HT on First Filter Cap in Standby Mode

[David Root]

Is this "good practice", as the Ancients would say, or not? If not, why not?Fender thought it was OK, but Marshall didn't do it, except on the JTM45, which copied Fender anyway.

I can see it might be good to have the power tube plate supply cap all charged up and ready to go, and since there's no load on it, why not? I haven't yet consulted the Ancients (i.e. Radiotron Designer's Handbook), so I'm ready to stand corrected.

Also, I'm curious about standby switch and HT fuse placement in general.

[Mickey_C]

I fuse the HT center tap, on the transformer, and put the standby on the B+ right before the filter cap. The first filter cap is often the high side of the choke and the OT center tap, so I'd rather not power them up with the amp on standby. It can make trouble shooting an amp blowing fuses much easier - that's my only reason. For that matter, a standby switch is really only a convenience anyway.

Mickey

[David Root]

I'm inclined to agree with you on the standby switch position, that's where I usually put it, exception being a cathode biased design, then I put the standby switch from PT CT to ground. Putting the HT fuse in that spot in fixed bias mode is interesting, I've seen that before but never done it myself, usually put it between the standby switch and the rectifier B+.

[Rattlecat]

Where would put the HT fuse in a cathode bias design?

[David Root]

Same place(s) as in fixed bias. Putting the standby switch between CT and ground is easier on the switch contacts, and quieter because it's a make/break to ground, not HT. Never do this with a fixed bias amp however, that would cause serious potential power supply problems.

[Mickey_C]

Putting the HT fuse in that spot in fixed bias mode is interesting, I've seen that before but never done it myself, usually put it between the standby switch and the rectifier B+.

Use the fuse, the cheapest part in the amp, to protect the power transformer, usually the most expensive part.



Of course, if you're not using a grounded center tap, this won't work, in which case I would put it where you said.