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On Wednesday, February 17, 2016 at 7:31:55 PM UTC-5, Patrick Turner wrote:
On Monday, 1 February 2016 05:37:06 UTC+11, John L Stewart wrote:
Measurement of Screen Resistance Example John L Stewart Jan 2016

The large amount of John's text re screen resistance is below my reply :-

A tube amp might have a 6U8 triode-pentode used for a driver tube but I don't recall too many, although the RCA inspired Dynaco type circuit comes to mind where pentode is input tube and triode is concertina phase inverter.

I have not found it important to know what the dynamic screen resistance actually is, and which could be calculated as signal Vg2 / Ig2 ac after measuring current and Vac across a screen feed resistor of low value. The Rs is different from the DC input resistance which is just Eg2 / Ig2 dc.

If the B+ rail = +300V, probable Ea would be say +150V, and Eg2 can be between +120V to +150V with little change of gain performance. Cathode Rk will determine the idle Iadc, and the gm, and gain, and Rg2 supply resistance may be 3 times the RLdc for anode, but Rg2 needs to be adjusted by trial and error to get Ea 150V, Eg2 120V and with whatever Rk is needed to get wanted Ia dc which is best measured by Vdc across anode RLdc / RLa dc. When these interactive voltages are correct, then the pentode is set up right. Usually, Rk will be two series R, say 220r + 47r, with 220r bypassed with say 470uF, and NFB brought to unbypassed 47r. So how to bypass screen? Most circuits show screen bypassed to cathode, and for LF stability with NFB the the bypass C value should be larger than the accountant allowed low value of say 0.1uF; better to use 1uF, and it won't be the screen circuit phase shift causing LF oscillations if there are any, it'll be the high open loop gain extending to too low a LF, and having insufficient primary inductance on OPT. This problem has always been common in many tube amps which prompted incompetent manufacturers to stipulate that their amp never ever be turned on without a speaker connected - they'd oscillate badly without load, because open loop gain goes high in output stages with no load. But even Williamson's amp was dodgy in this regard.
Just read my schematics at my site with critical damping networks for stability.

But some makers bypassed input pentode g2 to 0V, and when cathode FB is used, there's largish Vac between cathode and g2, so the effective gain of pentode changes and the WHOLE operation of the schematic in terms of OLG and amount of applied NFB changes and one needs to fully analyse a specific schematic before relying fully on any simplistic discussions here. There are just too many variables involved. But I do recommend that ppl interested go to their workshop, solder up a circuit, document it fully, post all results with schematic at a website, and then everyone will know what are the facts, well, pending presentation without errors.
Patrick Turner.


When designing or building an ordinary vacuum tube audio amplifier with
a pentode front end we can for the most part ignore the screen supply
resistance. Simply set it to be in the range of 3-4 times the plate
resister & the circuit will perform satisfactorily. As long as the
screen resister is adequately bypassed there are no significant
problems.

That all changes when the intention is to use NFB. Most folks would
assume that the RC time constant is simply the screen resister times the
screen bypass capacitor. That can lead to problems since the screen
supply resister is actually in parallel with the resistance of the
screen grid itself as seen looking into the tube. Some calculations
using the available published tube data indicates the screen resistance
of common audio voltage amplifier pentodes to be in the range of 40K.

This simple setup makes measurements leading to the incremental screen
resistance of the pentode section of a 6U8 vacuum tube while in
operation. The tube is connected to a plate supply of 300 volts. The
screen is fed from this supply thru a total of 730K resistance. But
interposed on that is a means of applying an interfering One KHz test
signal. Any audio transformer of high impedance primary & secondary can
be used to couple the audio signal generator to the high voltage on the
screen. I used an old Hammond 447 Interstage Transformer.

Just two measurements are required. Using a differential probe the AC
voltage drop across the 730K is measured, then the AC voltage from
common to screen.

The results are as follows-

First Pass- Drop across 730K was 0.9V

So Ig2 is 0.9 V / 0.73 M, 1.23 microA And Eg2 measured 0.043V

So rs is delta E / delta I rs = (0.043V / 1.23 microA)K or 35.0K

2nd Pass- Drop across 730K was 2.83V

So Ig2 is 2.83 V / 0.73 M, 3.88 microA And Eg2 measured 0.133V

So rs is delta E / delta I rs = (0.133V / 12.83 microA)K or 35.0K


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--
John L Stewart

Thus spake the Oracle!!

I would have thought Phil A would have made some comment by now. Hey Phil, lets hear your opinion!

Cheers to all, John L Stewart