Need help identifying location for standby switch

[thehoj]

In this power supply for the amp I'm working on, we have a voltage doubler getting us the 480v b+. Where could I put a standby switch so that I cut power to the b+ rails, but leave it for the bias section (in red)?

Also, if anyone knows, what is the .22uf capacitor for in the bias section?

[Old Tele man]

...the 0.22ufd cap is the AC-coupling capacitor between the HV-winding and the voltage-divider resistors, which down-scale the voltage to the diode.

...it's an "AC-coupled" circuit and is only possible because the bias circuit (ideally) does not have to supply current (remember NO grid current with most amplifiers).

...although the voltage will be VERY high, the obvious place for the STBY switch is the the +450V leg just before the 1.8K resistor. While another possible location is the spot "between" the 0.22ufd cap and the diodes...it'll leave voltage to the bias circuit but stop the voltage doubler input...however, in doing so, it will "unload" the voltage that the bias circuit "sees" disrupting the bias setting.

[thehoj]

...although the voltage will be VERY high, the obvious place for the STBY switch is the the +450V leg just before the 1.8K resistor. While another possible location is the spot "between" the 0.22ufd cap and the diodes...it'll leave voltage to the bias circuit but stop the voltage doubler input...however, in doing so, it will "unload" the voltage that the bias circuit "sees" disrupting the bias setting.

I'm confused about how putting the switch between the .22ufd cap and the diodes would leave voltage to the bias circuit.. Isn't that disconnecting the line to that circuit?

If I put the switch like your first suggestion, is it okay that that first 450v (supplying OT) will not be switched off when I have the amp in standby?

I guess the goal is to keep the high voltage off of the tubes, so it's fine right?
I was thinking normally though I wouldn't have any voltage on the OT either, but if I put that switch before the 450v leg then the voltage doubler circuit is disrupted, and I don't think the bias circuit will work in standby mode.

[R.G.]

Do yourself and anyone who comes after you a favor and put bleeder resistors on those two first filter caps. Something between 100K 1W and 220K 1W would do nicely.

Without these, you can turn off the standby switch operate the power switch, and have full voltage on those caps after you unplug the amp.

I've been slapped back into a wall enough not to want to do it again.

[thehoj]

Do they just go in parallel with each filter cap?

Do yourself and anyone who comes after you a favor and put bleeder resistors on those two first filter caps. Something between 100K 1W and 220K 1W would do nicely.

Without these, you can turn off the standby switch operate the power switch, and have full voltage on those caps after you unplug the amp.

I've been slapped back into a wall enough not to want to do it again.

[tubeswell]

yep

[Robert M. Martinelli]

...the 0.22ufd cap is the AC-coupling capacitor between the HV-winding and the voltage-divider resistors, which down-scale the voltage to the diode.

...it's an "AC-coupled" circuit and is only possible because the bias circuit (ideally) does not have to supply current (remember NO grid current with most amplifiers).

Hi OTM!
That 0.22 uF cap does more than simple AC coupling IMHO, as it's also part of the voltage divider, due to the Capacitive Reactance ( Xc ) it presents @ mains frequency ( more or less 14.5 KOhm @ 50 Hz and 12 KOhm @ 60 Hz ).

I' ve seen something similar working on some Marshall amps before.

Cheers

Bob

[Steve Conner]

See attached pic.

[thehoj]

See attached pic.

Great. This makes sense.
OTM mentioned that doing it that way would ""unload" the voltage that the bias circuit "sees" disrupting the bias setting"?
Is this that big of a deal?

[Old Tele man]

...no, the standby switch should NOT be located "between" the +450V point and the 1.8K dropping resistor. That KEEPS the plate voltage going to the output tubes.

...the purpose of the standby switch is to remove the high-voltage either: (A) from all the tubes or (B) from just the output power tubes.

...placing the standby switch directly in the +450V-note path will accomplish "B" while placing it as you show ("between" the +450V and 1.8K resistor) only removes the high-voltage from the screen of the output tubes and the preamp tubes and leaves the plate voltage "hot" to the output tubes--but with no screen voltage. Not a very "safe" condition.

...Steve Conner's illustration is a preferred location--it accomplishes "A."

...and, R.G. is 100% correct about those bleeder-resistors across the voltage-doubling capacitors!

[Steve Conner]

It seems to me the bias voltage will be a bit higher with the standby switch turned off, which shouldn't be a problem.

The switch doesn't break the bias circuit, there's still an AC path through the main filter cap to complete it.

[thehoj]

Okay, so I think I'll wire it as Steve Conner's diagram shows it. I appreciate the help everyone.

As for the bleed resistors on filter caps, is 1w resistors enough?

[tedmich]

~300v = IR
=220,000 ohms (I)
I = 1.4mA

1.4mA x 300v = 0.4w

plenty!

[Old Tele man]

...is 1w resistors enough?

...short answer, yes. The equation to use is: P = (E^2)/R

...so, if E = 450V/2 (because each capacitor "sees" half the total voltage) and R = 220K then P = 0.23W each resistor, but the real world "rule-of-thumb" is to use 150% of that value, ie: 0.35W(minimum), but 1W is the next closest larger value.

[tedmich]

safer still use a 330k resistor, the time to discharge (~5RC) isn't too much slower.

220k x 40uF x 5 = 44s
330k x 40uF x 5 = 66s

You have to be real fast with the screwdriver to get the case off and hit the caps in these times...