[AMRadio] substitute RF capacitor in 3-500Z amp

John Lyles jtml at losalamos.com
Sat Apr 5 10:22:15 EST 2008

As others have said, the tuning range may be limited by this change, either the bands covered (if it is the first cap, nearest the tube in a pi network) or the range of loads covered (if it is on the antenna side). If it is tuning, you might loose something like part or all of 10 meter range. But thats just a guess, you could just try it with some mica caps temporarily and measure looking back into the thing at the antenna port, if you have an antenna analyzer. You should see a decent match of 50 ohms at the frequency it is tuned and loaded, if you stick a fixed resistor in from the anode of 3-500Zs to ground, equal to the plate impedance. That is calculated from the plate voltage, current, or power that you plan to run. See ARRL or West Coast handbook on power amplifiers. 

That 500 pF 20 kV cap sounds a lot like the plastic doorknob capacitors which were used to filter the B+ for the HV to a television CRT. I would be very cautious in using those for RF duty. They tend to break and smoke, as the dielectric is a HV DC dielectric, not notworthy for reasonable RF losses. They are also inductive at HF.  An RF ceramic doorknob about 3/4 inch diameter will be rated for 5 kV or sometimes 7.5 kV in a slightly larger package. Remember that the current in the first capacitor of a pi isn't tiny. As others said, the RF current rating of the capacitor should be known, or at least use a good cap you can trust. 

Worst case, try 'em and see. 


> My question is since I don't have two two 200PF caps,
> would one 500PF @ 20,000KV doorknob do the job? With
> this change, I will have 750PF instead of 650PF at C1.
> Something in the back of my mind tells me that one can
> run into trouble when paralleling variable caps and
> fixed caps because it changes the minimum capacity
> available from the variable cap.  What are your
> thoughts on this based upon your knowledge and
> experience? I can provide you with a schematic of the 
>  tank circuit directly as I have scanned that part of
> the tank circuit.

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