Bassman recto/filtering/stdby-switch question

[rawnster]

I've been researching Fender's 6G6-B circuit. I'm curious as to why the first filtering stage in the power section is hooked up in parallel with the standby switch from the rectifier circuit. The novice in me is asking why isn't the DC current hooked up to the first filtering caps, and then passed on to the standby switch? Does it matter? In looking at other Fender schematics, it appears that Fender did it both ways with various types of amps.

Thanks for your help.

[FYL]

Different drawings, same circuit.

IMO the guy drafting the blueprints reused some existing templates and couldn't draw the caps after three diodes but before the standby switch.

[paulster]

In the attachment you've posted the standby switch is switching DC, which is going to be harder on the switch since switching arcs aren't self-extinguishing, and also means that the voltage rating on the first bank of caps has to be able to handle the full unloaded voltage.

Moving the first filter caps to after the standby switch means you'll be switching rectified AC, so the voltage will reach zero volts 120 (100 in Europe) times a second, and the standby switch will have an easier life because any arc will be extinguished at one of these zero volt points if not before as the voltage drops.

The first filter cap can get away with being rated at the loaded voltage (not that I'd do that in case someone fired it up with no power tubes in) and will get quite a current inrush though each time the standby is switched into play mode, so as with everything it's a trade-off.

[rawnster]

Thanks!

So, in the 6G6-B schematic above, the current bumping up against the standby switch is getting filtered by the caps? It looks a bit confusing at first glance. You can see how the schematic of the TW express is drawn differently, and makes more sense to my slow mind.

[paulster]

If the 1K resistor on the Express schematic was a choke then those power supplies would be pretty much identical.

It's just how they've drawn it on the Fender one which isn't the clearest.

Both of them are switching filtered DC.

If you had the standby switch before the diodes (and it was double-pole) you'd be switching AC, and if you had it between the diodes and the cap you'd be switching rectified AC. As soon as that rectified AC hits the cap it is then DC.

[rawnster]

If the 1K resistor on the Express schematic was a choke then those power supplies would be pretty much identical.

It's just how they've drawn it on the Fender one which isn't the clearest.

Both of them are switching filtered DC.

If you had the standby switch before the diodes (and it was double-pole) you'd be switching AC, and if you had it between the diodes and the cap you'd be switching rectified AC. As soon as that rectified AC hits the cap it is then DC.

Thanks Paulster. I understand. I stress over the more simply items at times. I'm brainstorming a bassman hybrid, and just making sure I understood the recto/cap wiring correctly as it was originally posited.

Thanks