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

Thu Aug 12 19:30:56 PDT 2010

I haven't played with an 833 but from what I remember reading in some of the
old articles on them, they don't work well in GG or passive grid driven. In
GG I think that they are not stable. Passive grid requires too high a value
of grid resistor and stability goes west. I think that the best way to run
them is grid driven and neutralized. And the neutralization can't be the
simple bridge type either. You need a full push pull (double ended) plate or
grid circuit to neutralize them. 
All from long ago memory.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Fuqua, Bill L
> Sent: Thursday, August 12, 2010 4:15 PM
> To: Amps reflector
> Subject: Re: [Amps] 833A 160 meter amp Phase 2 fit-up
> 
> 
> 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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