Hi David,
> Maybe my nomenclature of plate bypass capacitor is incorrect. It is
> capacitor connected to the DC feed end of the plate choke.
I would have called that a power supply bypass capacitor, or to be
clearer, a plate supply bypass capacitor. Anyway, now it's clear what
you mean.
> I have never
> seen a capacitor of that large value used for that application. In my
> experience, they are usually a 0.001 uF.
When in doubt, it's best to calculate what the real situation is. Since
I don't know the exact conditions in the radio you are restoring, I will
assume a "typical" case. You can adjust this to what that radio actually
uses.
The plate will carry a certain RF voltage when transmitting. Maybe 1000
volts rms, to take a nice round value. For 6146 tubes, it would be about
600V rms. It depends essentially on the plate supply voltage (times
0.7). The plate choke has a certain inductance, let's assume 100uH as a
typical value (maybe I'm far off here). At 80 meters, 100uH has a
reactance of 2.2 kiloohms. At 1000V RF, that would make 0.45A of RF
current going through the choke, and this is the current that the bypass
cap has to conduct to ground, unless there is something else helping it.
If you use a 0.001uF capacitor as a bypass, this cap having about 50
ohms reactance on 80 meters, that would leave about 22V of RF on the +B
supply line! I would say that this is too high for comfort. It could
lead to feedback, instability, maybe even to damaging the high voltage
diodes, because they are too slow to rectify RF, and will enter a
somewhat resistive sort of operation (much like an incompletely switched
PIN diode).
In practice, I would expect the power supply filter caps to short out
much of this RF. They are somewhat inductive at this frequency, but not
very highly so. But then, your bypass cap wouldn't be doing much, whith
the bulk of the RF current bypassed by the power supply electrolytics!
Also, the ground return of the electrolytics might not be at the best
location, RF-wise, leading to further feedback problems.
In short: There IS a very good reason to have a relative large capacitor
at that place. I would not recommend replacing it by a smaller one. A
0.001uF capacitor at that location will be about the same as having no
capacitor at all there!
Manfred.
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