"Kenneth Scharf" wrote in message
...
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!

Now that's really, really scary!

Old Chief Lynn, W7LTQ

"coffelt2" wrote

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,

Try 1000 watts for the combined plate dissipation.


and one's first though would be "you'll blow up the receiver!".

Just wait until it goes unstable and oscillates. Bye-bye receiver!

Terry wrote:
"coffelt2" wrote

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,

Try 1000 watts for the combined plate dissipation.


The 572B actually had only 150 watts plate dissipation and the SB200
used two of them. The T160L version of this tube had a 160 watt plate
dissipation. Many SB200's grossly overloaded these tubes and got away
with it.

and one's first though would be "you'll blow up the receiver!".

Just wait until it goes unstable and oscillates. Bye-bye receiver!


The odds of a pair of 572B's in grounded grid oscillating were very
slim, at least at HF. VHF parasitics were another story, but the SB200
was well built in this regard. I'm sure the rig was stable even without
an antenna connected. IIRC we thought of putting in a pair of back to
back 1N4007's on the rig side during receive just in case.

"Kenneth Scharf" wrote

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,

Try 1000 watts for the combined plate dissipation.


The 572B actually had only 150 watts plate dissipation and the SB200 used
two of them. The T160L version of this tube had a 160 watt plate
dissipation. Many SB200's grossly overloaded these tubes and got away
with it.


Sorry - I misread the OP as an SB220