Hello,
I got a pretty old amplifier with 2x 813 tubes in grounded grid. It
runs
nicely with 700 to 850 watts from 80 to 10m. I already did some
modifications (vacuum relays, fan control, 8 band operation (80-10m)
and now I am looking for some ideas for an input circuit. I meassured
the input impedance of the tubes with about 120 to 130 Ohms which gives
a bad SWR on a 50 Ohm output stage of a modern transceiver. I also
tried a 1:4 UNUN. Results where slightly better but the SWR is still to
high. As I want to avoid the use of an external tuner I am looking for
someone out there who has build an input circuit (e.g. 1 collins
pi-filter for every band) and can give me some advice how to calculate
or build.
tnx vy73 Rene DF9GR