Heater wiring - DC or AC

[überfuzz]

I'm about to rewire a heater circuit. There was AC going to the power tubes and DC to the preamp tubes in the old wiring. I'm use to wire the whole heater section with AC... Is this a way to prevent? Do I need to bother about rectifying the heater current?

[JoeM]

What amp is this? Is it the same one you posted about regarding a new board?

[melvin]

DC theoretically reduces hum in the preamp. I guess since a push-pull power amp is hum-cancelling already, it might be a good idea in a high-gain pre-amp. It takes more components, though, and of course, those components need maintenance and can fail. I just repaired a jcm600 that had such a circuit. After retubing and changing the main filters, it came back with a blown heater fuse. It may have been a fluke, cause it behaved with a new fuse, but I should probably change the filters in that section next time I see it.

[überfuzz]

Joe - Yes it's the Carvin 33...

[defaced]

DC = PITA if you do it wrong. AC + elevation is where it's at (and I build high gain amps). Do a search on it here, you'll find circuits a plenty and instructions on how to do it.

[überfuzz]

defaced - Thanks, that's the way I normaly do it.

[R.G.]

... since a push-pull power amp is hum-cancelling already, it might be a good idea in a high-gain pre-amp.

Push pull power amps are hum cancelling only for hum injected through the B+ line to the output transformer. The ripple in the B+ is injected equally in both half-primaries. Since these are out of phase the ripple cancels. This does not cancel any hum which comes in as signal from the preamp because the preamp picked it up.

You have to kill hum at the source of injection. Once it's mixed in with the signal, it's nearly impossible to dig it back out. There are some tricks, but they are just that - tricks.

After retubing and changing the main filters, it came back with a blown heater fuse. It may have been a fluke, cause it behaved with a new fuse, but I should probably change the filters in that section next time I see it.

Fuses are funny animals. If the element in the fuse moves visibly during the power up surge, this will eventually flex and break the element mechanically, or cause cracks; the cracks reduce the cross sectional area of the element there and can lead to early burn-out of the fuse.

This is not to say that the filter caps were not also partly at fault. Just something for you to tuck into the bag of facts.