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Shrad

Sun Apr 26 2015, 09:24AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

haha given the pics mine are reserigraphied as they are definitely smaller than a 6SN7 kind of tube (or my memory is not what it was)

mine are approx. 35mm diameter for 60mm height with the smaller endcap so I guess I've been ripped off ;)

thanks for the resource though, I may inspire from your work!

Ash Small

Sun Apr 26 2015, 04:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

He says look for PL36'S, they are dirt cheap and low impedance, but filament voltage is 25V.
I'll post a photo of his 2.1 single ended when I get back.

EDIT: As promised, Here's a couple of photo's of his 2.1 single ende PL36 amp:

He also said you could probably get away with using a 24V step down transformer, as they only have 300mA filament current, so a low loaded 24V transformer may well put out ~25V anyway.

They are originally used as line output valves in the late '50's and early sixties.

Typical prices on Ebay are ~Â£20 for six. Obviously, boxes of 'new old stock' are the deals to look out for.

Shrad

Mon Apr 27 2015, 12:55PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

yeah that's the kind of thing I'm looking for actually

I also looked at the GU-50 or the same kind of bigger russian tubes...

I'll have to find output transformers though... I'll have a look at some 440Hz control transformers I have somewhere as they should have enough bandwidth for audio, but I don't even know what they are worth to the ear

that's a vast field of experimentation

Mads Barnkob

Mon Apr 27 2015, 01:16PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Shrad wrote ...

yeah that's the kind of thing I'm looking for actually

I also looked at the GU-50 or the same kind of bigger russian tubes...

I'll have to find output transformers though... I'll have a look at some 440Hz control transformers I have somewhere as they should have enough bandwidth for audio, but I don't even know what they are worth to the ear

that's a vast field of experimentation

I assume you are located in Belgium as you wrote earlier. You should really take a look at Dagnall electronics, located in UK, manufacturing on Malta. Good quality to a competitive price, I think it was around 80 Euro for a custom 250VA power transformer, with shipping.

Ash Small

Mon Apr 27 2015, 05:37PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I think he also mentioned something about if driving them as triodes they have a lower imedance, or something...I think it was these he was talking about. I'm sure he said his 2.1 PL36 amp had an impedance of ~100 Ohms, at least that's what I think he said.

It was well worth the trip, as he gave me a couple of 10 Henry chokes he'd found during a recent clearout. I may now have enough of them to series together for the Parafeed on my single ended project

EDIT: I'll be trying a mains transformer on mine, as the OT.

As far as I can work out, I'm going to need a high impedance one. I've not really spent anything on parts, just what I can salvage or picked up dirt cheap at hamfests.

I've not really got into the discussion between high or low impedance, except that you generally need an OT to match the speakers to the valve.

I figure I can 'fine tune' it using a parallel/series array of speakers, and that if I use assorted ones, I'll maybe get a 'rounder' sound.

The transformer I'm intending to try first does resemble a large 'C' core OT. I've seen a Partridge that 'looks' identical, and seen others online. It's a 6.3V 27A mains step down transformer, so has a turns ratio of ~40:1, but I doubt the laminates are the same quality, or that the coupling is the same quality as an OT.

Lots of people on the internet say they've tried mains transformers as OT's and they sound awful, but if you use a mains transfoumer, or even a push-pull OT on a single ended amp ypu must use Parafeed.

For a push-pull there's no need.

Shrad

Tue Apr 28 2015, 07:18AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

then I'm at the same level as you are...

I have read and understood that the most important parameter is the reflected impedance and that you can achieve the same result with a different setup regarding impedance (a bit like communicating vessels) and you can also play on power stage impedance with parallel tubes or pentode driven in triode mode etc...

I'm really interested in the 6080 and other regulation triodes though, as pure triodes usually sound nicely and you don't need much to get a low enough impedance

what I also think is that small toroid power transformers could be a potential good candidate for OT if they are of quality and with multiple symmetric taps (I have some with 32-0-32 + two times 10-0-10 and 110-0-110 primary)

Ash Small

Tue Apr 28 2015, 02:32PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm of the opinion that you wan't as much iron and copper as possible. You also want thin laminates and good coupling/minimum leakage inductance, etc

You do generally get very good coupling with toroidal transformers, though.

Genarally, bigger is better, though, as the core doesn't saturate so much, at least, that's how I understand it. Keep the volts per turn down, then you get better bass response.

I've yet to start experimenting though, but I may now have enough chokes for the parafeed.

I do have some smaller OT's, so it will be interesting to see which work best.

EDIT: My main interest is in single ended amps. They do require more iron in the output stage, either a large, gapped OT, or Parafeed choke plus non'gapped OT.

In my opinion, the whole single ended V push/pull thing has more to do with the gapped OT/parafeed thing than the fact that you get more power from two tubes.

It's just that with push/pull, you don't need as much iron in the output stage. Therefore, it's cheaper per watt of music power.

If you were to parallel two single ended amps you'd get just as much power as push/pull but you'd need significantly more iron in the output stage.

This is just my opinion, though.

I think it's generally agreed that you do get some unavoidable distortion with push/pull, but I can't remember the exact details off the top of my head.

Shrad

Tue Apr 28 2015, 07:44PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

that's the reason pure triode amps with GU-81 in a underrated range are sounding astoundingly... because in that region of class A there is almost no distortion in the power stage

interesting designs also are DC feedback amplifiers and OTL self-balanced designs

I'm personally interested in getting the most out of what is in my drawers (mostly scrapyard sourced), and that can sometimes yield excellent alternative designs (without getting more watts than what is pleasant to the ear)

Ash Small

Tue Apr 28 2015, 08:03PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

That's exactly what I'm aiming for with mine

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