SS rectifier and stand-by switch

[menger66]

When using diodes in this case i'm using a FWBR, should i be running a wire strait from the diodes to the stand-by switch? or should i have a electrolytic cap also before the stand-by switch with a bleed resistor? seems like running the SS rectifier strait into a closed switch would be unhealthy in some way but i'm not really sure thanks mark

[Stevem]

Well the switches we amp builders and amp manufactures use do not hold up well long term to dc voltage do to contact pitting.
A good thing a bridge recto lets you wire in is a standby switch that opens and closes the ac output from the PT to the bridge by the use of a double. Pole double throw switch.
With this set up the switches will last forever!
Adding a thermistor will provide a soft start that the power supply filters and the PT will love also.

[gui_tarzan]

I have a bunch of these from a phone system power supply I stripped, is this what you're talking about? Where exactly would you put this, on the positive output lead on the bridge?

[Paultergeist]

Adding a thermistor will provide a soft start that the power supply filters and the PT will love also.

I have seen a Metal Oxide Varistor (MOV) which looks like the photo posted above......I am not sure, however, if a MOV provides any sort of "soft start...?"

[johnnyreece]

A thermistor, not a varistor (which is what is pictured, I believe). I used one in my most recent build. I put it in line from my rectifier to the first filter; no standby switch.

[teemuk]

A standby switch interrupting the B+ line isn't such a good idea to begin with; it's suspect to introducing huge voltage and current surges and it is dubious if it even "extends" you tube life - instead of actually doing the opposite. You can build mute circuits other ways and they will likely be more reliable in the long run.

Well the switches we amp builders and amp manufactures use do not hold up well long term to dc voltage do to contact pitting.

For a switch, it's less demanding to interrupt AC than DC. DC sustains arc much better than AC.

[pdf64]

No reason not to use 2 thermistors, on the primary to limit the magnetising current inrush and on the B+ secondary to limit the inrush from charging the reservoir cap.
This allows the use of fast blow fuses, which should then blow rapidly in response to a fault current.
The downside of that is if the amp is switched off momentarily, then the fuses/s can blow when switched back on again, should the thermistor not have had long enough to cool down.
Pete

[Structo]

It's curious that the vintage amp designers would switch the B+ for a standby.

What I do on my amps is turn the power on, then switch the standby switch on.

When I power down I just turn the power off, thereby saving the standby switch from one cycle of operation.
So in theory, the switch should last 50% longer.

[Phil_S]

The standby switch isn't needed. Skip it and save yourself the work and cost of the switch.

[Bombacaototal]

The standby switch isn't needed. Skip it and save yourself the work and cost of the switch.

I am curious why you say the standby switch is not needed. I might not put one on my new amp but curious about the theory behind it

[xtian]

Let me point you at the latest round of discussion on this topic:

http://www.thegearpage.net/board/index. ... y.1761176/

Short answer: standby was needed to prevent cathode stripping for high (read 1000+v) systems. Cathode stripping does not occur in our builds where we see only 300-600v on plates. Second, cathode POISONING can happen if you leave amp in standby for long periods.

[Bombacaototal]

Let me point you at the latest round of discussion on this topic:

http://www.thegearpage.net/board/index. ... y.1761176/

Short answer: standby was needed to prevent cathode stripping for high (read 1000+v) systems. Cathode stripping does not occur in our builds where we see only 300-600v on plates. Second, cathode POISONING can happen if you leave amp in standby for long periods.

Interesting! thanks a lot for the info