Hello Scott:
Congratulations on the 813 amp. I had one many years ago and worked about
150 countries on 75 meters. The tubes are instant on - and that is worth a
couple of db when you hear a rare station.
The electronics of a tuned input is relatively easy - you simply need to
match the impedance of the tubes to 50 ohms. Unless the amplifier you took
the input from used the same tubes, it would not match, so you might as well
build your own.
The most difficult problem in retrofitting a tuned input to an existing
amplifier is trying to figure out a way to switch the input with the output
switch. I have done that successfully on a couple of occasions and it
involves skill and good luck. So, plan on having a separate switch for the
tuned input and remember to switch it with the output.
If you wish to reduce designing to a minimum, get a pi-network antenna tuner
and connect it right at the input - with zero length cable. Then adjust the
match for the bands you want to operate on. You can then simply dissect the
antenna tuner and determine what value parts you had when you had the best
input swr. Then replace those measured or calculated values with mica caps
and small coils. You can use a two wafer rotary switch to select the band.
Or, you can design your own. I am guessing that the input impedance of an
813 is about 375 ohms. Two in parallel would be 188 ohms. Use one of the
filter design programs to design a pi type filter that has 50 ohms input and
188 ohm output. Wind a toroid for the inductor and use mica compression
trimmers for the capacitors and you should be able to hit it right on the
money.
On my 813 amp, I wound a broadband toroid step-up transformer and loaded the
output with a bit of resistance to make the load constant and called it
good. The driver was happy and I did not have to switch the tuned input. I
did not have much equipment to measure the performance, but never got death
threats from anyone so assumed it was ok.
73, Colin K7FM
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