Pentode Vs Triode Mode (Power Tube Life)

[WackyT]

Hi all. Have a small question. Is it easier on the power tubes in an amp when running it at lower apartment volumes to run it in triode (~17 watts max) vs pentode (~40 watts max) mode, or it doesn't make any difference? I have noticed on my Pignose that triode mode sounds a lot smoother than pentode mode does, so I'm running it that way all the time now.

[duhvoodooman]

I'm out on the thin ice here, but from what I've read, it looks to me like the tube will have to work harder in the triode mode, using more power at idle relative to its peak output. So I'd guess that would shorten life somewhat. OTOH (1) you like the tone better in triode mode ("warmer" and "smoother" seem to be common descriptions of triode mode tone), and (2) unless you're buying pricey NOS tubes, power tubes aren't all that expensive anyway. I'd use the mode that sounded best to me....

[WackyT]

Thanks DVM. I didn't think triode mode would be harder on the tube, but I guess I think too much in the integrated circuit world. Guess I'll keep a spare pair of tubes on hand then. I'm currently using Winged C (SED) SV6L6GC (http://www.tubedepot.com/sv-6l6gc.html) that go for $40 / matched pair, which I think isn't too bad. Thanks again.

[markb]

One one hand you're running at a lower plate voltage in triode mode, so your tubes may not burn out so fast. BUT, the main reason it sounds warmer and smoother is that you're driving the power tubes harder (lower wattage = more power amp drive). Swings and roundabouts, I think. If you prefer the triode tone, go ahead. I've never come across a piece of consumer electronics with a "self-destruct" feature yet.

[Tone2TheBone]

Voo's right tubes are tubes and they'll wear anyway. It's all good. Makes no difference in my 25/50. Besides lower wattage sounds bettah.

[tunghaichuan]

The thing to consider is that the screen grid is the weak link in any tube. In triode mode, the anode is usually connected to the screen grid through a small value resistor, say 100 ohms. The 100ohm resistor limits the current to the screen when the amp is cranked. The anode is pretty sturdy and can take heavy current draw. The screen is pretty fragile, under heavy current draw, it will actually melt if it dissipates too much wattage.

So if you're playing at bedroom/low volume levels, I wouldn't worry about it.

tung

[WackyT]

Thanks Tung. The grid resistor in the Blue Guitar mod I did for the Pentode:Triode switch is a 1K ohm 2 watt resistor.