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Re: [Amps] Plate Bypass Capacitor

To: Manfred Mornhinweg <manfred@ludens.cl>
Subject: Re: [Amps] Plate Bypass Capacitor
From: "David C. Hallam" <dhallam@rapidsys.com>
Reply-to: dhallam@rapidsys.com
Date: Wed, 05 Aug 2009 16:24:47 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Oops, I left out 0.1 in my reply.
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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