The past few weeks I have read a lot trying to understand better

[catnine]

 I was thinking about standby switches and ran across one on a fender amp that had the standby break the HT secondary center tap yet it was a cathode biased amp . Now I know Princetons didn't use a standby switch and if the did and the fixed bias comes off one of the red HT secondary leads and if the center tap was broken you would have no bias at the power tubes until the switch was on play and it does take a bit of time for the bias voltage to build up so looks like the output tubes would cook before the bias was proper. Even if am amp had a 50 volt bias tap and you broke the HT center tap the result would be the same.

 I have a standby on my P-P build don't know why I added it , I guess it seemed cool at the time but it breaks the B+ off pin 8 on the rect tube to before the res cap . I haven't used the standby in years simply because all it seems to do is allow the 6.3 volt heaters to warm the tubes yet you still don't have any plate voltage on the power tubes , doesn't it take a bit of time for the plates to come to voltage too or is it since the rect is already warmed up the plates get to voltage fast enough ?

 I have seen some standbys with a filter cap or even two after the rect and before the standby switch . The ones with two say 20uf 600 volt caps seem to be on amps with SS rectifiers and the ones with one seem to have a small value cap with dual  rect tubes , is the small cap just there to keep the amp from making a pop noise when switched to play . these were all fender twins.

 I'm just trying to figure this stuff out.

[TIMBO]

Hey catnine, Standby is just that, it keeps the amp in a state "ready to use" By this heaters are hot and the bull is at the gate and then it takes a split second for the amp to use.

I always use then for that reason, with out one the sudden inrush of power to heaters and tubes in a cold state i don't think that it is a good thing. You warm up your car before you try to do the quarter mile under 10s
As for fixed bias amps it could be a problem for the power tubes but i think that the bias would see power before the tubes had a chance to warm up to playing temp

For SS rects the standby is best placed between it and the first node or in some amps resivour caps are used ,it is sometimes best to have then fully charged before you let the electrons fly.

For rect tubes the standby i think is best after it has rectified the power and ready to power the circuit. These are my ideas and i think that standby's are a must, but...........

[jojokeo]

SS recto amps can have standby switch in a number of places w/out any issues, some places are better than others. Typical is after diodes & before resevoir cap however this places DC on switch which is not ideal for switch life but c'est la vie. You could use a DPDT and place it btwn PT B+/diodes to keep AC on it. You could break the CT as mentioned, and you could place it after the resevoir cap. Whatever...

Tube rectos however can be blown by the standby switch being btwn it and the resevoir cap by the sudden inrush current when switch is thrown because the resevoir cap hasn't had power on it to sufficiently charge it first. When this happens it's as if there's a short circuit for brief milliseconds as the entire circuit load placed on the recto tube & it takes the brunt of this. I like to place a 220K/1w resistor across the standby switch which allows the caps to charge fairly slowly while in standby (a second or two is slow in "electrical time"). I find that this value limits the voltage a bit & current enough so that you don't hear the amp when it's in standby mode & when you hit the switch it's immediately ready to go - and no issues placed on tube recto. The second option is to place the standby switch after the resevoir cap. This would be btwn nodes A & B for a PP amp and after the resevoir cap & btwn the choke or resistor and node A on a SE amp (not like how the old Fender amps did it as Leo didn't use an extra cap for the SE amps & better filtering back in the day)

[smackoj]

big thank you to jojo for the tip on putting a resistor across the stndby switch....i didn't even know i could be blasting the rect tube and causing failure....very good helpful hint.

 

[catnine]

 I just don't use the standby switches . On Champ build there is a place for one in the chassis so I put one in and then my resevoir cap is 47uf so I have a 47 ohm 2 watt resister between the rect and cap so to slow the inrush . I just leave the standby on play.

 I suppose a larger value across the standby switch would be even better .