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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