On Dec 29, 1:57*pm, Kenneth Scharf wrote:
Many years ago a friend of mine was repairing a Heath SB200 linear he
had picked up. *The antenna relay needed work and he was replacing the
stupid RCA connector in the input side of the rig with an SO-239 (why
did Heath use those RCA connectors for the antenna in all their ham
rigs?).

Anyway we were talking about how the linear improved the transmit signal
strength, but did nothing for the receiver and he wanted to also build a
small receiver preamp to put inside the linear cabinet, and we both got
the samw brainstorm, at the same time. *Why not wire the linear up so it
would work in BOTH directions? *I suppose a grounded grid preamp using
tubes with a combined plate dissipation of some 300 watts seems crazy,
and one's first though would be "you'll blow up the receiver!". *But
remember that those tubes have a power gain of only 13db and that's
about the same as a 6BA6 rf amp tube. *Of course the noise figure of the
572B as a preamp probably isn't quit as good as a 6CW4, but you won't
notice it much on 20 meters. *Also at the signal levels we are talking
about the usual cutoff bias for the tubes STILL leaves them in class A
for use as a small signal preamp!

To make a long story short, we wired the relay up so it would do the
required switching and tried it. *The linear did boost received signals
by about 13db, just as we expected. *Made a difference in the DX contest!

There's something to be said for an amplifier with high IP3. One way
to get that is to use high-power devices in the amplifier. It's not
to hard to find modern solid-state devices with fairly high power
dissipation and also low noise figure. One of the obvious problems is
that such an amplifier may have the potential to blow up things
following it...

Ulrich Rohde published an article in Ham Radio magazine some thirty
years ago about a way to get a good noise figure and extremely good
IP3 in an amplifier; his example was based on CATV transistors of
modest dissipation. The article seemed to leave quite a bit to the
imagination of the reader, and I've never seen any commercial products
that come close to the performance he suggested should be possible --
not even from the company in New Jersey of which he's a director. I'd
be really happy to find such an amplifier just now... (NF ~ 2dB, TOI
+70dBm, on the order of a watt bias power, per Rohde).

Cheers,
Tom