On 22 Jul 2006 08:39:03 -0700, wrote:

Hello,
I got a pretty old amplifier with 2x 813 tubes in grounded grid. I runs
nicely with 700 to 850 watts from 80 to 10m. I already did some
modifications (vacuum relays, fan control, 9 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 to 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.

If resistive I always figured anything under 3:1 was good.
I've fed a pair of 813s in GG at close to the legal limit with a
fairly modern transceiver with good results, but I never checked to
see what I had for an input impedance. I guess that was one of those
cases where ignorance is bliss :-)) I didn't know it was high so I
didn't worry about it.

For input impedance matching, look at a schematic of the Henry 2K4.
They use a separate matching network plugged into an octal socket for
each band. The ratios are different, but the design could easily be
modified and the layout is easy to experiment with. I don't have the
manual handy, but as I recall it's just an adjustable transformer with
a tap.

I think you'll be able to find the whole manual at BAMA

Good Luck es 73

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com (Get valid address from

tnx vy73 Rene DF9GR
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com