Re: EL503 interest
Ash Small, Wed Apr 22 2015, 02:39PM

If you can't find any EL503's, maybe this page (and the rest of the 'Champ' website will be useful, for converting to EL36/6CM5/EL360 tubes?

Re: EL503 interest
Shrad, Fri Apr 24 2015, 12:28PM

Hi Ash,

I had a look already and know of much designs which would suit my needs, but the problem of those EL503s is that their low impedance and the particular output transformer of that amplifier make that it is not practical without completely modifying the amplifier (replacing the complete push-pull)

while the champ example seems a simple solution, the guy is actually redoing all the power stage with nearly a custom power transformer for each tested tube and all things like that while keeping things simple in appearance as he writes things very well (that's the reason his site is such nice to read)

sadly I don't have that knowledge of tube designs myself, so I'm looking for something simple enough ;)

I think I'll remove the other one from the pair which poses problem and duplicate the bias circuit to be able to put in non paired tubes, and perhaps make another preamp based on tubes instead of transistors, but not a complete redesign

I still have 8 6080 which I plan to use as OTL amplifiers for stereo 30W amplifiers which will fit nicely with that 200W beast and a subwoofer, and optional guitar amp

I have a friend who has a spare EL503 but he is 1000km away, I'll have the opportunity to meet this summer when vacation in his area with the family and was looking for quick alternatives

sadly vacuumtubes.net is out of stock and others are 100€ (actually one is 36€ in germany but shipping included it is not interesting)

thanks anyway

Re: EL503 interest
Ash Small, Sat Apr 25 2015, 03:22AM

I didn't realise you'd need to replace the OT.

I know someone who has thousands of tubes, I can ask him if he has a 503, or even a matched pair if you are interested.

I presume I'd need to post it/them to you?

Re: EL503 interest
Shrad, Sat Apr 25 2015, 01:01PM

yeah that's not an issue as shipping from UK is usually 2 days to Belgium

if you can find me a matched pair that would be really nice as I could restore it in its past glory, but as it's not an exceptional amplifier (that's a town hall sono amplifier rather than a hi-fi marvel) I wouldn't spend more than it cost me at start, or say less than 100€, because I could go for cheap and do a conversion with cheaper beam tubes

if you could ask your friend about a list of common higher wattage beam triodes, tetrodes and pentodes he would have it would also be interesting as I'm looking at a 2.1 tube setup for a long time now, and why not jump in now

Re: EL503 interest
Ash Small, Sat Apr 25 2015, 04:29PM

I'llprobably pay him a visit tomorrow. He designed the 'Mighty Atom' I posted details of a while back.

I've not come across bean triodes or pentodes before. I always thought thr Beam tetrode was just a way of getting around the pentode patent.

I'm building a single ended amp using a beam tetrode voltage regulator tube.

He said he doesn't think he has any EL503's, I'll ask him about the common higher wattage ones though. I'm pretty certain he'll have something suitable. Any idea what impedance you want?

Apparently there is a Russian equivalent, the 6P45S, but I've not looked into it. I just found it while googling.

[Moderator edit by Mads Barnkob: merged posts]

Re: EL503 interest
Shrad, Sat Apr 25 2015, 09:23PM

I've some 6P45S but the tube is approximately one third in size (or they are reserigraphied and I would be disappointed as they were in a kind of original 'CCCP' package)

the interesting thing about beam tubes is that a yoke is nearly the same as an OT and as they are cheap and designed for 1000's of hours of continuous use instead of the hundreds of classic audio tubes, they are gold for sturdy amplifiers provided a non conventional design

same for triodes used in regulated PSUs and voltage regulators, such as the 6080 (6AS7) which has low impedance and can drive a 8 ohm speaker with 8 tubes in a row without an OT, and is ultra cheap

for beam pentodes for example a matched pair is in the 30€ range, and even a 24LQ6 with 30W is less than 20€, so those can only be interesting

if you friend has a box of such tubes he wants to get rid of, that would be really nice for me to have a bunch of experimentation power tubes (I already have 100+ various small tubes for the rest, preamps etc...)

Re: EL503 interest
Ash Small, Sat Apr 25 2015, 11:01PM

I'll ask him about the above valves.

The beam tetrode I'm using in my single ended amp is from some old military equipment and high impedance (max 800V anode voltage, but I'll run it at ~500V. It's good for 35 watts or so. I think I have a suitable OT, I'm just waiting to find a suitable parafeed choke, or a core I can wind one on).

He knows a lot more about valves than I do.

I'll see what he has, what he wants to part with, and how much he wants.

Probably sometime in the afternoon/early evening. I'll try to post on here while I'm there.

Re: EL503 interest
Shrad, Sun Apr 26 2015, 07:57AM

that's very nice of you

in terms of choke, I would certainly not go myself for a conventional design for the B+... I want to try the PC controller route as it delivers the range needed for a triode with the suitable load characteristics

I really have to read my books but I feel this is a road to go for saving money and performance

a mosfet drive for the powere tubes could be nice too... but I'll stick with conventional first (

Re: EL503 interest
Mads Barnkob, Sun Apr 26 2015, 09:10AM

I have built a 100 Watt mono block amplifier using 6P45S tubes:

Re: EL503 interest
Shrad, Sun Apr 26 2015, 09:24AM

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!

Re: EL503 interest
Ash Small, Sun Apr 26 2015, 04:25PM

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

Re: EL503 interest
Shrad, Mon Apr 27 2015, 12:55PM

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

Re: EL503 interest
Mads Barnkob, Mon Apr 27 2015, 01:16PM


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.

Re: EL503 interest
Ash Small, Mon Apr 27 2015, 05:37PM

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.

Re: EL503 interest
Shrad, Tue Apr 28 2015, 07:18AM

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)

Re: EL503 interest
Ash Small, Tue Apr 28 2015, 02:32PM

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.

Re: EL503 interest
Shrad, Tue Apr 28 2015, 07:44PM

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)

Re: EL503 interest
Ash Small, Tue Apr 28 2015, 08:03PM

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