Pentode/triode switch

[jmcd]

The attached schematic shows an implementation of a pentode/triode switch that includes a 100R grid stopper for triode mode. 
I have two related questions about that resistor:

1. Should it be included?
2. What should its power rating be?

[K O'Connor]

Hi Guys

If you look carefully at the schematic in post-1 there is no screen-stop. It is a typical inconsistency of thought that leads to a design like this: For some reason, protecting the screen in triode mode is important, but not in pentode mode, even though pentode mode is far more hazardous to the screen.

Readers of The Ultimate Tone books know that the screen grid in an output tube in a musical instrument amplifier must have a significant-value screen-stop, typically 1k-5W. The wire-wound type is specified to assure that the resistor is a flame-proof type. The 1k value protects the tube during clipping.

When we take our standard tube+1k screen-stop as a "unit", the free-end of the 1k becomes the "screen connection" for every other purpose. For example, in the above schematic, the 1k is inserted between the tube and the switch wiper, making the 100R redundant.

Anyone questioning this should look at the filter cap feeding the switch contact for pentode mode. What does this cap do? Yes, it is a simple supply filter and places the output of the dropping resistor at AC ground. Remove the cap and the resistor becomes a screen-stop albeit not a good one as it is separated from the screen - the 100R has a purpose in this scenario.

Note the absence of a grid-stop. We hope this is just a part of keeping the schematic simple for exposition rather than the intended way to wire the amp?

In practice, the 1M resistor is not needed across the triode-pentode-UL switch. If you flip the switch while playing there is zero 'pop', but may be slight click without a signal. This is because mechanical switches and relays are good at switching AC but not DC.

[jmcd]

This schematic was just to get the conversation started (and to ask a specific question about the 100R grid stopper. My implementation had the usual 1k grid stopper in there, but ultimately I removed the switch because it was unnecessary as a volume control and not worth the additional noise in triode mode (without the LP quiet supply module, at least).

[uzerman]

Thats looks helpful, thanks!