually doing with the amp running under load. It needs a good going over but it seems to be a very solid amp, I want to have it ready to go for some recording soon.