Output wattage opinions?

[Buckwheat]

Just finished an amp(4 6v6) and probed up some numbers. Some people have told me that they are too high, while others said I was fine...

Plate voltage: 330v
Plate current: 36mA

giving about 12watts per tube....
No noticeable plate glow, unless its pitch dark, where there is a faint red line across each plate.

I'm using a 150Ohm/15W cathode resistor

Any opinions or cathode resistor recommendations would be great!

[stokes]

Sounds like you are right on the edge,some tubes will be able to take it some not.If the faint red line you describe is at idle,when you start adding signal it is going to get worse.Do you like the way the amp sounds and are willing to sacrifice tube life for that tone?If not bring it down some with a bigger resistor.Do you have a bypass capacitor on that resistor?

[guitician]

Don't Class A(Cathode) biased amps run hottest at idle?

[Buckwheat]

Thanks for the reply

I have a 50uF bypass cap with the resistor, and the tubes are Tung Sol...not sure if they can take the beating..anyone? The tone sounds amazing, but I would prefer the tubes to last a couple of years.

And yea, not sure if class A amps run the hottest at idle..

[Enzo]

Don't Class A(Cathode) biased amps run hottest at idle?

Class of operation is not determined by source of bias. MAny cathode biased amp are class A amps, and all the single ended ones (with a couple exception), but it is not the fact they are cathode biased that makes them so. Point being that class A and cathode bias are not the same thing.

[stokes]

Class A or not isnt really at issue here,its a matter of the tubes taking the beating.I have a couple of 6V6 amps that idle at close to 12 watts,but I dont see them redplating at all,at idle or full tilt.Watch the tubes while you play the amp at full bore and see if they redplate.Even if they dont,you are still biased hot and tube life will be shorter than if you were to bias them at,say,70% max.How much shorter is impossible to say.The two amps I mentioned before,one is a dual 6V6 the other a quad,get,to me,satisfactory life.The tubes have been in there over a year,they get regular use from both my sons,and are still good.One has a pair of NOS Tung-Sols,not the so called "re-issues" and the other has a quad of NOS Marconi's.Before that I had NOS RCA's in both for over 2 years.If you dont see a marked increase in the redplating and like the tone much more than if you bias them a bit colder,you are probably not in any immediate danger,assuming the tubes are good to begin with.But you should definately cool them off a bit to see how they sound at a lowerr dissipation,you may like it and can put your mind at ease.

[pdf64]

"Plate voltage: 330v"
Is that plate to ground or plate to cathode?

"Plate current: 36mA"
Is that plate current or cathode current, ie are you taking screen grid current into account? And is that the average current of the 4 tubes, or the current of the tube with the highest current?

"not sure if class A amps run the hottest at idle"
Depends what is meant by runs hottest. The load on the power transformer will be about the same from no-signal to max output (the screens take more current as the output goes up). Whereas the plate dissipation of the power tubes will be highest at no-signal, and maybe halve at max output.
But, as Enzo implies, this amp probably isn't operating in class A - the cathode voltage will most likely rise significantly at high signal levels.
If you want the tubes to last, fixed bias would be better. With a cathode resistor, the tubes have to run hot in order to avoid moving into class B at high signal levels.
ChuckH reports that putting a suitable zener in parallel with the cathode resistor stops this happening.

[Buckwheat]

The 330v was plate to cathode, and I did take the screen grid current into account for the plate current... And the 36mA was an average...

Think I'll just try upping the 150ohm cathode res to a 200.

Not sure if the tubes are supposed to give a certain "scent" when hot, but it seems mine do slightly. Not too comforting...

Anyways, thanks for the help everyone!

[Bruce / Mission Amps]

I'd go a little higher then 150 ohms for these four power tubes but it should be OK with four good ones, if the the four power tube's screen voltages are not too high.
What is the screen voltage of this setup?

[Buckwheat]

Screen voltage is 298v (Pin4-Pin8) and 324v (Pin4 to GROUND)

[Bruce / Mission Amps]

Screen voltage is 298v (Pin4-Pin8) and 324v (Pin4 to GROUND)

OK, those voltages ought to be fine with a 150 ohm resistor on all four cathodes.
I think you just happen to have some "soft" tubes in the amp right now.
Soft means a tube wants to draw more current at idle then a hard tube does... with everything else being equal.