Single-Ended Pentode Redux

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Nick
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#16 Re: Single-Ended Pentode Redux

Post by Nick »

Just to point out that that feedback will make the drivers job considerably harder than if it was a pentode (both in terms of the load its driving into and the voltage swing needed), so an unbiased cathode resistor which will only increase the drivers ra is maybe not ideal.
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andrew Ivimey
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#17 Re: Single-Ended Pentode Redux

Post by andrew Ivimey »

So diode, cap and resistor, just a resistor, battery... what is best? Looks like a cap in parallel with a resistor but then we argue over the type of resistor and forage for a blackgate.

All to be tried, no one has mentioned grid chokes 😉 there's only so many variables.
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
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#18 Re: Single-Ended Pentode Redux

Post by Nick »

I was the driver stage I meant as the feedback on the power valve will make it much harder to drive.
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#19 Re: Single-Ended Pentode Redux

Post by andrew Ivimey »

That's what I thought too. Grid choke on the output valve but what does it sound like!?
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
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#20 Re: Single-Ended Pentode Redux

Post by Nick »

It would break the feedback. Or at least increase the amount there is.
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Paul Barker
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#21 Re: Single-Ended Pentode Redux

Post by Paul Barker »

Nick wrote: Thu Jan 13, 2022 9:44 am 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?
I was watching a youtube of a guy building a kt88 amp, in which he had schade feedback and u.l. and a triode switch. I wanted to shout at the ipad and say to him why he hadnt tried triode mode without feedback. The feedback resistor was 220k though so not too much feedback.

On his distortion test with the schade and the u.l. I think, mostly below 0.4% and only 1% at frequecies below 30hz above 15khz going off memory.

But hes definately not in my mad professor try all options to evaluate every alternative by ear, those very things Andrew mentioned.

a whole new generation of valve amp builders are now doing the teaching on youtube and theyre on a different page to me at any rate.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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#22 Re: Single-Ended Pentode Redux

Post by Paul Barker »

so to recap the guy hasnt evaluated his diode bias on va stage, hasnt compared it to battery bias, to different diodes, to fixed bias to bypassed resistor bias or to unbypassed resistor bias. And thats just stage one. He hasnt evaluated pure triode no feedback. He hasnt tried no schade feedback at all. He made a comment he liked sound of u.l. with a glint in his eye the only emotional sign. Its all very unexperimental and emphasis on electronics without evaluating alternatives.

Noone seems to use choke input any more either, its not even in their knowledge bank!

Waley Waler, I feel like Chicken Licken, “the sky is falling down” I suppose in my retirement in couple years Ill have to repeat all above that I learned long ago on utube to counter all this electronics based junk.

It was primarily Jonothan Noble on a private email group run by Ned Carlson which also had Stephie and many other good skilled people that gave me my excited to try different things. Johnathon would ping me a private email “ You have to try choke input” same with every option. I have a knowledge bank from those days of things that will never let me down and that are always the better of two options. Im like a ratchet, I ratchet my practice to those items. There dont seem to be the people inspiring these practices forums are much more contentious full of adverse unfounded facts reachid without experimentation, and self claimed opinionated with no practical journey to form the opinions. And thats all before you start to experiment with tube rolling, capacitor rolling, resistor rollng transformer rolling topology rolling, direct coupled rolling capacitorless rolling et al. Dare I say, ac heating with cancellation rolling, various dc filament heting rolling! So much today is left out! Everything was on the table and there werent all the twatts having spats on forums. It was private email groups. Just wish Id kept the emails!
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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#23 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

These YouTubers are operating in the mainstream of tube amplification and just enjoying themselves, as are 99.99% of mainstream folk into tube amps.
For example, most 300B amps we see for sale, use the much maligned (in our circles) 6SN7 cascade as VA and driver. It gives that warmth, and romantic vibe people want. Hell, they even use 6SN7 cascades to drive Chinese 845s, for those who want an even more powerful, amber lit, loved up, romantic and expensive audio experience from their ‘real world’ speakers. 🥰

Then you get the 6SL7 into 6SN7 phase splitter driver, into push pull KT120 and KT150, for those who want bass. This is what we have these days. I would estimate that there are only a few dozen people on the fringes still at it, and that knowledgeable bank is dying in both a literal and metaphorical sense.

As for those of us who sit in what can be quite the uncomfortable place between the two extremes, dodging flak from above and below, we use what we have in our parts bins and shove our imperfect, often laughable circuits out on the interweb (feck knows why really) just because we enjoy a bit of tinkering. We make little or no progress, (frustrating I know) happy in the comfort zone, happy to just build and enjoy bathing in the sound of something we made ourselves….until we make something else. There’s no destination with this; just a hobby, just the journey. There is an ultimate destination for us all, but that’s got nothing whatsoever to do with tube amps, so I say let’s just enjoy what we do.

Don’t forget to check back in a few minutes Phil, I’ve already edited this post twenty times :wink:
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#24 Re: Single-Ended Pentode Redux

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I think Steve;s correct, he does it for fun, in the past we were (I think) trying to strive for a goal that was to a large extent illusion. There was a strong sense that there was real gold out there, it just needed us to discover in some old paper or circuit. But I came to realize that it was all manly a dopamine addiction. The fact that we can hear all these differences between bipassed/unbipassed cathodes, diodes, and power supplies and inductive and restive loads I came to understand was just a sign that they were all wrong, just wrong in different ways. The moment for me was the 300b amp that was so good at some things, but just plain sucked at others. If I wanted it all then I decided that valves (at least in the output stage) had too many problems. Ed was (IMHO) right all along, there was nothing wrong with solid state, it just had to be good solid state. In the same way that digital showed what was wrong with vinyl and allowed those faults to be fixed improving vinyl to where its at least as good as digital (just less practical), I think for me valves have shown whats wrong with solid state and (at least for me) I am at a point where I can get solid state to do all I want without loosing anything that wasn't illusion (and there is nothing wrong with illusion unless it had drawbacks, which much of it did). A preamp that does little or no damage, and sources that are good enough, which in my case with valves in to add a little bit of that distortion that still sounds nice. Loudspeakers are still the hard part hence my messing with Lowthers in OB, but in general I am happy with what it all sounds like.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
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#25 Re: Single-Ended Pentode Redux

Post by Ant »

Realisation for me came with the f5 amp i got off ali.
I had a couple of amps that i was playing with at the time, (kt77, el34, kt88 amp and an el84 simple push pull) which i liked most of the time but annoyed me at other times. I was constantly faffing with them chasing something.The f5 came along and didnt annoy me at all, it just played and sounded like the sound i had in my head that id been chasing.
Havent built an amp since.
I have however, as you lot know gone full on ocd with vinyl instead.

I got a wierd idea the other day for a fully skeletal lenco. An interesting thought experiment on how to make what i have in my head is going on in idle moments
Also starring Rex Hamilton as Abraham Lincoln

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#26 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

Converted to ultralinear 6V6 this afternoon and bypassed the cathode resistors on the input valve.
I made up a turret board with different values of Mills 12W wirewounds to use as cathode resistors. With this facility, bias values for KT77, EL34 and EL84/6V6 can be dialled in by putting different combinations in parallel, using simple wire links.
A small dropper resistor and a 5R4GY rectifier for maximum voltage drop, gave the right HT value. Perming a couple of the Mills wirewounds gave a spot on book value for the bias. 11.3W of anode dissipation later and I must say 6V6s are very good. Massive soundstage, which is interesting given the small size of the valves themselves.
I have a pair of NOS, French Mazda 6V6 from the 1950s in the amp at the moment and they are lovely.

I have 2 NOS black glass Brimar and 2 grey glass Sylvania, plus a pair of new production JJ 6V6S, so plenty to be going on with.

A nice, sweet and detailed 4 and a bit Watts or so, vs the 9..ish Watts of KT77. The 6V6 in ultralinear Schade config is quite close to a 45 soundwise.
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#27 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

Yes, I think the 6V6 SEUL with Schade feedback is certainly a definite candidate for putting in a proper enclosure. That's three amps then:

2W DHT SET
4.5W 6V6 SEUL
60W solid state with built-in phono stage.

Quite a productive and enjoyable few weeks. Lovely.
Valve buffer for the NVA thingy anyone? :)
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pre65
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#28 Re: Single-Ended Pentode Redux

Post by pre65 »

Cressy Snr wrote: Fri Jan 14, 2022 7:23 pm Yes, I think the 6V6 SEUL with Schade feedback is certainly a definite candidate for putting in a proper enclosure. That's three amps then. Lovely.
IF I had UL output transformers I'd be tempted. :wink:

But, I don't need any more amps, my one remaining ambition is to get the revamped 833a up and running. :)
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#29 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

Yikes! :shock: :D
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#30 Re: Single-Ended Pentode Redux

Post by Cressy Snr »

Paul Barker wrote: Fri Jan 14, 2022 7:40 am…..So much today is left out! Everything was on the table and there werent all the twatts having spats on forums. It was private email groups. Just wish I’d kept the emails!
This quote above from Paul is interesting.
I have, what I think is a superb ECC81 type valve in the form of a GEC A2900: I found it in a job lot of small pentode valves I got long ago from Stuart Dean, who used to post on here around 2010, but not seen anything from him since.
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Indeed, the valve is so good sounding that I’m scared to death I’m gonna use it up, wear it out. Consequently, I keep it strictly for best.
Langrex wanted £200 for one last time I looked and guitarists kill for them to use as reverb drivers.
I decided to stick in in the breadboard, driving 6V6 and sure enough, it makes a beautiful combination.

I don’t know about you, but I like to look for other people’s experience of these things and I found a guy on a blog (Audio Cognoscenti) using A2900s in a VTL amplifier. He wrote this:

In conclusion, the Gec A2900s are simply incredible. In my opinion, the Gec A2900s are unequivocally the best 12AT7s in the world. They take total control of your Power tubes ( I'm using Ge 6550a tubes ) which provides you greater headroom and output performance which is why I highly recommend the A2900s in the signal tube driver position of your amp. They demonstrate an unbelievably musical, balanced, dead quiet, smooth, and detailed presentation. They take the highly accurate signal from my E81CC's and impart a musical warmth that is undeniably "MAGICAL"

OK so it is a bit OTT for my tastes in writing, but look below the obvious enthusiasm, and in general he’s right on the money. His experience in his amp matches what I hear in my completely different amp pretty well. IME of them, for what they are, A2900s are bloody good, superb even.

So the very first comment on his blog after his post, goes like this:
You are a nutcase.
I would test your hearing, then double blind test you, to prove that everything you write is not only a heap of total bollocks but you can't identify any of the things you claim to hear.


Nuff said Paul.
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