Oddly enough with a 50 ohm swamping resistor the amplifier should be stable (
not oscillate) on 160 meters.
The forward voltage gain from grid to plate is only 12 or so using the tube
specs. And the reverse ratio Vg/Vp due to internal capacitance and and
50 ohm swamping resistor voltage divider is less than .08 so .08x12= .96. If it
is near 1 or more it could be unstable and oscillate. So with two tubes it is
barely stable gain wise. But the phase is wrong since it has a resistive input
impedance. The amplifier is inverting so you will not get positive feed back,
but under the right conditions you will. A grid driven ampifier becomes a TGTP
oscillator when the forward gain and reverse gain product becomes greater than
one and the input reactance is inductive which combined with the feedback
capacitance throws in enough phase shift to create positive feedback. This
seems to risky to me. And Vic points out the high loss in the swamping resistor
as well. You could the swamping resistor but seems a bit risky to me.
Rather than risk the possibility or things going bad I also suggest cathode
dirve. I used to run a pair of GG 250th's 40 years ago and they have similar Mu
values. It is just that the input Z is much higher than
50 ohms (use an antenna tuner to match) and the power gain will not be so high
as with hi mu tubes.
Now saying that, I am concerned that the plate to filament capacitance is
8pF per tube on the 833. Can that be right? That of the 250th is only 0.7pF
If it is really 8pF it may not be so stable in GG operation. Because now you
have an non-inverting amplifier and a forward and reverse gain product greater
than one.
I wonder if they measured it with the grid floating. Other wise is seems that
the grid offers no shielding at all if it really is 8pF. How can the plate
filament capacitance be greater than the plate grid capacitance?
Bill
wa4lav
On________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Vic K2VCO [vic@rakefet.com]
Sent: Wednesday, August 11, 2010 10:39 PM
To: Greg Weinfurtner; Amps reflector
Subject: Re: [Amps] 833A 160 meter amp Phase 2 fit-up
The schematic shows a swamped grid-driven circuit. I think you will need to
develop about
225 volts across that 50 ohm resistor in order to drive the tube in class B,
which means
that you'll need to hit it with about 1KW to drive it! All but 20 or 30 of
those watts
will be dissipated in the resistor.
I suggest that you use a parallel-tuned input if you want a grid-driven
circuit, or use a
cathode-driven circuit. The 833A has a mu of about 35. Usually, higher-mu tubes
are used
in cathode-driven circuits (the Eimac 3-500Z has a mu of 130). I think this is
because the
higher mu means the grid is denser and will provide better shielding, but I'm
not 100%
sure of this.
On 8/11/2010 7:08 PM, Greg Weinfurtner wrote:
> Hi all,
> I promised that I would tell when I had made major leaps in the
> 833A, single band, 160 meter project. Latest pictures are at
>
> http://oak.cats.ohiou.edu/~weinfurt/833/833A.html
>
> No wiring is done as this is just the 'fit-up' to see how things
> fit. Heavy? You bet! The two HV microwave transformers are in parallel and
> diode isolated from each other. Grounds are common, though. There are 4
> square inches of in each of the transformers center leg, making a total of
> 8 square inches total. I think they will supply enough power.
>
> Anyway, check it out if you'd like to.
> 73,
>
> Greg Weinfurtner AEE BSS
> NS8O
> 40192 State Route 689
> Meigs County
> Albany, Ohio 45710
> United States of America
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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