Where to insert Standby Switch?

[winterturtle]

Greetings and salutations friends. Still playing with my Lafayette no-model amp similar to a Univox U255r & looking to add a standby switch. I've of course seen the debate and for me frankly it's a want to have rather than a need to have =)

At any rate, I'm not entirely sure where it insert it. I've heard you want to have the first Cap energized when in standby so when you shift to ON it's already there to help buffer the inrush. With my circuit I think I need to change where the first resistor before B2 branches off to accomplish this. I would also then also be leaving the bias circuit energized in standby which feels odd, but is also how my U-235B came from factory so I suspect isn't a huge issue having some negative DC on the power plates.

Does this look right?
https://imgur.com/xb2cIdB

[sluckey]

It's a good idea to have bias voltage even when in STBY mode so don't mess with that.

Your circuit shows a typical STBY switch. Looks good. However, I'm concerned about the 350V rating on that first filter cap. The voltage on that cap will rise to 395V when in STBY mode. An easy way to test this is to pull the output tubes and measure the voltage on that cap. I bet the voltage is higher than 350V. I suggest replacing with a 500V cap. In fact, I would replace all those 350V filter caps with 500V caps.

[pdf64]

See Merlin http://www.valvewizard.co.uk/standby.html
This looks a good option. There's no issue hot switching solid state rectifiers.

[winterturtle]

It's a good idea to have bias voltage even when in STBY mode so don't mess with that.

Your circuit shows a typical STBY switch. Looks good. However, I'm concerned about the 350V rating on that first filter cap. The voltage on that cap will rise to 395V when in STBY mode. An easy way to test this is to pull the output tubes and measure the voltage on that cap. I bet the voltage is higher than 350V. I suggest replacing with a 500V cap. In fact, I would replace all those 350V filter caps with 500V caps.

Thank you for the advice! -- the caps on the B+ line have been replaced with 450-475v new capacitors but I'm going to follow your instruction to see if it exceeds or even approaches those numbers. I definitely want this little gem to be reliable =)

[winterturtle]

See Merlin http://www.valvewizard.co.uk/standby.html
This looks a good option. There's no issue hot switching solid state rectifiers.

Oh boy this is food for thought. I hadn't really pondered the DC arcing the contacts but totally makes sense. I'll have to see how much current is drawn by the non-hearer part of the circuit... I suppose 1-2 watts... and I do have diodes sitting around!