Triode / pentode switch: can it be switched "live"

[Mostro]

Well, that. Can you switch it with the amp on? I´ve read people saying no, it has to be off or in standby. My question is, why?

Is it because you will get a big POP? Or will you bust something? Assuming the switch is a break-before-make type, I think this: when pentode connected all is fine. When the switch breaks the contact, you basically put the valve into standby because the screen gets no voltage. When the switch makes contact with the plate supply, it will get a rush of voltage / current, but if there´s a screen resistor it will be current limited. In my case I have 1K R´s.

So, any technical reasons why I should or shouldn´t do it? Experiences?

[tubeswell]

For triode-pentode switches, Merlin's pre-amp book advocates against having a switch that breaks contact with the screens completely - citing a possibility of sparks from switch bounce - and he recommends bypassing the switch contact with a Large (~1M) resistance to overcome this. This still has the effect of a 'triode-mode' because the high resistance effectively shuts the screen current off. There are a couple of ways he recommends implementing this in the book.

[Mostro]

Tubes, thanks for the suggestion. That book talks about the EF86, right? I supposse the value would be different in an EL84? Sounds as something I should try but will it be the same on an EL84? I mean, increased current, voltages...

[tubeswell]

Yeah you would need the Rg2 sized for EL84 screen current. However 1W 1M would still be sufficient for the pop resistor

[Mostro]

OK guys. i tried the switch with a 1M R as suggested by Tubeswell. a small sliding 2PDT. First turning the amp off and then, with some trepidation, with the amp on. No problem at all, no sparks, shorts, blown parts or elctrocuted people (me). Just a mild pop.

Sooo, I guess you can do it "live". Will the switch break with use? Maybe... I really don´t see it happening, after all the currents are low... is the R helping? Well, if it ain´t broke don´t fix it! Meaning, I won´t try it without the R.

Thanks all for the help.