From: info@turneraudio.com.au   
      
   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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   > |Filename: Incremental Screen Resistance Plot w Captions 5W.jpg     |   
   > |Download: http://www.audiobanter.com/attachment.php?attachmentid=424|   
   > |Filename: Screen Resistance rs Determination 5W.jpg                |   
   > |Download: http://www.audiobanter.com/attachment.php?attachmentid=425|   
   > |Filename: G2 Resistance Test Setup 5W  E.jpg                       |   
   > |Download: http://www.audiobanter.com/attachment.php?attachmentid=426|   
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   > --    
   > John L Stewart   
      
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