Manfred,
Maybe I have left out a piece that is pertinent to this discussion. The
SBE-33 does not use a transformer in the HV power supply. It has a
voltage quadrupler from line voltage to a nominal 430VDC on transmit.
Maybe that's why they used uF bypass capacitor.
David
Manfred Mornhinweg wrote:
> 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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