Single-Ended Pentode Redux

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Cressy Snr
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#1 Single-Ended Pentode Redux

Post by Cressy Snr »

Been messing around with modern production KT77s and EL34s again and having a good time. It's a while since I played around with this kind of configuration.
I've got the circuit below running at the moment:
Screenshot 2022-01-11 at 16.27.37.png
Screenshot 2022-01-11 at 16.27.37.png (56.3 KiB) Viewed 5728 times
The circuit is SE ultra linear with plate to grid feedback around the output valve, via the 51K/510K potential divider and the 2u2 DC blocking cap. It works well.
The idea is to see if I can make a nice amp using modern production, easily obtainable valves.
Apart from a couple of sets of 45, four 13E1 and a pair of mesh MH4, I have nothing else worth a hoot, in terms of the old valves.
This circuit uses Russian New Sensor Genalex reissue KT77s and an Electro-Harmonix 12AT7EH (long plate version). There are also Chinese EL34B and New Sensor, Tung-Sol reissue EL34B to play about with if the fancy takes me, but at the moment the KT77s are playing quite nicely.

I've always liked the sound of single ended pentodes/tetrodes in partial feedback configuration. Playing with them a few years ago was a lot of fun.
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Cressy Snr
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#2 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

I can't use it in the front room at the mo' as it's a dangerous lash-up, but it has the potential to become a third working amp.
Transistor, SET and SEP/SEUL. Three different flavours of the aural matrix.
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Paul Barker
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#3 Re: Single-Ended Pentode Redux

Post by Paul Barker »

Oh Waily waily
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Paul Barker
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#4 Re: Single-Ended Pentode Redux

Post by Paul Barker »

I’m with youre sentiment to build a second valve amp cheap enough to play as long as you like.

But you only need 2 watts. I’d go EL84 Russian version triode set. Knocks those higher powered valves out the park at 1 watt.

Also I’m building the Bates 6v6 cf output amp from 1951. See how that turns out with the cheap Russian 6v6’s.

I have no idea yet but I’m hoping the Bates does it for me as looking at 4 801a’s lit up all the time is like watching the Jag V12 fuel gauge! I keep thinking_ “oops, dropped a clanger here, only 801a option is nos nos equipment pulls and nos! What a Wally”!
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#5 Re: Single-Ended Pentode Redux

Post by Paul Barker »

Otherwise, anyone know a Russian equivalent for 801a’s ?
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#6 Re: Single-Ended Pentode Redux

Post by steve s »

Old a/c pen's I'm told are a nice valve too, I have a small box full awaiting the inclination to do something with them ?
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#7 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

Paul Barker wrote: Tue Jan 11, 2022 6:18 pm I’m with youre sentiment to build a second valve amp cheap enough to play as long as you like.

But you only need 2 watts. I’d go EL84 Russian version triode set. Knocks those higher powered valves out the park at 1 watt.

Also I’m building the Bates 6v6 cf output amp from 1951. See how that turns out with the cheap Russian 6v6’s.

I have no idea yet but I’m hoping the Bates does it for me as looking at 4 801a’s lit up all the time is like watching the Jag V12 fuel gauge! I keep thinking_ “oops, dropped a clanger here, only 801a option is nos nos equipment pulls and nos! What a Wally”!
I might have a couple of those Russian EL84s somewhere.
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#8 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

Little bit more progress today:
KT77SEULPFb2022-01-12 at 16.23.58.png
KT77SEULPFb2022-01-12 at 16.23.58.png (55.11 KiB) Viewed 5553 times
Got shot of the LED bias on the input stage after finding some 390R half-watt resistors in a bag and left the stage un-bypassed, as is the more usual practice with this sort of 2 stage SEP setup. Reduced the plate-to-grid feedback a smidge, which brought the gain up a bit.
It's excellent, with no aural nasties that might indicate the presence of high order odd harmonics.
A smooth, detailed presentation, completely silent background and nice 'n easy, as IME good amps of this type seem to be. I've proved to myself that making a decent sounding and cheap valve amp to use as a daily driver is perfectly possible with a bit of thought and a following wind. Single-ended ultra-linear with additional local feedback seems to work quite nicely in this context.
Might do some additional experimentation with 6V6s as output valves next week. Circuit will need altering and the mains transformer will need to have a lower secondary voltage. I have a 1:1 mains isolation transformer that would be just the job for either 6V6 (or EL84 if I can find them)
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Nick
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#9 Re: Single-Ended Pentode Redux

Post by Nick »

I just realised. The point of a pentode is the extra grid shields the g1 from the anode. Adding that feedback is just turning it back into a triode.
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#10 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

Yes, it is doing that and more.

I’ve no idea about the theory, (OH Schade in the late 30s) but apparently with this you get highly linear triode performance with the power of a pentode. https://www.dos4ever.com/uTracer3/Schade.pdf (p362, 363. figs 34 and 35.) I’m aware that I’ve added the ultra-linear connection to the output transformer into the mix, so who knows what is going on with that lot, as I have no means of tracing or measuring, but I really like how the combination sounds. The Schade paper did give me a start when I first got into all this SEP malarkey
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pre65
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#11 Re: Single-Ended Pentode Redux

Post by pre65 »

Steve, you changed the last post a few times. I have to go back and check your posts several times now. :wink:
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#12 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

You should see all the drafts of my collection of short stories. Not many of them bear any resemblance to how they started. I could have written two novels from the stuff I discarded along the way. Things evolve. This one has already been edited three times and it’s only a few minutes old.
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Nick
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#13 Re: Single-Ended Pentode Redux

Post by Nick »

Yep, the plate feedback to g1 and UL to g2 are in effect doing the same thing. I wonder if the 1938 Schade paper was a response to the Alan Blumlein introduction of distributed loading in 1937 and US patent in 38, Maybe that's why there is no coverage of g2 operation in the paper?
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#14 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

Nick wrote: Thu Jan 13, 2022 9:44 am ….I wonder if the 1938 Schade paper was a response to the Alan Blumlein introduction of distributed loading in 1937 and US patent in 38, Maybe that's why there is no coverage of g2 operation in the paper?
Could be. I’d not considered that before.
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andrew Ivimey
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#15 Re: Single-Ended Pentode Redux

Post by andrew Ivimey »

Ha ha ha, talk about playing with a loose tooth. How can something so simple, your implementation of ecc 81 - not even paralleling both sides - actually, why not!!! work so well. I too am fond of el84 though and they are so easy to drive but you are making the bigger brothers to be easy too!!

6v6 are the electrical equivalent of el84 but that doesn't mean they sound the same. So many guitarists are reduced to tears about the virtues of 6V6 but not el84 ... ( Vox AC30 for example at the moment - fashion changes) . I only have a few Brimar 6V6s but would like to hear how the cheap Russian ones sound. I also have some 6W6, ancient US NOS.

erm... what's wrong with using 6Sn7GTx ( fess up, I have two ecc81 and loads of 6sn7s)????

looks like I've got to get cutting, drilling and, regretfully bleeding, pain and frustration again.... Onwards!
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