Strange Standby Arrangement

[Natman]

Hi all, I am trying to copy an amp from gut shots and there's a cramped little corner where I can't tell what's going on.

It involves:

1) the standby switch
2) a 15 W wirewound resistor
3) a 450 V filter cap

All 3 wires are red so I'm hoping someone can shed light on the most likely scenario. I drew a crappy diagram in paint, so it's a 'connect the dots' game:



And here's the actual photo:



What you can't tell from this angle is that 2 wires are twisted and the third is hidden underneath. I know how a stanby switch is usually located before the filter bank but this one might be different? I know there is an inductance in a wirewound resistor so it would act more like a choke.

Thanks if you can help!

[tubeswell]

Looks like you'd have to unscrew the switches to see how they are wired. If you do, then be careful putting them back in that you don't cause a short to the chassis from any of the switch terminals

[Natman]

To me, it looks like it's probably doing the following:



But that places the standby switch AFTER the first filter cap and resistor and the OT only gets connected to the second cap. Seems a little weird but I don't see why it wouldn't work. Sadly the amp is not mine and I can't take anything apart. I have to judge by these pics. Here's a schem I drew of the above:

[guitarmike2107]

My guess is its shorting out the resistor, so that the resistor allows a small current to flow during warm up…
see merlins ( Valvewizard) website about putting a resistor across the standby switch..

The Valve Wizard

or it’s a sag switch rather than standby switch…..

Mike

[Natman]

Hmm -very interesting read Mike. The resistor here is only 150 ohms, so I doubt it's high enough to reduce the current enough for that application.

[Enzo]

It is just a standby switch, not part of the signal path, so unless you want museum acuracy in a reproduction, for pratical use, just wire up the power supply so it makes sense and wire the standby switch in any way you want that works. Whether the original has it before the first filter or after doesn't matter to the tone of the amp. When the switch is on, it disappears from the circuit.

Having the B+ on the OT all the time and breaking the B+ for standby after it is the way Peavey standby's most of their tube amps. You remove the screen voltage from the power tubes and they stop conducting.

[Natman]

Good info -thanks very much Enzo!