1/2 power pentode/triode switch

Discussion in 'Amp Central Station' started by figaro, Nov 25, 2010.

  1. figaro

    figaro Tele-Meister

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    Should a 1/2 power Pentode/Triode switch cause an amp to break up earlier even in full power mode?

    A friend just bought a 1966 Princeton Reverb that has a half power Pentode/Triode switch installed on the back of the chassis and it does lower the volume. But even when switched to full power, it breaks up much earlier, (about 3 on the volume) than my Princeton Reverb reissue. I have swapped tubes and checked all the voltages. The plate voltage in it seems low, about 399. My PRRI plate voltage is about 435.

    I think the lower plate voltage is causing the earlier break up. Could the Pentode/Triode switch cause the lower plate voltage?
     
  2. Cliff Schecht

    Cliff Schecht Tele-Holic

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    Triode/pentode switches aren't really half power switches, they typically reduce the power by a factor of 3-5x. Pentodes are up to 5 times more efficient than their triode counterpart. With that said, in pentode mode the switch should be "invisible" (i.e. power stage in stock condition) and in triode mode you usually connect the screens to the plate through 100 Ohm resistors. This shouldn't affect the B+ voltage but you obviously will lose a lot of headroom (and usually presence) in triode mode. If the amp is breaking up too early, that means you are driving the inputs too hard or there is maybe a problem internally (or the tubes themselves).

    Reducing the B+ by 35V will give a slight drop in volume without too much deterioration of the clean/dirty sound. This wouldn't cause a night and day difference.

    To answer your question clearly, the pentode/triode switch shouldn't be causing the lower plate voltage. Either the '66 needs new electrolytics (high esr with old caps will cause B+ to droop) or the tubes could be shot. Have you tried a fresh set of tubes? A bad rectifier can also drop excessive voltage and bad output tubes would have no headroom.
     
  3. figaro

    figaro Tele-Meister

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    I've replaced all the tubes with no difference. I think I'm going to disconnect it and see what happens. It could be a bad switch or it could be wired wrong. Anybody have a diagram of the correct pentode/triode wiring?
     
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