> It's pretty clear that the first thing I need to do is make sure that the
> filament voltage is right. One of the ways I've heard to do that is via a
> rheostat on the primary side of the transformer. That sounds great for an
> isolated filament transformer, but the transformer in the AL80A contains
> windings for the filament, 12 VDC supply, and the B+. So, my thought is
to
> check the filament voltage when the line voltage is highest (which I think
> is in the evening or morning) and insert appropriate resistance,
> appropriately rated, in the filament supply.
Those amplifiers contain buck-boost windings on the primary and had
carefully selected secondary winding placement and design plus a step-start
circuit.
You can measure the filament voltage right at the tube pins, and it should
be within the rated value if you have the primary taps set correctly. Inrush
is also no problem. It wouldn't hurt to check voltage though and be sure.
73 Tom
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