The recent postings about the SB220MHz high power oscillator brought to
mind an amp I came to own as part of a job lot of other goodies. This is
a totally homebuilt 50MHz amp using 2 off 4CX250B tubes in what looks to
me to be a rather old fashioned and overly simplified configuration.
I've posted some pics at
http://s1301.photobucket.com/user/hs0zed/library/50MHz%20amp%20HS1WR so
hopefully you can see what I mean.
It's beautifully made for a homebuilt amp and clearly the builder took a
lot of pride in his work, right down to the silver plating, it's just
that it seems all wrong somehow. Signal is fed on to the grid by a link
coupled inductor with twin capacitor to resonate on 50MHz. A trimmer is
placed between the two grids. Bias is fed to the centre of this coil
(not the link) through a 1K resistor with a cap on the supply side.
Screen volts is fed through 100R resistors at each tube base.
On the output side you can see the anodes of the tubes directly
connected by short strap to the twin gang tuning cap the far side of
which is feeding the output coil which has B+ fed to the centre through
a choke of about 40 turns on a 3/8" rod of some description. Not sure
but the brass securing screw might run right through it . The bypass at
the B+ end of the choke is 100pF and the B+ exits the PA compartment
through a large feed through made of paxolin sheet with large aluminium
discs, a half inch PTFE washer fills the hole between the paxolin
pieces. One end of the output link grounds through a 100pF variable
whilst the other end of the output link goes to a LPF in a brass box.
I can find no reference to this configuration which I would say was
built around 30 years ago let alone a schematic. Anyone recognise it?
It's very simple and even the metering omits any sort of grid or screen
current monitoring!
With the output fed to a large 50R load and with no input signal any
attempt to swing the grid and plate tuning is met with copious thumping
and banging suggesting this 25kg amp is about to leap of the bench
accompanied by violent and hair raising excursions of the plate current
meter. It's not so much taking off as orbiting! NASA might be interested :(
Thoughts and suggestions please. I don't mind finishing the job off to
make it as it was meant to be even if it's not the ideal arrangement
it's deserving of some effort to tame it I think without ripping it up
and starting again.
73
Martin, HS0ZED
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