Very high screen current?!

[mario]

I finished my Champ style build and it sounds great. I've found it sounds best with 6l6 and el34 power tubes. My only concern is screen current and disibation. I mesured the screen current at max signal and found it peeks at nearly 30 ma! (when I play a really loud E chord) Given the 400v on the screen this would put screen disibation at around 12w, far too high for these power tubes. I'm using a srceen resistor( 1 k) in serries with the b+ supply that goes to the OT for screen HT supply. Is this ok or is it the reason I'm drawing so much screen current. Should i make a screen tab with a 1k resistor and say a 20uf from the b+ supply rather than the series resistor method?
Thanx!
Mario.

[pdf64]

Given the 400v on the screen

Is that really the case? When the screen draws current, the screen voltage will sag.
Measure the cathode voltage under those conditions and subtract it from the 'sagged' screen voltage to obtain a more relevant screen to cathode voltage.
What meter are you using, on what range, what's the basis for its displayed value? eg ac, dc, ac+dc, true rms,average or sine equivilant rms reading?
My feeling is that only meters capable of 'true rms ac+dc' readings are suitable for these type of measurements; other types only offer a rough indication and may give a false impression if taken as gospel.
It would be beneficial for tube life to reduce the B+ down from 400V, though Champs are often reported to have voltages well above the schematic values and 400V isn't unusually high.
What rectifier are your using?
You may be able to see whether the screen grid structures in the tube are glowing hot under overdrive conditions.
If you can't, and the tubes are surviving, then if it sounds good don't worry about it; it's very likely that all guitar amps push their output tube screen grid dissipation well above the data sheet limiting value when overdriven.
Pete.

[mario]

Hi Pete,
Thanx for your reply. I made the measurement with a DMM in series with the screen. On idle it was very normal about 4ma on full tilt it was nearly 40ma. I re mesured the voltage on the screen and it was 415v compared to 387v on the plate. I was using a solid state rectire made out of 2 1N4007 diodes. I switched to a GZ32 today and took the screen voltage from the tab for the preamp tube and made measuments again. Now it starts of at 3ma and goes to about 13ma. I monitored the voltage at the same time and there was significant sag so based on this a gather the screnns are now in the safe zone at full signal. Voltages on idle are 387v on the plate 350v on the screen and 23v on the kathode. Anode current is 38ma so plate disibation is about 14w, alitle high I know ( i switched back to 6v6). anyway the amp roars now put out 5watts. Was my higher than the anode screen voltage the reason for the high current?
Thanx again!
Mario.

[ThermionicScott]

The higher the screen voltage, the more current it will draw when the tube clips (control grid goes to 0V).

If we can assume that the preamp current draw is constant (or close to it), then I think the best way to measure the average screen current is to measure the drop across the first PS dropping resistor (10K in the 5F1 Champ, 1K in the BF/SF), and subtract the preamp current. The average screen current is what we need to be concerned with for tube life.

- Scott