> My 77DD amplifiers drove only the cathodes and it only takes a single 1 amp
> RFC per tube to keep RF off the B-. (Radio Shack sells a nice 100uh RFC for
> the job) The filament just needs to be bypassed with a .01 and a .001 (for
> good measure...) on each side to ground...
>
> This only works BELOW 30 MHZ.
>
> Above 30 MHZ there is considerable coupling to the filament and the unit
> needs a simple filament choke just like a 3-500... But I wouldn't be in
> such a hurry to couple RF to it... JUST Bypass across the thing then and a
> choke with caps to ground behind the choke.
160 meters on the Alpha 77DX/SX has always appeared to me to be an
after-thought. They can be made to work with several mods. One of them is, as
Dr. Schmidt suggests, is a filament choke in the heater line. When I started
tackling the first 77SX I had, I had gobs of 160 meter RF all over the power
supply area, and all over my house wiring. After adding the choke and proper
value bypass caps on every lead that passes from the RF deck area to the power
supply area, the problem was solved. Luckily, the 77series amps use an
oil-filled filter capacitor; I am sure all that RF in the PS would have done in
a bank of electrolytic.
I always assumed that the coupling of both sides of the filament through
capacitors came from the glass tube days where the cathode was internally
strapped to the filament. (4-1000A, etc.) Certainly not applicable to tubes
with
heaters!
I agree...a lot of this stuff has proliferated down through the ages, such as
large value plate blocking caps, small value bypass caps, parasitic suppressors
not matched to the tube, etc. The addition of WARC bands, 30 and 160 meters
changed the ball game quite a bit for the amp builder.
(((73)))
Phil, K5PC
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