[Premium-Rx] Preselectors and 7360

Tue Dec 4 19:55:12 EST 2007

Very nice page, Adam, thanks for the link!

There are possible "dumpster fodder" sheet beam tubes used as sync detectors 
in color TV sets.  Among them are teh 6AR8, 6JH8, and 6ME8.  Any can be 
bought for a few dollars, but I'm not sure how they perform in mixer service 
as compared to the 7360.  Certainly none are pin-for-pin compatible.

My HB RX uses a 7360 as the first mixer directly driving a bank of Racal 1.6 
MHz xtal filters.  2 IF stages, and an infinite impedance AM detector.  It's 
almost silent with no antenna connected and I have never noticed any 
cross-modulation, even with a 10 uV signal and a 0 dBm signal (from 2 
different generators) 20 KHz away.

I've torn it down to rebuild the VFO, which I was never happy with.

73 John

----- Original Message ----- 
From: "Adam Farson" <farson at shaw.ca>
To: "Premium-Rx" <Premium-Rx at mailman.qth.net>
Sent: Monday, December 03, 2007 11:19 PM
Subject: FW: [Premium-Rx] Preselectors and 7360

Agreed - varactor-tuned preselectors are subject to parametric modulation of
the varactor diodes in the presence of very strong signals. At least 6 dB
attenuation must be inserted ahead of the preselector to reduce this effect,
which is a superb IMD generator.

In 1964, I designed and built a 20m SSB/CW transceiver using one 7360 as a
combined RX front end and TX balanced modulator. The tube cost £2 ($5) back
then. The cross-mod dynamic range was excellent for a radio of that era.

http://www.ab4oj.com/1st/hb/main.html

But try and find a 7360 nowadays at any price! The 7360 was probably the
pinnacle of high-performance receiving tube technology. But it may have been
its  swan-song too.

Cheers for now, 73,
Adam VA7OJ/AB4OJ

-----Original Message-----
From: premium-rx-bounces at mailman.qth.net
[mailto:premium-rx-bounces at mailman.qth.net] On Behalf Of w3jn
Sent: 3-Dec-07 17:21
To: premium-rx at mailman.qth.net
Subject: Re: [Premium-Rx] Preselectors

The RF-551 was used in fixed full-duplex HF data circuits.  I'm not sure
what the frequency separation was, but they were used with RF-590s, the
matching exciter, and the Harris 2KW amp.  The RX used a RF-551 as a pre and
the exciter was used as a post selector.  Another key to the success of this
was the very low IMD rating of the Harris amplifier.

I'm of the school of thought that looks with extreme suspicion on any
preselector circuit using varactors or elements switched using PIN diodes -
or any solid state elements for that matter.  A well-designed mixer using
the 7360 sheet-beam tube will handle tens of volts of RF before crumbling.

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