[Amps] Another 50MHz oscillator

[Vic Rosenthal]

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 at 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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