I would expect screen resistance to change based on G-K volts, according
to some kind of general curve, and it's probably NON-linear at that,
based on the actual screen volts, plate volts, plate current, blah blah
blah.
But yeah, get it wrong and you howl and screech like a poorly configured
regenerative receiver.
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OK Bob, you bring up a good point. But things are not as ominous as that. See below-
Terman’s Explanation of Change of rs with Change of Screen Current
Translated it looks like this-
If Ig2 the screen current increases by 25% we would take the 3rd root of 1.25
(1.25)^(1/3) = 1.077
Then the reciprocal 1 / 1.077 = 0.928
If the screen resistance rs was 40K then at the new current it is 0.928* 40K
rs = 37.1 K at the new current
Fortunately the test setup is relatively simple. It is easy to make measurements under any set of conditions. If one has an ordinary scope there is no need for a differential probe as I have used. Just need two channels set for AC in, x10 probes & A-B to measure the difference resulting.
Some measurements are a long way off ground with large DC offsets so be sure to set AC inputs. With x10 probes the circuit loading will be 20M so in most cases that will not have to be taken into account.
I gave my analogue scope away several years ago. The Pico Tech provides a lot more information. And that can go right to a file for further use. But I forget to check the diff probe input Z, it is 4M. So I would need to go back & take that into account for the measurements across the 730K resister in the example. That is easy on an HP Calculator running RPN. So the 730K screen resister in the example becomes 617.3K & on from there. But the basic procedure is the otherwise the same.
The step in frequency response provided by the partially bypassed screen is a convenient adjustment one can make to get LF stability in a NFB amplifier. If it were a triode front end then a partially bypassed cathode resister could perform the same function. For the HF something else needs doing.
Cheers to all the Faithful still on here, John L Stewart
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