[AMPS] hot switching

[Pete Smith]

At 12:05 PM 8/19/98 -0500, David J. Riehl wrote:
... (describing K4XU's TR switch circuuit ....

>Disadvantages:
>1)Requires a lo-capacitance connected to the hi-Z end of the amp's
>  pinet, for the receive tap.  INSTALL WITH CARE, NOT FOR NEOPHYTES!!
>  Also this connection is frequency sensitive, but should be good for
>  a 20-and-up, or 2:1 frequency range.

Not my experience (see below) as long as the amplifier pi was tuned to the
band of interest.  Otherwise, suck-out could be pretty severe.

>2)Requires separate receive antenna port on the transceiver or 
>  separates.  Most modern rigs provide this.
>3)There is some attenuation on receive, but an inexpensive pre-amp
>  should overcome this as well as provide another "layer of protection"
>  for receiver front-end.

Also, raw sensitivity is rarely the limiting factor below 10 meters --
better to avoid the preamp route if possible, because of potentially
decreased dynamic range.

>4)There is possible damage to the receiver port if the protection fails,
>  but this is not likely, if done properly.

See below

>      
>QSK TR switching for the receiver port (well under 100w) could be
>implemented by a medium-power RF reed relay, a simple SPST pin-diode
>switch, or an electronic TR switch (tube or FET).

For whatever it's worth, I had one of these on a single-813 Class C
amplifier back in 1959, using a 6AK6 (I think) cathode follower as the
switch to isolate the receiver.  It was a commercial unit at the time,
cheap and worked very well, until one DX contest when the length of RG-8U
we were using as the coupling capacitor failed (probably arced through).
After the loud bang and flash we cautiously reviewed the situation.  The
receiver was OK, there was nothing left inside the envelope of the 6AK6
(!), and inside the base of the TR switch there was nothing but carbon and
wire ends where the passive components had explosively exited stage left.
Impressive!

Bottom line is that it seemed to work pretty well, but an event like that
could be pretty hard on a modern front end, particularly if the switching
element failed shorted instead of open.  Dick shows up on the amps
reflector sometimes, so maybe he'll surface and fill us in on what he
thinks of this kind of design today.

73,  Pete Smith N4ZR
In wild, wonderful, fairly rare WEST Virginia

NSI# 1,000,048	NQRPI# 1,000,004

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