New SE With Triode Mode KT120/150

[IslandPink]

Top notch !

[steve s]

Heh heh!
Larks
Schematics? I don’t know why I bother!
Hytron 807s from 1966, in Menno Van Der Veen, ‘super triode’ mode (UL with cathode feedback for us plebs)

I do know about the 300V screen voltage limit with 807s, but I chose to ignore it. I’m over by 60V. No ill effects or glowing screen wires

Nothing stays the same for long in your house steve..

[Cressy Snr]

Nothing stays the same for long in your house steve..

It’s this damned lockdown. I have to keep doing stuff, otherwise I would have thrown the telly out of the window, every time I saw Johnson, Hancock or any of the other clowns, spouting their non-stop bullshit! Anyway....wrong thread.

As this amp is set up for the 6L6 variants, with different degrees of plate dissipation available, depending on which rectifier you choose, I thought I’d have a play with 807s and a high-drop 5R4GY rect. I have some used 807s that Paul B gave me years ago when I was messing with single-ended pentodes. I’m using NOS Hytron at the moment, which I bought for a tenner each off Langrex, sometime around 2010. The only action they saw was as series pass valves in my 6B4G monoblock power supplies.
I tried the used 807s Paul gave me, in tetrode mode, with regulated screens and Schade type feedback, but never got on with them. No matter what I did, there was a persistent glassiness and a nasty edge about them, like a poor solid state amp, so they were given pretty short shrift and I reverted to modern production EL34s and KT *** types. It was a pity, as 807s just look so cool when lit up. I later found out that in the wrong circumstances, and with very little provocation, 807s like nothing better than to release a lovely spray of high, odd-order harmonics, for the listener’s delight and delectation, and that these can be a complete b’stard to deal with. That probably accounted for the listening fatigue they induced when I tried them.
In the ‘super triode’ mode however, they seem to be something of a different proposition. Set up like this, they give a liquid and detailed presentation, with good bass and, importantly, unlike in tetrode mode, they don’t induce a headache. They sound like a triode strapped PT15, but seem to be more tolerant of cheap iron, so they are better in the bass than PT15s if cheaper transformers are used. And I like the top caps. I used petrol lawnmower copper spark plug leads for the plate connections.

There’s also the Cossor ATS-25 / VT60A version of the 807 to be had, if I want an all-British amplification chain.

[Cressy Snr]

Here are the schematics for the 807 version of this amplifier:

Compared to the KT120 version, the schematics show the range of voltages available from the output stage supply, as the rectifier is changed.
Going from a GZ34 on the one hand, to a 5R4GY type, there is a 42V difference we can use. The 5U4G or GZ37 will get you into the middle of this range, so all the 6L6 derivatives from 6L6G to KT120 can be accommodated. It'll also accommodate an EL34, dissipating 21W, with a GZ37 or 5U4G as the rectifier, so all in all, this design is quite flexible in scope. Whether it gets the best out of that large a range of valves or just encourages across-the-board mediocrity is debatable, but never mind.
The circuit certainly encourages tube rolling, which can be either a good or bad thing, depending on your attitude. It was a bad thing for me as I simply couldn't settle on one type and be done with it. Using the 807, with its UX5 socket and top-cap, cuts me off from the other valves I have, and stops the aimless wandering about and plugging and unplugging of different rectifiers and power valves. It's 807s and 5R4GYs or nothing now and I'm happier, now that the compulsion to tube roll has been, as it were, surgically removed.
Somebody on the DIYAudio forum reckons that tube rolling is a mental illness. Indeed!

[Dave the bass]

A question from the hairy kid with the funny accent at the back of the class....


Sir, sir! What's the role of the 10nF cap from the junction of the 2 glow tube regulators to Grnd?

Noise shunt?

[Cressy Snr]

...Noise shunt?

Yes.

[IslandPink]

Funny you should say that !
I've generally put a resistor and cap across a pair of VR tubes - as advised years ago by Gary Pimm. However I never spent a lot of time measuring anything to establish if I'd got the values spot on, just took the suggested values I'd seen on circuits. The idea is you can null the effect of VR tube inductance this way. I also think it reduces the white noise they put out. It certainyl sounded cleaner when I did this on my phono amp.
Maybe someone else has put in more groundwork though.

Oy Cresswell ( no ) ! - you just took away the comment I was replying to !

[Cressy Snr]

Sorry Mark.
Here it is.

I expect somebody will tell me I should have connected the cap across the whole of the stack

[IslandPink]

He he

[Cressy Snr]

Over the past couple of days, I’ve done a double transformer transplant.

The rear toroid is an ‘Antrim Transformers’ 280V - 600mA job, I bought from Andrew L, years ago now. It also has a pair of 6V 4A heater windings. This transformer powers the output stage, giving 330V at the plates of the 807s.
By the time the cathode bias is taken into account, there is 309V anode to cathode and 308V screen to cathode, which is near as dammit to the 300V screen voltage limit of the 807. This makes me happier, as Robert Pirsig said, the machine now gives ‘peace of mind’

The front toroid, is a ‘Scandinavian Transformers’ 230V/100VA, isolation transformer, which gives 300V for the shunt reg tubes work at.

I was initially a bit reluctant to change out the EI mains transformers, but I wasn’t really happy with either of them. once I moved over to 807s. Nothing physically wrong with the transformers, but they were simply too high in voltage. Fine for KT120s but way too hairy for 807s. The smaller one powering the input stage was also a bit ‘edge of seat’ when the MH4s only need 200V. I was having to drop 200V across a resistor, before the juice even got to the glow tube stack. Wasteful of power.

OK so the look of the project has been somewhat diminished, but proper functioning and everything operating within limits has to take precedence over aesthetics. Just need some covers for the toroids.

[IslandPink]

Sorted ! - and remarkably shoddy into the bargain if I may say so.

[Cressy Snr]

Sorted ! - and remarkably shoddy into the bargain if I may say so.

Cheers Mark,
As is usual, this project has evolved into something completely different to what I started out with. But this time it has also evolved beyond a thrashing about aimlessly and ‘change and hope’ kind of exercise.
As Nick said in your thread, I was finding myself hidebound by dogma and needed to break free of it. I’d built a dogma amplifier....again, and those globe MH4s deserve to be driving something better! God knows how many times I’ve fecked about with EL34s and KT*** stuff and they still don’t give me what I want, in a consistent and reliable manner across a wide enough range of music. Maybe my expectations are too high for my budget. In fact I may as well come out and say that yes they are. There I’ve said it!

On a more positive note, the old 807s in ultralinear, with cathode feedback seem to be giving me what I want, in terms of tone, detail, speed and sweetness. The fact that they also look dead cool, glowing behind the globe MH4s and are highly synergistic with them is an added bonus.

By changing to the pair of toroids, I’d lost the 5V windings, so I pressed a pair of 5V, Hammond 266M5 transformers into service for heating the two valve rectifiers. These are hipot tested to 2000V, so having 300-odd volts of pulsed DC sat on the secondaries was no sweat. With the toroids in service and the rectifiers on their own transformers, the background noise levels have dropped to virtual silence, as has the mechanical noise. Barring a lottery win, I’ve probably gone as far as I can and am going to have to be satisfied.

[steve s]

There’s also the Cossor ATS-25 / VT60A version of the 807 to be had, if I want an all-British amplification chain.

I was once told the cosser 185bt made a very nice audio amplifier steve..

[Cressy Snr]

There’s also the Cossor ATS-25 / VT60A version of the 807 to be had, if I want an all-British amplification chain.

I was once told the cosser 185bt made a very nice audio amplifier steve..

Yes, I think you mentioned that one at one of your meets at your place. It’s a nice looking valve too. I would be wary of the screen rating for ultralinear use, but that is a characteristic of all the line output pentodes I’ve looked at and can be overcome, or ignored according to the guy on DIYAudio ‘Tubelab’ who has, over the years, thrashed a huge range of line output pentodes (sweep tubes) to within an inch of their lives, and built an 833A guitar amp.
He’s also blown quite a few TV pentodes up too, so you have to read carefully through his stuff to find out which ones will stand up to the abuse of ultralinear or triode operation. PL519s and 509s are in his and Bruce Rozenblit’s “tough as old boots” category, but I’ve been there, done that with them and not been that impressed. They’re great in OTLs with a ton of feedback. The late TdP made a good go of them in one of his EAR push pull amps, so good sound can be got out of them. But I was underwhelmed with them SE triode operation and no feedback, where they got a bit relentless for my liking; probably high order distortion, but I’d no idea about that sort of thing at the time.

I’m surprised at how sweet the 807s sound to be honest. Still cheap as chips too. They tend to get bad press on places like DIYAudio because of high odd order harmonic distortion and crap screen ratings for ultralinear operation. I can’t hear any nasties from them with UL and cathode feedback: quite the opposite actually. These valves are giving me the thing I want out of a hi-fi system, good across all my digital music sources, and excellent with vinyl. The British options around the 807 types or similar look like a good place for future exploration, and for not much money.

Might make my ramblings less “Oh FFS not again” too.

[steve s]

Early Vt 75 is the pick of uk tv valves
I think prices are creeping up for them though steve
All the best..