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Hello All,

I have a question about just exactly where in the power circuit I should insert a soft start switch. The "switch" is basically Tom McNally's idea about using an Amperite delay tube with a relay.

Mostly, people seem to switch the B+ before or after the filter stage. But in my case, I'm trying to switch a PS for two channels with only one relay contact something like this:



Why, you ask? Because I have a very nice 3PDT plug-in relay on hand (Rated at 15A/120VAC). One pole is needed to switch the Amperite heater off, one is used to activate the 12vdc relay coil, which leaves one pole to accomplish what I'm trying to do in the first place.

What do you think? My preference is to switch the PT CT, but who knows? Am I trying to blow myself up here?

Any comments appreciated, as always.

Chris

I'd go with the Switch DC Ground circuit on the left.

Not even needed with the circuit shown. No current will flow until the tube warms up.

Cheers!

Geek wrote:Not even needed with the circuit shown. No current will flow until the tube warms up.

Cheers!

Thanks, I agree completely. But...I'm trying to build flexibility into this amp so I can play with it. I might want to try SS rectifiers and try different tubes: 5AR4, GZ34, 6550, Coppertops, whatever, to learn more about the adjustments that have to be made for each combo.

What convinced me to add a soft start option even with the 5U4BG's is that different rectifiers can have shorter warmup times. Can't find my references, but somewhere I read that 5U4 warms up in 15 to 20 secs, whereas KT88/6550's take somewhat longer. Nice rationalization, huh? But maybe I'm gilding the lily with that one.

Another reason is to build in flexibility and an easy path to SS use in case someone might want it that way in the future.

Thanks,

Chris

mesherm wrote:I'd go with the Switch DC Ground circuit on the left.

Thanks, that makes sense to me now. With this particular arrangement, the electrons don't care what's being switched, pos or neg.

Regards,

Chris

I have switched the B+ before using the resistor across the contacts to slow the voltage rise and it worked ok but most people go with switching the ground to avoid the high voltage.

mesherm wrote:I have switched the B+ before using the resistor across the contacts to slow the voltage rise and it worked ok but most people go with switching the ground to avoid the high voltage.

Well, switching the ground it shall be then.

Thanks,

Chris

This isn't unlike the setup used by CMLabs, but they put it on the primary side (it was SS equpment and was before surge surpressors or high intital surge rectifiers were available).

Cheers!

Geek wrote:This isn't unlike the setup used by CMLabs, but they put it on the primary side (it was SS equpment and was before surge surpressors or high intital surge rectifiers were available).

Cheers!

Thanks, Geek.

Sounds interesting and I'll check them out.

Regards,

Chris