To: <amps@contesting.com>
> Date: Mon, 4 Aug 97 22:43:55 +0000
> X-To: boatanchors <boatanchors@theporch.com>
> Does any one know the reason why Heath would chose to float the grids
> above
> ground on the 500z tubes.
That was suggested as a method of adding negative feedback to the
tube. It was done without proper measurement, and proved to be a bad
idea.
> they are bypassed for rf thru a couple of chokes
> and capacitors, it seems to me it would make more sence to ground them
> directley to the chassis, for better isolation and stability.
Your gut instinct is correct, it decreases grid isolation and reduces
stability. At the same time it creates phase shift problems as drive
and tuning is changed.
Some people claim it "series tunes the grid", but if you look at the
values used the capacitors only reduce feedback in a very narrow
range between 30 and 60 MHz, depending on layout and lead length.
At the frequency where the tubes tend to take-off, 100 to 200 MHz,
the longer leads actually INCREASE grid impedance. If you look at
lower HF where we want good input to output isolation, the caps
greatly decrease the isolation.
You can measure this effect with a transmission loss function on a
network analyzer, and I've done just that. You can also measure it
with a signal source like a MFJ-259 and a RF detector.
Grounding the grids directly to the chassis through lugs right near
the socket tabs will actually allow you to remove turns from the VHF
suppressor with no loss in stability, and improve IMD performance.
73, Tom W8JI
(My opinions are my own. I am an independent consultant for several
manufacturers and government agencies. I have no vested interest or control
beyond supplying data and making suggestions. I do receive royalties tied to
the sales or use of certain paten
ted RF
medical devices.)
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