screen grid resistor and electron potential

[Andy Le Blanc]

I was a bit surprised too...

One observation doesn't cast it in stone... it was like turning the presence down.

The observation was centered around "0", I had the chance to grab a tight
measurement.

And I only had a range of -3 to +7, the operation point was fixed at the same predetermined
current figure for all conditions measured, the driver circuit did use feedback
and although my observation was centered on the difference between the
plate and g2 I'm pretty sure your correct about other factors involved in the the observation.

If you get a chance measure the difference between the plate and screen
put the meter between socket pins, as opposed to comparing figures in
reference to ground or the cathode. See what you find.

[Andy Le Blanc]

Here's a quick bench scribble with some voltages.
The diodes on the screen circuit are incidental.

But I can make some generalizations...

I have to say it again, I made an arbitrary decision to set the operating point
at 50% of the power transformers ma. rating, noodling with the tubes bias
makes any voltage readings, and resistance values irrelevant.

The readings and values change when you alter the bias or tube type.

It was interesting the 400r closely matched the voltage drop across the OPT.
It seems that if the bias setting is known (irregardless of the tube type)
it may be the only value that has to be changed with a change of tube type.

The variance between TP1 and TP2 reading on either side of the P-P circuit
is a reflection of the tubes in the circuit.

The 100r on the cathodes are the simplest way to satisfy the stated need
for either separate adjustable bias controls or separate bias resistors.
In this case I opted for a combination bias...

Measuring the difference between TP1/TP2 while biasing an
amp offers interesting possibilities. The plate voltage drops as the bias
is increased while the screen voltage stays relatively constant.
Dropping the plate voltage below that of the screen creates an AC current
as the plate swings positive relative to the screen. Even if the difference is
quite small, the added current may be all it takes to exceed the screens
current limitations under extreme conditions and exasperate the difference
between matched tubes.

It does seem critical that the circuit values be chosen after a bias point is
decided. The bias adjustment is there to maintain the chosen bias
(circuit voltages, resistances, etc..) despite the variance with in the tube
type, Its there to maintain the circuit relationships and might be veiwed
to have little more to do than compensate for varience in plate resistances
of a give tube type.