No Center Tap and No 100 ohm R's in Filament Circuit

[hasserl]

I'm working on a Harmony 306A I picked up recently and I've noticed that the power transformer does not use a center tap for the filament coil, but there are no 100 ohm resistors on each leg of the heater circuit either. The circuit goes out to the tubes like in most other amps, including the pilot light, then one wire off of pin 9 of one of the 12AX7's connects to the power tube cathodes, and that's it. I've always seen either the center tap or the two 100 ohm R's; this is a new one on me. The amp is fairly quiet, but does have a bit of noise when cranked up, and cranked up is how this amp begs to be played. But the noise is more of the common white noise, not a low freq hum, which is what I'd expect if the filaments were contributing to the noise floor.

What do you think, could I expect any appreciable difference by rewiring this and adding the resistors?

[stokes]

Sounds like the old way of wiring filaments,where one side goes to 6.3v lead and the other side goes to ground.But instead of ground it is connected to the cathode of the power tube to raise it above ground to reduce hum.

[hasserl]

BTW, here is a link to the schematic: http://schematicheaven.com/bargainbin/harmony_h306a.pdf

[Enzo]

What stokes said...

DOn't add anything or change anything, it is ALREADY grounded by way of that power tube cathode. It is grounded and lifted by the cathode voltage. This is a perfectly legitimate way to do it.

[hasserl]

Thanks guys!