[Amps] Plate coupling capacitor value

[Manfred Mornhinweg]

> 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.

Manfred


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