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[Amps] Tank Circuit Problem.

Subject: [Amps] Tank Circuit Problem.
From: Mel <>
Date: Thu, 7 Jul 2011 08:41:37 -0700 (PDT)
List-post: <">>
An individual who does some transistor amplifier repair gave my friend a basket 
case mono-band sweep tube amplifier, it was missing tubes and, the circuit 
board under the antenna transfer relay was cooked beyond recognition.

I found a couple of 20LF6's and, I cut out the burned circuit board and hard 
wired the connections to the relay and the amp took off and started to put out 

I know what this amp should put out, but this one does not; the first stage of 
the amplifier does not tune, there is no peak to the tuning.  

I am looking for ideas/troubleshooting tips.

With the original coil in the circuit, the amp has maximum output with the 
tuner fully meshed.  I piggybacked a 75 pf @ 5Kv doorknob and now, the amp puts 
out it's max at minimum capacity.  I changed the DC Blocking cap and, the RF 
choke is the stock 22 uh choke that came in the amp.

I started out with the original tank coil on the first stage (tuning gang was 
fully closed) and, I changed the tap on the coil up and down, with and without 
the external capacitance added and max DK was only about 150 watts.

Using the original coil as a model, I have wound 5 different coils of various 
lenghts that I substituted and tapped in an effort to get the watts up to an 
acceptable level.  I changed the circuit from a coil grounded and tapped to a 
coil straight between the two stages of the amplifier.

I cannot get this first stage to resonate and put out max power.  I by-passed 
the whole first stage and went into the five final tubes with 15 watts drive 
from my FT-101 and got more watts out than with the 2 drivers into the five 

Any suggestions would be appreciated.

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