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Are you sure its PENTODE?

Posted by Slick (A) on March 11, 2006 at 14:17:02

In Reply to: Question on triode pentode switch hookup posted by MIchael Samra on March 11, 2006 at 09:36:06:

My experience with triode/pentode changeover switches has not been positive. It is difficult to change both the G2/Plate or G2/screen grid power supply connections without also changing the bias point. Screen voltages typically being substantially less than the HT supply. I have an amp whose current doubles when switched to triode from pentode without adjusting the bias.
BUT, sinced you are talking about my favorite amp (especially with a Hedge 6CG7 circuit with 12BH7 constant current cathode followers), I think you mean Ultralinear/triode operation. I have had nothing but success whenever I've tried that. The G2 voltage really doesn't change, so the bias current reamins the same. Even the stock 12BY7 circuit seems to have no problem driving the KT88/6550 in triode mode.
I've always used DPDT switches, one per channel, but I understand your need for simplicity for your customer's sake. Remember to include 100 ohm resistors in series with the G2 connections.

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