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Re: [Amps] Plate coupling capacitor value

Subject: Re: [Amps] Plate coupling capacitor value
From: Manfred Mornhinweg <>
Date: Sat, 09 Sep 2017 16:53:24 +0000
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Its only 1.275 amps, and that?s peak, not rms.

Please be aware that on the highest bands typically much or most of the tuning capacitance is the tube's stray capacitance to ground. For this reason the plate coupling cap does not just conduct the _input_ current to the PI tank, but it conducts most of the _coil_ current on those bands, which is higher by a factor of the tank's Q!

Specially if tubes are used that have a lot of internal and stray capacitance, the loaded Q of the PI tank typically has to be rather high on 10 meters (which requires those 10m coils made from thick copper tubing rather than wire, or from copper strap). Almost that same current flows through the plate coupling cap. So this cap needs to be able to handle a pretty high current. Like 10A, maybe even 20A, depending on the power, plate impedance, tank Q, and ratio of capacitance in the tuning capacitor to stray and internal tube capacitance.

On the lower bands the current in that capacitor gets lower too.

Since it?s a monoband 136 khz amp,  you have some wiggle room.....
you don?t have to worry about higher freqs.

In that case life is easy indeed! But I still thought it would be good to point out the current requirements on high bands, for forum members building all-band amps.

As to the capacitance, I would make it large enough so that its reactance on the lowest frequency of operation isn't more than, say, 20% of the plate impedance. But even higher reactances can be tuned out in the PI, if designed to do so. Be aware that the RF voltage across the cap rises when its capacitance is smaller. So, a high value capacitor can get away with a relatively high loss dielectric, while a lower capacitance ones needs to have a lower loss dielectric.


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