Since it's a single-band amplfier, it would be simple to use a balanced grid
tank circuit
and connect the 'bottom' to a neutralizing capacitor to the plate.
I'm using a grid-driven circuit in my 813 amplifier, and I'm neutralizing it
with a bridge
circuit. That would work, too. All of these neutralizing circuits are in the
ARRL
handbooks, even the new ones.
On 8/11/2010 10:51 PM, Bill, W6WRT wrote:
> ORIGINAL MESSAGE:
>
> On Wed, 11 Aug 2010 19:39:51 -0700, Vic K2VCO<vic@rakefet.com> wrote:
>
>> 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.
>
> REPLY:
>
> I agree with Vic but I will take it a step further and say you will
> have the same drive problem if you go to cathode driven (grounded
> grid). This tube needs a very large cathode-to-grid voltage swing
> whether grid or cathode driven, but with the low impedance of cathode
> drive, your drive requirements are going to be excessive. It would be
> roughly the same as with a 50 ohm swamped grid configuration as Vic
> mentions above.
>
> I've looked at the data sheets from RCA, Amperex and GE and none of
> them mention grounded grid operation. I think there's a message there.
> If anyone here has experience with an 833A in grounded grid, please
> jump in and share your findings.
>
> I believe your best bet is to use a conventional, old fashioned,
> un-swamped grid driven circuit, but you will have to add
> neutralization. A handbook from the '40s or '50s would be a good
> reference.
>
> Don't be discouraged - this sounds like a great project. The 833A is a
> classic old bottle. Good to see someone still wants to run them.
>
> 73, Bill W6WRT
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--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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