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placement of a standby switch

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12/27/1999 10:09 PM

placement of a standby switch

is a better place for a standby switch after the first filter cap? my original idea was to put the standby right after the rectfier but I have since read some articles about how the tweed bassman(which uses that design)has some arcing problems somtimes due to that rapid charge of the first filter cap. I wasnt sure if a 5E3-ish type amp would have that same problem. thanks, Ricky

12/28/1999 1:21 AM
Benjamin Fargen	Ricky, Just to be safe I would go ahead and put it after the first PS cap.......just make sure your first filter cap is rated above the full B+ voltage with no load on it! Besides the arching.....you can sometimes get weird noise artifacts for a couple of seconds when dropping all that B+ voltage on the first filter cap. Better to go with the safer design. Benjamin

12/28/1999 4:19 AM
stokes	Someone once suggested placing it between the c.t.and ground.I've tried it and it seems to work well.I never see this set up on any schematics-anybody know if there is any reason the c.t.to ground might not be ok to use?

12/28/1999 5:22 AM

I've seen it used by Pete Traynor many times in the old tube amps.
Plus a few homebrew amps I've looked at.
It works good and I can't think of any reason to not use it.
Oops,
I just thought of a reason when not to use it.
If your amp uses a negative bias supply that also needs the center tap to get a ground reference, then when the center tap is grounded again, the power tubes will be supplied with it's B+ at the same time the bias supply is building up negative voltage.
If the bias supply's raw AC voltage is taken before the rectifier, off the high voltage secondary, it means the power tubes will be running with no bias for a few seconds while charging the bias supply filter cap(s) through it's 100K to 150K current limiting voltage divder resistor.
Hope that made sense to those that have not seen this.

Bruce

12/28/1999 7:57 PM

Mark Buckingham

I've used the c.t. style standby switch on the amps I've built, and it works well. So far, I've used cathode bias, so I haven't run into the problem Bruce describes.

The only problem, I've had, is that when you switch off the standby, the amp isn't instantly silent. It can take a couple of seconds to totally drain the filter caps. Also, the reverse is a problem, because when you switch the standby on, it takes a second or two for the B+ to build again.

Actually, the above might mitigate the problem Bruce sees. If it takes x seconds for the B+ to build, then the fact that it takes that long for the bias to build may not be a problem...

12/29/1999 3:42 PM

I have built a couple of amps with the standby
switch in the center tap. The bias comes from
the high voltage winding. When the amp is
on standby, the bias voltage builds up
to about DOUBLE the normal voltage. This
is because the circuit is completed
by a bleeder resistor across the filter caps,
so you have the full PT secondary voltage
instead of just half, charging the bias
supply. Doesn;t seem to be a problem but the
caps need sufficiently high voltage ratings.

12/29/1999 3:47 PM

The problem stems from the large value dropping resistor on the high voltage AC line to the rectfier diode in the bais supply.
That resistor and the bias supply's filer cap form an RC circuit with a long time constant.
Much longer then the time to charge a filter cap in the B+ rail.
As an example, it might take three or four seconds to charge up the B+ rail but the same amp might take 20 or 30 seconds to charge up the bias supply rail.
If the the amp is not cathode biased, then the B+ will be on the power tubes for a length of time with virtually no or very little bias voltage, as the bias -DC supply is charging it's filter cap through that large value resistor.
The tube will be conducting too much DC idle current during this and cause unneeded wear and tear on the power tubes.

Bruce

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