[flipper]

On Wed, 23 May 2012 02:13:59 -0700 (PDT), Patrick Turner
wrote:

snip,


It doesn't work too well
because screen input resistance is fairly low, and where you have RG2
high, then VG2 becomes high, but screen FB applied this way is a non-
linear way of applying FB.

That's the second time you made an arbitrary declaration of
"non-linear," after 'always wondering' about it, and with no
explanation of what the heck that means. Non-linear how? Like Ia is
non-linear?

I often type stuff which baffles many people, sometimes for a second
time, just as you mentioned,

That's because you often babble gibberish.

so to be sure about what you are doing,
get away from PC and be the the Real Scientist for yourself, in your
workshop, and thus findoutabout all things confusing involving more
than 4 interactive variables.

You're the one who "always wondered" so get to it.

I don't have any data one way or the other but it was apparently
considered 'linear enough', whatever that criteria might have been,
for self split PP amps. And while that's not entirely equivalent it's
a heck of a lot more substantive than you declaring it 'sort of works
like' Rk and then never again speaking of screen feedback in the
previous rambling.

Anyway, I'm a bit confused by just what you mean now. what was "it"
that was apparently considered "linear enough"? please try to extend
your descriptions and definitions so all the rest of the dummies
reading your post will have some idea what you are saying.

The "it" is what's been the topic of this thread: the voltage signal
generated by an unbypassed screen resistor.

Look up self split push pull amp.

According to RH4, using a screen resistor was done to get 'less
distortion' than using a divider off the plate signal.

The conventional UL OPT supplies the screen
input power, ie, Vg2 x Ig2 input from a relatively low source
resistance.

If you are externally *driving* G2 then you need a low source
impedance but a large value Rg2 is not 'driving' G2, the voltage
produced is the *result* of G2 current. G2 does the 'driving'.

Unlike G1, G2 has low input impedance. And current flow into isn't
marvellously linear with an applied linear voltage.

You miss the whole point in that one is not 'applying' an external
voltage *to* the screen. Screen current results in a voltage.

The larger the
added series R between a voltage source and G2 becomes, the more the
G2 acts independantly and its voltage change is due to Ig2 x series,

That is 'the point' of it.

and that isn't a very linear application of NFB.

That claim is obviously pulled whole cloth from thin air because the
first blooming thing you said about this subject was you "always
wondered" how good it was.

I'm saying :- don't
use large series R between any voltage source and G2 because it leads
to worse general operation of the tube in a number of ways.

I know what you've been 'saying' and it doesn't make sense to "always
wonder" and then claim to know what you have "always wondered" about.

The screen
tap on the OPT at say 40% of P turns would have source impedance of
pentode Ra x 0.4 squared, because there is an Ra transformation here,
so Rout from the g2 tap for 6550 with Ra at 32k = 5.12k. However, as
soon as one connects G2 to the G2 tap at 40% on OPT then whatever
voltage change is applied to the G2 works to reduce Ra, perhaps to
about 3k5, so the G2 tap output resistance becomes 3k5 x 0.4 x 0.4 =
1.k4 approx, and in fact the UL connection lowers the circuit
impedance from any point to 0V. But the screen does have to be driven,
ie, supplied power, both current and voltage to do things to Ia,

You just said a series screen resistor won't do anything to Ia because
it doesn't 'drive' G2. So much for the claim of 'non-linearity'.

Stop babbling.

whereas G1 does not have to be driven because its input power is
negligible because its input impedance is many megohms, and Iin is
tiny. But this changes with F because of Miller and stray C.

Nobody made a comparison to G1, except your nonsense.

A series R does not drive anything, because its a passive element,

You're the only one stuck on this 'driving' nonsense. Everyone else
has known from the get go it doesn't 'drive' G2 and, as I said, G2
current is what causes the voltage change.

but
where there is a series R between G2 tap and G2 on OPT, the anode
drives the OPT, and the tap applies voltage change and hence current
change to G2 and the series R tends to reduce the effectiveness of
what the G2 tap is there for. The ultimate screen NFB occurs when the
pentode or beam tube is triode connected. One never ever sees a high
value R between G2 and a for a triode connected tube. But you should
try it anyway, just to see why nobody uses high value R between a
voltage source and G2, without bypassing G2 to cathode.

Your the one who "always wondered" so get to it and find out.

That's akin to bitching that Rl on a triode is 'too high an
impedance'
to 'drive the plate'.

I doubt you.

A change in Ia causes a voltage across the load resistor and that
voltage on the plate is the cause of 'inherent triode feedback'.

The 'large' load resistor is not, however, 'driving the plate'. The
voltage, and resulting feedback, is the result of Ia through the load.

Whether that 'works' for an unbypassed screen resistor would likely
depend on how well it tracks plate current and, as I said, I have no
data one way or the other but at least I don't pretend to know what
you "always wondered."

Patrick Turner.