Nobody but the OP has seen what this thing looks like, or a schematic, and the description leaves me wondering... there was a mention of two heater "taps?" does that mean there are two separate filament windings? Do they have center taps?Cameron wrote:Most of the "hum" you are hearing is probably the tube. Floating the filament will most always kill the hum. If you are hearing a "buzz" that would be more of a reason to use DC or adjust the layout.
Cameron, I think you are over-generalizing a bit... "Hum" would indicate that the source is 60Hz unrectified AC, and "buzz" would likely be from 120Hz ripple voltage on rectified DC. Either one could get into the audio path in a number of ways.
In the common set-up using AC heaters, the source of excessive 60Hz hum is likely to be poor lead dress, where the AC on the filament circuit is getting into the audio path, or the lack of a suitable ground reference. A half-wave rectified bias circuit is also a potential source of 60Hz.
If there is a 120Hz buzz, then the source is likely to be unwanted coupling of ripple from the HT. If a rectified DC heater circuit is introduced, then that is another potential source of 120Hz.
In this case, the OP has powered the filament circuit with a battery, and found that the hum went away. That identifies the source, but not the mechanism. I'd bet on a lead dress or grounding issue unless it's a very low level hum.
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