Sean Williams wrote:
Hi Jeff,
Yep, thats the design, with the obvious bloopers removed.
What bloopers?
I noticed from the picture that you replaced the output coupling
cap with a disk. The schematic calls for 79 pF (probably operating
at about 40 pF) because the intended circuit uses the L-C to shift
the impedance lower and that requires a larger coupling capacitor.
If you want to stay with the parallel disk type output cap, then you
have to raise the inductance from about 90 to 120 nH...or about a 30%
increase...basically increase the diameter by sqrt(1.3) or add 1/3 rd
of a turn. The tune capacitor drops supposedly drops in value from
about 70 pF to about 20 pF. The fact that it seems to operate at 20 pF
may indicate that it has quite a bit of stray inductance.
The larger inductor will bring the impedance at the junction up to a
higher value of inductance, requiring less capacitance to resonate and
less capactance to couple the output.
I have included a picture of the anode compartment if that helps.
Nice amp...
If you can look at post script files, here's two charts that show how
I think the circuit works. The first one show the intended operation
with small operation and higher value inductors, see the component list
at the bottom. The second shows what has to change to use about 6 pF
for the output coupling.
I can probably figure out how to turn them into .gif files as well.
jeff, wa1hco
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