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Patrick Turner Patrick Turner is offline
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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.