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
power.
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
finals.
Any suggestions would be appreciated.
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