[Amps] 833A 160 meter amp Phase 2 fit-up

[Fuqua, Bill L]

Has anyone got an idea of why the filament plate capacitance is higher than the Grid Plate in the 833A specs? does not make sense.
I suspect it will work ok in GG..
Bill wa4lav

________________________________________
From: amps-bounces at contesting.com [amps-bounces at contesting.com] On Behalf Of Fuqua, Bill L [wlfuqu00 at uky.edu]
Sent: Thursday, August 12, 2010 1:17 AM
To: Vic K2VCO; Greg Weinfurtner; Amps reflector
Subject: Re: [Amps] 833A 160 meter amp Phase 2 fit-up

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 at contesting.com [amps-bounces at contesting.com] On Behalf Of Vic K2VCO [vic at 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 at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps


--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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