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Pentode Screen Resistance (rs) Estimation Example



 
 
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  #11  
Old February 18th 16, 12:31 AM posted to rec.audio.tubes
Patrick Turner
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Posts: 3,964
Default Pentode Screen Resistance (rs) Estimation Example



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
>
>
> +-------------------------------------------------------------------+
> |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|
> +-------------------------------------------------------------------+
>
>
>
> --
> John L Stewart


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  #12  
Old February 19th 16, 10:50 PM posted to rec.audio.tubes
[email protected]
external usenet poster
 
Posts: 13
Default Pentode Screen Resistance (rs) Estimation Example

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
> >
> >
> > +-------------------------------------------------------------------+
> > |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|
> > +-------------------------------------------------------------------+
> >
> >
> >
> > --
> > 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
  #13  
Old February 24th 16, 03:43 AM posted to rec.audio.tubes
Big Bad Bob
external usenet poster
 
Posts: 366
Default Pentode Screen Resistance (rs) Estimation Example

On 02/17/16 16:31, Patrick Turner so wittily quipped:
> 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.


ack, there are plenty of curves and equations described online in
various sources describing the behavior of pentode amplifiers, and they
can have VERY high gain if you set them up right, but maybe you're
simply explaining why I prefer to use triodes.

12AX7 good enough for Leo Fender, should be ok for me, heh.

A bipolar transistor single-stage amplifier can have a gain of 1000. I
wouldn't want to hear the audio quality of that, not at all. use an
emitter resistor (like unbypassed K resistor on pentode) for NFB and
keep the actual stage gain a bit lower, and all that gain might keep the
signal:noise and distortion down.

hard to say which is better. build and test, measure with good
equipment, use low noise metal film resistors and high quality
capacitors, shield the tubes, and additional low capacitance filtering
in the power supply in parallel with electrolytics...


  #14  
Old February 26th 16, 01:21 AM
John L Stewart John L Stewart is offline
Senior Member
 
First recorded activity by AudioBanter: Jan 2011
Location: Toronto
Posts: 301
Smile

Patrick is correct, we don’t see many audio amplifiers making use of the 6U8. However, I did not have to look far to find one. It is one of Patrick’s very own early efforts. It has two 6U8s. The schematic is attached so that everyone may marvel at it.

NTL an interesting topology as anyone who has tried it will agree. Patrick manages to get about 95% of the signal at the plates of the output tubes all the way back to their grids in the form of NFB. If one looks carefully we can see an anode follower.

Without the MU followers as plate loads one is lucky to get ˝ the signal all the way round the loop. In Patrick’s amp the result is the output tubes look very much like very stiff triodes driving the OPT. That will help the LF response very much. But not so much the HF since the signal still has to get past the OPT leakage inductance & winding capacities.

Each of the mu followers present what looks like a constant current load (Hi R) to the pentodes. Under ordinary circumstances that often creates a problem wherein the operating point (Q) is said to be ‘undefined’. Very small changes in either the pentode or mu follower result in large shifting of the Q point. The circuit could be unstable. In Patrick’s circuit the plate 470K NFB resister will keep things under control. Refer to the attached.

The circuit utilizes many parts to accomplish this. Some would say too many. Unfortunately the Law of Diminishing Returns sets in rather quickly. A simpler circuit using one less tube & fewer parts can accomplish the same thing.

I used the 6U8 for the screen resistance tests simply because I had one. But any ordinary small signal pentode would have done just as well to illustrate how the measurement can be made. The data is useful when designing an amplifier that might use the screen of an amplifier input tube to establish an LF step in the frequency response.

And I used a soldering iron as Patrick often suggests. But I also frequently use modern tools such as simulation software to get some idea if a particular circuit is worth pursuing. Before I plug in the iron! And it is better not to build the entire amp to measure only a part of a circuit. Best to look at each section in isolation to better understand what its function will be in the whole.

And I don’t use my slide rule a lot anymore either. The HP calculator running RPN is a hell of a lot faster. For anyone interested in RPN there is a less than one MB download at this link-

https://sourceforge.net/projects/fwcalc/

RPN does the calcs from the inside out, just as we do with pencil & paper. It stores the intermediate results so you won’t need the pencil until the final result. No Drucker!!

Cheers to all, John L Stewart
Attached Images
  
  #15  
Old February 26th 16, 07:59 PM posted to rec.audio.tubes
Big Bad Bob
external usenet poster
 
Posts: 366
Default Pentode Screen Resistance (rs) Estimation Example

On 02/25/16 17:21, John L Stewart so wittily quipped:
> NTL an interesting topology as anyone who has tried it will agree.
> Patrick manages to get about 95% of the signal at the plates of the
> output tubes all the way back to their grids in the form of NFB. If one
> looks carefully we can see an anode follower.
>
> Without the MU followers as plate loads one is lucky to get ˝ the signal
> all the way round the loop. In Patrick’s amp the result is the output
> tubes look very much like very stiff triodes driving the OPT. That will
> help the LF response very much. But not so much the HF since the signal
> still has to get past the OPT leakage inductance & winding capacities.


biggest problem with high NFB from downstream of the output transformer
is the phase shifting at the edges of frequency response. Careful
choice of component values (and possible filtering/peaking reactive
components) might be needed to avoid the inevitable oscillation that
would take place without it.

but yeah, if you can get away with it, it would be like a typical op amp
with very high open loop gain, and relatively low 'stage gain'. The end
result is very low distortion.


  #16  
Old March 4th 16, 01:28 AM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,964
Default Pentode Screen Resistance (rs) Estimation Example

I reply to John L Stewart...

Patrick is correct, we don't see many audio amplifiers making use of the
6U8. However, I did not have to look far to find one. It is one of
Patrick's very own early efforts. It has two 6U8s. The schematic is
attached so that everyone may marvel at it.

I could not see the attachment address, so here is is, see the schematic at top of page....
http://www.turneraudio.com.au/miscel...chematics2.htm

I also found many 6U8 around, they have useful sharp cut-off pentode, a bit 6AU6, and triode with lowish Ra medium µ. Probably best might be 2 x 6BX6 with above triode "CCS" being 1/2 12AT7.


NTL an interesting topology as anyone who has tried it will agree.
Patrick manages to get about 95% of the signal at the plates of the
output tubes all the way back to their grids in the form of NFB. If one
looks carefully we can see an anode follower.

**I don't get 95% local FB to output grids. The circuit in above link has rather low ß, ie, the fraction of N&D at anode appearing at grids is probably not above 0.5; its years since I build the test circuit in about 1997, in response to what Allan Wright was doing with a secret module for power amps ( Forced Symmetry ) and involving shunt FB with a secret j-fet. He'd pinched the idea from RDH4.

**But where output pentodes / beam tetrodes are used, they will have enough open loop gain to render a ß = 0.5 as being quite enough to do more than 15% CFB or any UL connection and with more linearazing effect because of 2 reasons I can think of.


Without the MU followers as plate loads one is lucky to get 1/2 the signal
all the way round the loop. In Patrick's amp the result is the output
tubes look very much like very stiff triodes driving the OPT. That will
help the LF response very much. But not so much the HF since the signal
still has to get past the OPT leakage inductance & winding capacities.

** I've used my balanced shunt FB idea ( stolen from RDH4 ) in this SS amp.....
http://www.turneraudio.com.au/solids...ono-mosfet.htm

Each of the mu followers present what looks like a constant current load
(Hi R) to the pentodes. Under ordinary circumstances that often creates
a problem wherein the operating point (Q) is said to be 'undefined'.
Very small changes in either the pentode or mu follower result in large
shifting of the Q point. The circuit could be unstable. In Patrick's
circuit the plate 470K NFB resister will keep things under control.
Refer to the attached.

** It all worked OK with a very poor quality OPT.

The circuit utilizes many parts to accomplish this. Some would say too
many. Unfortunately the Law of Diminishing Returns sets in rather
quickly. A simpler circuit using one less tube & fewer parts can
accomplish the same thing.

** Maybe, depends, I got 4 tubes in input amps; I think its better than the RCA inspired circuit with just one triode-pentode. Williamson with 4 x 1/2 6SN7 was a kind of gold standard and much better than RCA, Dynaco et all suggest, I like 2 cascaded LTP, as in http://www.turneraudio.com.au/RCA-reformed-30W.html
and then this one, with radical re-engineering, lotsa input tubes, but a most magnificent amp, which benefitted with its Chinese OPTs which were close to bein accidently good....http://www.turneraudio.com.au/Ming-D...-reformed.html


I used the 6U8 for the screen resistance tests simply because I had one.
But any ordinary small signal pentode would have done just as well to
illustrate how the measurement can be made. The data is useful when
designing an amplifier that might use the screen of an amplifier input
tube to establish an LF step in the frequency response.

** Possibly g2 of input pentode can be used for OLG shaping for stability, so at mid F the tube has high gain while at VLF and VHF (audio) the pentode operates as a lower gain triode.
A number of commercially made amps in 1950s used g2 on say 6SJ7 as a NFB port for GNFB. If the screen signal was equal to the anode signal driving the next stage, then the pentode is working as a triode; and because the anode signal at V1 is low and a small % of possible dynamic range then Vg2 FB signal could be easily twice Va, putting 6SJ7 into realm of supercharged triode.

And I used a soldering iron as Patrick often suggests.

**Currently I use 80W rated Chinese made, driven with switched windings from mains PT in box because with full 240Vac it runs way too hot for Pb-Sn solder, it is hotter for the lead free solder. With about 180Vac it runs just right and at about 40W for PbSn without oxide forming quickly. I can switch up heat for chassis and big leads. The iron clad tips are 8mm dia, and didn't tin easily at first, but did after some use. So these irons now last many years and are now better than old crap from old days....

I also frequently use modern tools such as simulation software to get some idea
if a particular circuit is worth pursuing. Before I plug in the iron!
And it is better not to build the entire amp to measure only a part of a
circuit. Best to look at each section in isolation to better understand
what its function will be in the whole.

** I never ever found time to learn to drive anywhere in a simulated vehicle; it IS GOOD, but my brain still works OK, rather like the guy who can think up a good chess move without needing to spend all day and night using 30 online programs to find a better move. But I was a hopeless chess player, all the club guys could easily beat me, but that's clubs for ya, fulla ppl wanting to beat you up on a table.
They were hopeless with electronics and use of any tools, and often extremely obsessive-compulsive and some were quite insane.
Horses for courses, eh?

Keep well, and sure that's a hard one, but do try to not go insane,
Patrick Turner.
 




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