Power triodes - biasing hot?

[pvsage]

We all know that if you bias a power pentode or tetrode too hot and the load line passes the "knee" of Vg=0, you risk (assure yourself?) of destroying the tube due to high screen current.

What about power triodes? The characteristic curves for triodes are quite different from pentodes, having a more logarithmic characteristic rather than the inverse log nature of pentode/tetrode curves. At what point do we have to worry about damaging a triode from biasing it too hot, and where would the failure occur?

I realize the use of triodes for power stages is extremely uncommon in instrument amplifiers, but please humor me on this one. There are quite a few inexpensive NOS tubes on the market that have triodes which may be suitable for low-power practice amps.

[Steve Conner]

I think you have that wrong. High screen current in a pentode happens when the plate voltage gets much lower than the screen voltage. This is caused by having too high a load impedance.

If you tried to bias yourself into an idle state with Vg=0, the tube would conduct heavily and die from too much plate dissipation, unless you lowered the screen voltage greatly. In any case, since the tube draws grid current when Vg > 0, and the usual guitar amp driver circuits can't supply grid current, it woud hardly work.

For triodes, the failure would presumably happen due to overdissipation of the plate, since there's no screen to burn out.

[pvsage]

Thanks Steve! I got the "screen current would rise almost instantly when the valve is overdriven" from http://valvewizard.co.uk...

Obviously biasing *at* Vg=0 doesn't make much sense, but with the tubes I'm looking at the bias is on the "warm" side - fairly far from center bias. I know that in preamps this results in a warm, "crunchy" type of distortion, but wasn't sure how this translates to power stage use. (If it matters, I'm looking at the 6AN8 in particular, using the pentode for a preamp.)

What about blocking distortion? Is this an issue in a power triode?

[Robert M. Martinelli]

High screen current in a pentode happens when the plate voltage gets much lower than the screen voltage. This is caused by having too high a load impedance.

Hi Steve, sorry, but I didn't get your statement, to my knowledge plate voltage drops heavily when the load impedance is too LOW and the current demand is higher ( too high a load ), did I miss something ?
Regards
Bob

[Steve Conner]

Nope, it's the other way round. We're talking about the instantaneous plate voltage on signal peaks.

Imagine you shorted out the OT primary, connecting the tube plate straight to the B+. No matter how much plate current you pull, the plate voltage stays at 350V or 400 or whatever. The plate cooks, but the screen grid stays cool, because the high plate voltage attracts the current away from the screen and towards the plate.

Now imagine you disconnect the plate completely, leaving it open circuit. Now the only thing for the cathode current to hit is the screen. All the current that would normally flow in the plate goes to the screen, and your first Pete Townshend power chord makes it go off like a flashbulb, melting into a lump.

All other impedances are points between these extremes. If you don't believe me, try cranking an EL34 amp with a 4 ohm dummy load on the 8 ohm tap, and watch the plates get red while the screens stay cold.

Then try again with an 8 ohm load on the 4 ohm tap, and watch those screens glow like lamp filaments. And maybe melt if you crank it too hard :O

[Enzo]

How about a John Mayer power chord instead of Townsend?


I think a lot of people envision the output of a tube amp the way they do solid state. In the solid state amp, the lower the impedance, the harder the output stage has to work. The load in a tube amp does not directly load the tubes the same way.

[Robert M. Martinelli]

Now imagine you disconnect the plate completely, leaving it open circuit. Now the only thing for the cathode current to hit is the screen. All the current that would normally flow in the plate goes to the screen, and your first Pete Townshend power chord makes it go off like a flashbulb, melting into a lump.:O

Ok, my mistake....thanks Steve, and Enzo, you're right, I was erroneously thinking in terms of the sole collector......ehm.....no, drain....oops.....plate current, forgetting screens...beg your pardon.

talking about chords.....time ago I restored an old HIWATT head, and the first thing I played through it to test it was the "won't get fooled again" starting riff ( quite obviously ) - God bless Townshend!