6L6GC in Push Pull Class A - how much power out?

[diaz028]

425V B+
6L6GC
4k center tapped ot
Push pull class A
Fixed bias

TDSL says this for cathode bias:
B+ 360
Vg2 270
Ia 50
Ig2 9.5
250ohm shared
impedance: 9,000 output power: 24w


If i were to ha e 425V at the plate and fixed bias I'm guessing 60-75ma per tube and maybe 30 watts? Just a rough guess here..

 

[cerrem]

If you have 425V B+ , need to keep the screen voltage under control at 250V in order to bias into Class A while respecting the plate dissipation....
The hardest you can push the 6L6GC into Class A Push-Pull within your requirements and without a meltdown... is probably 250V screen and 425V plate .... Bias somewhere in the 50mA to 60mA range.... roughly -19v to -18v area.... 21W to 26W plate dissipation at idle..
A good plate load fit is 8K to stay away from the knee...
With a good strong set of tubes you will just make 30W power output before clipping...
Plot the load line and see...
A couple things to note, the screen supply should be regulated at 250V, due to screen currents at full signal ....The Plate supply should be at 425V when amp is at full power output....possibly rising a bit at idle...
McIntosh achieved 30W from 6L6 pair biased much colder and in Class AB1 ....but a whole different OT situation...

 

[diaz028]

Thank you. What would happen if I were to keep the screen at near B+, and cool down the bias? I'll post a schem later.

 

[lcsaszar]

I am running 807 tubes (similar spec as 6L6GC) at B+ 425V, g2 tied to plate, i.e. triode mode. Output transformer 5k : 8R. Ik = 50mA, Pout = 12.5W @ 1% w/o GNFB. But Vg2 exceeds the specs, and I think it reduces the life of tubes.

 

[danFrank]

Thank you. What would happen if I were to keep the screen at near B+, and cool down the bias? I'll post a schem later.

You will be operating in class "AB1". You will get more power output though...
And, as for class "A1" operation, 15-20 watts output is more realistic, unless you drive the 6L6s grids positive for class "A2" operation, but this will make your output tube driver design more complicated.

 

[danFrank]

I am running 807 tubes (similar spec as 6L6GC) at B+ 425V, g2 tied to plate, i.e. triode mode. Output transformer 5k : 8R. Ik = 50mA, Pout = 12.5W @ 1% w/o GNFB. But Vg2 exceeds the specs, and I think it reduces the life of tubes.

+25 volts over for the screens isn't going to do much harm, if any. 807s are transmitting tubes, which can take a lot of abuse. +25 volts over isn't abuse for the screens.
Did you use screen resistors, maybe like 100 ohms in value? Screen resistors are a good idea when operating beam tubes/pentodes in triode mode.

 

[lcsaszar]

I believed Vg2 should be 300V max. Yes, I use 1.5k screen grid resistors, but again I believed they are only for preventing HF oscillation.
Would the 6L6GC be more musical in the same circuit?

 

[diaz028]

Thanks guys. I appreciate your knowledge!

 

[danFrank]

I believed Vg2 should be 300V max. Yes, I use 1.5k screen grid resistors, but again I believed they are only for preventing HF oscillation.
Would the 6L6GC be more musical in the same circuit?

Vg2 of 807 is 400 volts in Triode:

http://frank.pocnet.net/sheets/049/8/807.pdf

 

[TonyTecson]

running triodes at lower B+ means lower plate resistance and higher mu, plus grid is lower...
just sayin, based on looking at datasheets...