B Field collapse
On 09/23/17 00:05, Xtrchessreal wrote:
On Friday, August 4, 2017 at 11:37:24 AM UTC-6, wrote:
In common power supplies the only example where there might be a problem would be that using choke input. The choke likes to keep the current flowing, so a switch between the rectifiers & the choke input could cause insulation failure. But a switch on the primary side as it normally would be is OK, the choke still has a way to dump stored energy thru the PS transformer wdgs & rectifer.
But choke input is now uncommon, most PS are cap input.
In my case I installed the fix I spoke of in a regulated PS with DC standby I built many years ago so that it can be run either in choke or cap input. The max volts or current can then be used to advantage.
Cheers to all, John L Stewart
its a 2204 Marshall circuit that I built, I added an LED between the input filter and the standby, when switch closes and B+ hits the amp rail LED ON, such that when switch is later opened the Caps discharge through the LED to ground over a period of several minutes, for safety when opening the amp e.g. to bias new tubes or repairs, tweaks...
X
LEDs typically use up to 10ma, which is actually a LOT for a 400V power
supply (that'd need a 5W resistor in series with the LED).
A more interesting modification would do the following in standby:
a) short out the speaker leads on the output transformer (or use a very
very very low resistance) to prevent unusual oscillations, etc.
b) power the system with 50-100V and leave the fixed bias ON for the
output tubes
but if your system doesn't have fixed bias, then it probably won't work
right...
The reason why you want to apply at least SOME voltage in standby mode
is to prevent cathode damage. It's my understanding that tubes without
positive plate voltage have a tendency to form bizarre 'charge clouds'
around the cathode, damaging the coating among other things. I read
about things like this in an article on extending tube life, from the
1960's. I don't have a link handy, though...
So, in any case, you might consider at least doing 'b' but maybe
activate a relay to cause 'a' so you don't go into motorboating type of
oscillations in the power stage [with a big fat series resistor dropping
the voltage down like that]. Shorting the O.P.T. is generally bad, but
doing so while voltage is between 50V and 100V won't "cook" anything,
and would prevent oscillations. [merely opening the speaker leads won't
prevent oscillations - in fact it might make them WORSE]
You could also use a relay to make sure that the short is removed BEFORE
normal voltage is restored, a "break before make" condition using a
relay with multiple SPDT switches in it.
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