Best chassis location for OT and filter cap doghouse to minimize hum in reverb

[SolidbdyJazzfan]

Hi all,

First post here. A question relating to reverb tank positioning with respect to the output transformer and the filter cap board. This amp build will be a mirror-imaged Twin Reverb, but consisting only of the "Vibrato" channel, and having no vibrato circuit, but only a reverb one. It will be mounted with the tubes, transformers, and filter caps on top in a Marshall style cabinet.

Here's a work-in-progress image of the inside of the chassis, to provide a rough idea of what I'm doing:



I have purchased a type 8 (short) reverb tank, having been advised by Mojo that I can avoid induced hum by mounting the tank more toward the output transformer end of the chassis (less of an AC field down at that end).

My question is this. Which of the two following chassis-top layouts would tend to result in the least hum induced in the reverb circuit, assuming I can get the reverb tank mounted (against the inside of the front cabinet panel) all the way at the right end of the chassis?

My initial design (note that the OT is all the way to the right):



My more recent design, which more closely follows the Fender Twin Reverb chassis layout with the filter cap dog house to the right and outside of the OT:



The first design, it struck me, would keep whatever AC field that surrounds the filter cap board further from the reverb tank, but will cause the OT wires from the output tubes to be longer than they perhaps should be. Obviously, there's a compromise involved here, but I thought I'd ask for opinions on which of the two layouts might work best with a small reverb tank mounted at the right end of the chassis.

Thank you!

[Steve Conner]

The most important consideration is to get the reverb tank's pickup transducer as far away from the PT as possible, given the confines of the cabinet. Both of your layouts are equally good in that respect: the PT is in a corner of the chassis, so you can arrange the reverb tank with its pickup coil in the opposite corner.

The other components don't really matter. They don't radiate any significant magnetic field at line frequency. The OT does generate a magnetic field, but it sounds like a copy of the speaker signal, so it would blend right in if picked up by the reverb recovery circuit.

[SolidbdyJazzfan]

Thank you very much Steve! That is precisely the sort of reply for which I'd hoped, and I learned something about the OT field.

[Chuck H]

I've even stuffed full size reverb pans in head type designs and had less induced hum than average. The EMF from the transformers and the sensitivity of the transducers isn't an even radiant field IMHE. You can often move the tank around to find a quiet spot and mount it there. It might be at an angle or even backwards from what you thought it would be.

[SolidbdyJazzfan]

Thank you, Chuck! The pan I bought is a "Revisit" brand with three springs. Given that I'm pretty happy with the short pan in my Fender Studio 85, I figured I wouldn't be too disappointed with the depth afforded by the Revisit pan.

The good news is that although the cabinet is the "middle sized Marshall style" (26 inches wide), there should be a number of places I can stick the pan and have it clear of the other parts. So your point about moving it around is well-taken!