It looks like a grid-driven push-pull amplifier. I don't see any evidence of a
neutralization circuit, which certainly could explain the oscillations. It
should be easy to add a traditional cross-neutralization circuit.
Vic 4X6GP/K2VCO
> On Jul 20, 2015, at 4:14 PM, Martin Sole (HS0ZED) <martin@hs0zed.com> wrote:
>
> 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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