> I have used this method many times with the 77 series amps to drop the
> heater voltage to a proper level.
Phil:
My line voltage runs 255VAC and after two 8877 replacements in three years,
I began to take this a bit more seriously. When I made the RFC mod,
everything is easily reversible if someone wanted to go back to the previous
configuration. I am using the spare chassis feedthrough cap and wire (as
would be used in the 77Sx) from the filament transformer since I'm only
using a single 8877. That made things even easier. Using a Molex
extraction tool, I re-positioned a couple of pins to bring the filament
return through the spare wire and feedthrough cap. Reversing this requires
only the use of the Molex tool again.
I decided to tap two #8 holes inside the 77Dx's bottom tube compartment but
they cannot be seen without a small mirror. The tapped holes are nearly up
against the side of the cabinet and the new RFC is cleanly placed along the
surface of the chassis and held in place with two Nylon circular clamps.
Changing or working on the RFC does not require removal of any other panels.
In my configuration, I am using three 0.02 uF RF bypass caps on the input
side of the new RFC: One side of each from filament to ground, and one in
between filament leads. A pair of Siemens GDTs are used from each side of
the filament to the chassis and a third GDT placed between filament pins.
On reflection, I am now thinking about using bypass caps on both ends of the
RFC. I left them off the output end of the RFC (i.e., at the tube) thinking
I may want to eventually tie filament together with the cathode using a
jumper (e.g., as in the AL-1500). But the amp has been running so well with
the filament isolated from the cathode (and the chassis), that I've given up
on that idea.
Paul, W9AC
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