Pentode/Triode switch

[MikeT]

What is the theory behind this mod? Anyone got a schematic?

[GregS]

When in pentode mode the screens of the power tubes are connected to the screen supply node of the power supply so they operate as normal pentodes. When switched to triode mode the screens are connected directly to the plates and the tube effectively becomes a triode. This reduces output power somewhat and supposedly results in more even order harmonic content.

I don't have a schematic to upload but will try to describe it. You use a substantial DPDT switch. The two poles of the switch connect to the screens of the tubes. Position 1 of the switch connects the poles/screens to the screen supply. Position 2 of the switch connects the poles/screens to the plates.

Hope this helps,

Greg

[tedmich]

its often combined with a UL mode where the UL taps (~40% for low dist, ~20% for power) are a 3rd connection. Can't switch while "ON" and while Hifi guys tend to love their class A triodes most guitarists who have this mod leave it on pentode, IME.

[Wilder Amplification]

Quite honestly I'm starting to think that guitarists who are seeking to drop the amp's total output power assume that the amp is putting out full power all the time at any volume.

[Merlinb]

What is the theory behind this mod? Anyone got a schematic?

Converting from pentode to triode will roughly halve the power (which is about 80% as loud). Most people report that the triode sound is kinda dull, and triodes have higher grid current too which leads to harder clipping and a more preamp-style distortion. Bit of a let-down really.

The oldest example of a triode-pentode switch that I know of can be found in this 1956 article:
http://thermionic.info/hedge/Hedge_C...ifier_1956.pdf

[MikeT]

Thanks guys..........

Mike