Ampeg G-15 Gemini ll

[Helmholtz]

Could it be possible to reduce the heater voltage via resistors to a center ground?

Certainly not. Especially as the amp employs heater elevation (heater supply sits on bias voltage).

But you could try to wire a 0.27R or 0.33R 5W wirewound resistor in series with (not across!) the heater winding.

[Glenn63]

you could try to wire a 0.27R or 0.33R 5W wirewound resistor in series

Is this calculated to drop 1 or 1.5v? As R= (V) x (I)
Did you calculate this by a known amperage of the heater filaments?
Or based on past experience with adjusting heater voltages?

And again, installing a new PT designed for 120vac would alleviate all high voltage issues.

[Helmholtz]

Is this calculated to drop 1 or 1.5v? As R= (V) x (I)
Did you calculate this by a known amperage of the heater filaments?
Or based on past experience with adjusting heater voltages?

.

I roughly estimated total heater current to 3.3A, using 0.9A per 6L6 and 0.3A for each of the 5 small tubes. You may want to verify using the tube datasheets.
I assume there is some variation of actual tube heater currents.
The goal is to get heater voltage within the recommended +/-5% range of 6.3V.

Nothing wrong with exchanging the PT. My earlier remark only referred to your hum problem.

[g1]

A small bucking transformer to drop the incoming AC would be another option (if there is room inside).