Hi All, Vic is probably exactly correct. Assuming this to be the case, the
amplifier likely has more than 20 dB of gain. That makes the need for
neutralization even greater. Also, depending on how you plan to use the
amplifier, there may be a need for input attenuation to prevent over drive.
If attenuation is needed, it may be more practical to add some resistive
loading across the grid circuit. This would lower the gain and result in
better stability at the same time.
Another consideration is the class of operation. If the amplifier is set up
for class C, then the grid bias would be a lot higher than class AB or B.
If this grid bias is derived from grid leak then there would be no bias at
all without rf drive. In this case, high plate and screen current would
certainly be the case.
I would suggest you unplug the tubes and do some d.c. tests to determine
the presence of grid bias voltage before going further. You could also
disconnect the heater voltage if that is easier than removing the tubes.
73,
Gerald K5GW
In a message dated 7/20/2015 10:28:53 A.M. Central Daylight Time,
k2vco.vic@gmail.com writes:
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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