> Speaking of the chassis-carrying current, I recently modified my Alpha
> 77Dx
> by adding a filament RFC, thus raising the filament return above chassis
> ground. I've always detested using the chassis as a high-current-carrying
> conductor. For a single 8877, that's 10A of AC current. For the 77Sx,
> that's 20A of current through the chassis. For me, another benefit was
> in
> the reduction of filament V as a result of high utility line voltage. It
> went from 5.6V before the mod to 5.0V after the mod due to the slight (but
> needed) IR drop through the new filament RFC. Arguably, even the newer
> Alpha amps that utilize the chassis as a filament return could probably
> benefit from a filament choke upgrade.
I have used this method many times with the 77 series amps to drop the
heater voltage to a proper level. Alpha assumed 230-volt mains when this
series was designed. Most of us now have 240-245 volt mains in the ham
shack. This results in excessive voltage at the tube pin(s). Instead of
using a resistor in the secondary of the filament transformer, I simply wind
a filament choke using the proper size wire to obtain the correct drop to
obtain 4.9-5.0 volts at the tube socket. An added improvement with this mod.
is that 160 meter RF is blocked from entering the power supply compartment
via the leads from the transformer, getting into the power supply
components, and radiating RF into the house wiring. This is but one change
that 160-meter enthusiasts need to make to get optimum operation on that
band. I think that top-band was an after-thought for Alpha on these models.
(((73)))
Phil, K5PC
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