I cant recall offhand any other receiver that used a 7360
as a mixer, but need to look in my vacuum tube receiver book,
it lists all the receivers with their tube lineups.
The ARRL sure liked the 7360, they used it in a lot of their
receiver projects, in the 1967 handbook anyway.
I suspect the cost of a 7360 was lower back then, not sure why
its so expensive now, must be very rare?
I still have a few that I bought new back in mid 70's for about $12 each.
There are a couple of beam deflection tubes similar to the 7360, made for TV
applications. I believe one is the 6AR8, and there is another one without
the wierd filament connection, but I forget the type number.
The 7360 was used as a mixer in the Tempo One (an early, very flaky, Yaesu
tube type SSB transceiver). In later models they replaced the 7360 with
some kind of solid state mixer. The tube is probably expensive because it
is rare, while there is equipment still in use that requires it. I once
built a SSB generator using one as a balanced mixer. I successfully got it
working, generating a near-hifi SSB signal at 64 kc/s using a Collins
multiplexing asymmetrical mechanical filter from Ma Bell as the sideband
filter. I lost interest in the project before building converter stages to
translate the low-frequency signal to the amateur frequencies.
I never realized just how noisy some receivers are till the
first homebrew was done and compared it to the R390A.
I guess you think its atmospheric noise, or just get used
to it, but I cant stand the R390a anymore.
The problem with the R-390A is that there are so many mixer stages ahead of
the selectivity. Every mixer contributes to noise.
The ultimate design for a practical receiver would be single conversion with
no rf stage ahead of the mixer. It would use a selective enough front-end
tuning network between the antenna and mixer to reject images, and the mixer
would be low noise enough to hear all the way down to the atmospreric noise
floor. The mixer would have high enough output level to feed directly into
the selectivity filter with no amplifier stage between the mixer and filter.
Following the filter, the i-f amplifier would be low noise and high gain
enough to boost the signal to the level needed at the detector without
raising the noise floor above that of the signal that exits the filter. One
requirement would be a selectivity filter with minimal insertion loss.
One interesting receiver design from before WWII used a separate tuned
circuit to null out the image frequency. It was written up in QST, I
believe (or was it RADIO?). Hallicrafters came out with a model or two that
used the design, but it was swiftly discontinued. I think the problem was
that the image null had to be a separate control from the main tuning
because of the difficulty of getting the image null to accurately track with
the rest of the receiver. This might be a worthwhile topic to research and
apply to a no-compromise homebrew receiver.
I am sure there is a tube to replace the 12SA7 in a miniature
type.
Zillions of them were used in the miniature tube version of the classic
5-tube ac/dc bc receiver. Isn't it a 12BE6? (My computer is in the house
and all my radio reference material is out in the shack).
Every commercially built receiver, ham or military, is one big compromise,
designed to best meet the needs of diverse users.
Don K4KYV
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