[AMRadio] Parsllel capacitors for C1

JACK C. SHUTT w9gt at verizon.net
Fri Apr 4 15:44:08 EST 2008

You might want to consider two limiting factors.
  1.  What is the current rating of the doorknob?  It might work OK, but it may not be rated for the needed current.  It could break down or heat up.
  2. The minimum capacitance would be affected as you stated, which would limit the tuning range.
  73,  Jack, W9GT

John King <k5pgw at yahoo.com> wrote:
  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.

Denny Hadd at Amp Supply really cut corners in the
design and manufacture of an otherwise good amplifier.
Your assistance will be greatly appreciated. 73, John,

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