first filter cap location

[labb]

For the Fender AB763 circuit why did Fender put the first filter cap before the standby switch? ie: rectifier, cap then switch.

[66Strat]

To mitigate the risk of hot switching.

[Leevi]

Maybe because the first filter cap is all the time fully charged and the amp is ready to operate.
/Leevi

[Willabe]

To take some of the stress off the rectifier tube from inrush current when you turn on the amp.

The 1st cap/stacked caps charge up when you turn the amp on, but leave the stand by off. Then when you hit the stand by switch the rest of the B+ filter caps charge up.

[Latole]

To save rectifier tube.
When you put standby switch on play position, the first filter cap are full charge and help rectifier tube from working too hard to charge other filter caps.

[dude]

I "didn't" put the first filter before the standby, I noticed a flash from the rectifier tube almost every time I turned on standby, luckily I changed that before any issues. Flash gone, amp happy.

[Latole]

I "didn't" put the first filter before the standby, I noticed a flash from the rectifier tube almost every time I turned on standby, luckily I changed that before any issues. Flash gone, amp happy.

So Leo Fender make a big mistake ?

[PRR]

I "didn't" put the first filter before the standby, I noticed a flash from the rectifier tube almost every time I turned on standby, luckily I changed that before any issues. Flash gone, amp happy.

So Leo Fender make a big mistake ?

Leo made many mistakes.

But here Labb seems to say Fender put a cap before the switch, Dude put the cap after the switch and saw fireworks, Dude moved his cap before the switch (which is seen on AB763 schematic) and seems happy? It seems that Dude made a mistake and quickly saw the problem and solution. Or saw that Leo had been here before and got a clue.

[Latole]

I never saw a issue with these caps before the stanby switch on many Fender amps or other brand I fix.
 
To resume ; 

Forum member pdf64 wrote same thing about that, Valve Wizzard too;

https://el34world.com/Forum/index.php?topic=23617.0

London Power, Kevion O'Connor write something interesting;

https://londonpower.com/standby-switch/

And DRZ ; write ; amp do no need stanby switch !

http://drzamps.com/blog-standby-switch/

[mresistor]

O'Connor states,
The Safest Standby Switch - Bypass the standby switch internally so that it does nothing.

[Latole]

As in almost everything, there is not just one truth or recipe.
Otherwise there would only be one type of amp, one model of guitar, one type of car, name it .....

[dwinstonwood]

Do any of you amp experts (whom I'm trying to learn from) ever use anything like a CL-90 inrush limiter on the hot line of the power cord? I understand that these can make life easier on tube rectifiers during startup.

Thanks,
David

[shooter]

No expert;
I figured out how NOT to use standby switches or tubes to rectify AC, never looked back 

[kagliostro]

Use a mute

same result, less controversy over its use

 

Franco

[66Strat]

Do any of you amp experts (whom I'm trying to learn from) ever use anything like a CL-90 inrush limiter on the hot line of the power cord? I understand that these can make life easier on tube rectifiers during startup.

Thanks,
David

Inrush current at startup is not a problem with vacuum tube rectifiers, if the first capacitor "seen" by the rectifier is correctly sized according to the rectifier's specification. When the rectifier is cold, it gradually begins to flow current as the tube heats up. This allows the capacitor to charge slowly and stay within the rectifier's peak inverse current rating. It's when you switch a hot rectifier on to a discharged capacitor that problems arise. As the rectifier is already warmed up, current flows immediately to charge the capacitor. If the first capacitor that the rectifier "sees" is too big, the rectifier can fail.

I've thought about using an inrush limiter like the Ametherm MS22 2213 between the transformer secondaries and the rectifier plates as a "cheat" to use larger than spec capacitors. But I never implemented the idea. A little resistance added to the secondary winding can be just as effective, more so when switching a hot rectifier.