[Collins] RF tank design help needed

[Dr. Gerald N. Johnson, electrical engineer]

There have been a few rig circuits published with 6550 and many with
6146. The single ended tube will need more neutralization because of the
relative lack of isolation between plate and grid pins, and the fact
that the 6550 was designated an audio tube may mean that it has less
internal shielding. Otherwise its a nice linear tube for SSB (as is the
6146).

Virtually every ARRL handbook from 1957 (give or take) to about 1986 has
had circuits for what you wish to do. Complete details. No design
required. Earlier handbooks will tend to use 807 or 6L6 for the PA.

The PA bandswitch is a problem. The output C of the tube is enough on 10
meters to cause more tuning capacitance than required for a decent
loaded Q thus making the coil smaller and the circulating current
greater. The small 10m coil means the leads to and through your proposed
relays may have more inductance than needed for the coil and irregular
leads as inductance have lower Q, thus greater loss than a true
inductor. You can lower the inductance of the relay by using the
techniques shown for a VHF solid state PA in many ARRL handbooks from
about 1976 on through the early 90s where the flex leads were removed
and the clapper contacts were barred together. Even surplus you could
end up with more in relays than for a surplus ceramic switch. Check with
Fair Radio for a sturdy bandswitch, say out of a BC-375, used for
antenna tuning. Its a little overkill since its been run at a KW but it
may not be too expensive.

What you need is a Heathkit DX-35, 40 or 60 for coil and bandswitch (and
everything but a plate modulator). Or a Viking Ranger, or Viking
Challenger or a myriad of 50s and 60s era ham rigs already built to do
what you wish. Or the output tank and band switch from any of them
having been scrapped.

Typically the plate bandswitch was a ceramic wafer and at the 6146 power
level, nothing special for contacts. Though the contacts on good
Centralab switches were more substantial than the current production of
import rotary switches.

73, Jerry, K0CQ, Technical Advisor to the CRA.
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.