Ampeg ET-2 Super Echo Twin bias issues...

[alchemy]

I am working on an old Ampeg ET-2 That is pulling between 45 and 50 mA on each 6V6. Plate voltage is around 410v. The schematic does not list voltages, but I seem to recall a lot of older Ampeg and Fenders running high plate voltages with 6V6's. I also recall quite a few of these old cathode biased amps running over 100% dissipation without issue, but if my math is correct 18-20watts seems a bit excessive for a 6V6. While the tubes are not quite red plateing, it is apparent that they are running very hot. Since I cannot find the voltages this guy is supposed to be running at, I can't decide if I should try to lower the current by making the 250ohm cathode resistor larger, or if I should try to get the plate voltage lower, say around 360v or so (for some reason I seem to remember some old 6V6 ampegs running around 360v on the plates. Could be wrong!).

It has sovtek 5y3's, and was running EH 6V6's, but I tried a bunch of different 6V6's with roughly the same results...I guess I could try changing the rectifier to drop a bit of plate voltage as well....

Any suggestions?

[alchemy]

Well, I just did some reading and came to find out that the Sovtek 5Y3 is not really a 5Y3 at all. I had always stuck with NOS 5Y3's and had not run into this before. Swapping a NOS 5Y3 in dropped the plates 45 volts!!! So I guess a full rectifier/power retube is in order, but I am still curious what kind of voltages these amps were running to begin with, just to get it dialed in....

[alchemy]

Still having some issues with over-dissipation. Changing to a NOS rectifier dropped the plate voltage to 366v and the current for each tube to around 44mA. Cathode voltage is about 22v so (366v-22v)x.044mA=15.1 watts. Still a bit high for a 6V6 with 14 watts max plate dissipation. I increased the cathode resistors (350 ohm, up from 250ohm, shared between 2 tubes) the idle current dropped down to 39mA, but the plate voltage comes back up to 384v, and cathode voltage to 27v. SO, (384v-27v)x .039mA=13.9 watts. There is one 1kohm resistor shared between 2 screens with 6 volts across it. (6v/1000ohms)/2=3mA per tube screen current. So if I subtract the screen current from the total cathode current, we get (357v x .036mA)=12.8 watts.

I realize that this is technically within the limits of the tube, but it stills seems a bit high. Seems like it could very easily go well over the max when there is a signal. I have always read that about 11-11.5watts is about ideal for cathode biased 6V6's. Trying to dial this amp in so that it will not eat tubes anymore and just want to make sure that everything is kosher...

PLEASE TELL ME IT'S OK!!!!!

Or what I should do to lower it a bit more.....