[Amps] Re: Amps Digest, Vol 12, Issue 33

[gw4dgu at blaenffos.org]

> I have been recently been able to look at a Rhode and Swartz 5 KW solid
> state UHF TV Transmitter, water cooled.  It sure works and works very
> very well.
> It is all in a single 19 inch rack, each module produces about 1.5 KW
> output and it is wonderfully linear and very stable in performance.  It
> is great to see level setting changes of several KW output with NO
> tweaking of anything, just turn it up and turn it down.

If it ever comes up as a scrapper, let me know!! I have a delivery address
in VE6..........

> For me, my concerns with solid state is to have an effective controller
> that will protect my investment.  After some consideraton what I want is
> a G3SEK type box that will look after my dollar investment in solid
> state RF modules.

Modern RF Fets are pretty robust devices and have an MTBF a couple orders of
magnitude better than thermionic devices, provided the junction/channel
temperature is kept to a sensible level, and the breakdown voltage and
dissipation limits of the device are observed. Monitoring the drain
currents, heatsink temperature and load VSWR is really all that is required.
A $2 microcontroller could be used to automate the monitoring/trip
functions......

Providing the amp. is properly designed, it should survive transient
open/short circuits on the output. In professonal amp. specs. it's normally
required that an amplifier will work without failing or running into
instability with any phase of something like a 5:1 vswr.

My one dread with my own station is a very close lightning strike, but then
that would be a serious problem with many tube PAs. Fortunately we don't get
big electrical storms in this part of the world too often. I try to remember
to keep the
transmitters disconnected from the antennas at times when I think we might
get one.

> I am basing this on my experience in the LF world, 136 KHz where I had >
an ex Decca amplifier, MosFets at up to 100V and 30 Amps and more, yet
> it was unconditionally protected, antenna ON or OFF it did not matter -
> the amplifier was protected by a wonderful protection design.

Hitting the nostalgia key, I was once given a tour of one of the original
WW2 Decca stations, at Bolberry Down in Devon, on the south coast of
England, courtesy of the the Station Manager, the late G3CHN. That would
have been sometime around 1982. The station was still using an array of 807s
in the PA. Some had been operating continuously in the same sockets since
1943. Presumably they were combined together, and I'd be interested to know
how. If I asked at the time, I can't remember the answer.

73

Chris
GW4DGU