[AMRadio] Parsllel capacitors for C1

John coleman wa5bxo at gmail.com
Fri Apr 4 16:42:32 EST 2008

I'm assuming by the large value that C1 is the output or loading capacitor
of a Pi tank but I'm curious as to why the fixed capacitor are in circuit
all of the time regardless of the frequency band.  That part doesn't
computer with me.  Maybe it doesn't cover all the HF bands?

Adding an extra capacitance to the output of a Pi circuit will lower the
value of minimum impedance that the circuit will load to for a single
frequency and coil setting.  But since it is a fixed amount of capacitance
then the highest load impedance that it will load into is lower than before.
For instance if it was designed for 40 to 700 ohms load then it may now be
35 to 625 ohms.  These numbers are not real, just representative and the
scenario is for a single frequency.
John, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of John King
Sent: Friday, April 04, 2008 1:37 PM
To: amradio at mailman.qth.net
Subject: [AMRadio] Parsllel capacitors for C1

Need a little technical consultation, if you can run
this through your head for me.

I have an AMP Supply LK 500ZA amplifier using two
3-500Z tubes. The tank circuit was designed and built
very rinky dink and light weight. Otherwise, it is a
fairly well built amp.

I have had to rebuild the tank circuit and put a heavy
duty band switch in it. The amp had a C1 in it
comprised of a 250PF variable paralleled by two fixed
200PF caps that stayed in the circuit all the time.

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.

More information about the AMRadio mailing list

This page last updated 16 Oct 2017.