HiFi Subjectivist ¦ Audio Forum ¦ NVA User Group

I got bored a few weeks ago so, built myself another amp with chassis made from 2 quid Poundland bamboo chopping boards. I found a pair of no name 5K, Chinese single-ended output transformers with ultralinear taps in a box of bits I’d forgotten about for 15 years, so I thought I’d put them to good use. Another old USAF potted choke and another large Hammond potting box to cover a pair of 20H Sowter chokes later and I had the basis for a decent amplifier.
Below is what I came up with: Underside: I thought I’d do something a bit different so this one breaks a few of the rules governing purist single-ended valve amplification. It is not a triode amp. It uses EL34s in single ended ultralinear configuration, to give just over 6W per channel. It also has additional feedback to the control grid of the power valve, in the form of a so called ‘E-Linear’ setup, where the high voltage supply to the driver is taken from the ultralinear tap on the output transformer, creating a loop from the anode of the output valve, back to its control grid via the driving circuit.
Rectification is solid state via a bridge of 1200V 3A SiC Schottky diodes, with an RC series Zobel network across the power transformer primary, to stomp on any remaining switching noise.
It sounds bloody fantastic. Cant fault it at all. It’ll play anything from 1920s acoustic recordings of Caruso and Louis Armstrong, all the way up to boring audiophile stuff without fuss, or favouring any music style over the other. It seems to get the best out of anything put in front of it.

You are indeed a man of many talents

I wish I had even the faintest inkling of how to do that.

slinger wrote: ↑Fri Mar 11, 2022 9:36 pm I wish I had even the faintest inkling of how to do that.

Likewise

Nice work Steve. I know next to bugger all about valve amps so can't comment on your circuit. You seem pleased with it and that is all that matters.

Very pleased in fact.
There’s nothing wrong with single ended triode amplifiers, but the major drawback with them is the output transformer. If, as is the tradition with SETs, you go for a zero negative feedback topology, apart from the inherent internal feedback mechanisms of the triodes themselves, then the output transformer has to be of absolutely unimpeachable quality. That costs serious money and it is an inescapable cost.
If you don’t have the money for such exotica (which I don’t) then a SET amplifier is always going to be compromised, soundwise, no matter how exotic are the caps and resistors in the circuit itself. Without the expensive output transformer, you are not going to realise the full potential of a SET amplifier - simple as that.

There are a group of guys on Audio Talk who do have the appropriate (read expensive) output iron, plus impeccable interstage coupling transformers (amorphous cores and silver wire) and frankly, the SETs they build are in a different league to anything I could come up with on my budget. Those who have been to any of our meets at The Owston Lodge will have heard these creations and will have heard the difference in performance they exhibit compared to some of the other stuff there. We’re all good designers, but as with any sort of craft, given similar design abilities, it is the ones with the best tools that will produce the finest work.

So after a very long time building SETs, I’ve given up on them. The 45 amp was my best effort by a long shot and sounds great, but still, I feel it is compromised by less than perfect output iron, so why bother? It’s my last SET.

Now this single ended ultralinear amp with feedback is more tolerant of cheap output iron than a feedbackless SET. To quote one of my AT mentors, James Doddington:
E-linear will be more tolerant of cheaper output transformers as the local feedback reduces the anode resistance making it a better option for a lower inductance transformer (better and more controlled bass) and for driving the parasitic capacitance at high frequencies both in terms of extending the frequency response and reducing overshoot by dampening it better (giving better midrange as well as h.f. and increasing transparency) It just doesn't help with the SE magic that some amps have - that's either there or not and only you know if it has it or not

So given this, why would someone like me on a budget, not want to build the amp I’ve just finished as opposed to making yet another fruitless attempt to get SET magic on the cheap?

The only thing you have to watch out for with single ended pentodes or ultralinear configurations and feedback is the inevitable generation of high, odd-order harmonic distortion products, caused by the non-linear transfer functions of the devices themselves when subjected to a feedback signal. These high-order distortion products need to be kept way below the 0.1% normally referenced for a low distortion amplifier. Even fractions of that 0.1% at 5th, 7th, 9th and above on the output will result in listener fatigue and then you might as well buy a good solid state amp, where feedback is par for the course and most designers have long ago minimised the high order distortion harmonics issue. You know, NVA for instance?

The E- Linear amp above sounds nothing like a SET, but it is low distortion, neutral, clear, dynamic and simply gets on with the job. That, to me, is what an amplifier should do. Enough of my ramblings.

Fascinating reading Steve

Well done again Steve, I'd love to hear that.

So how much are the best output transformers ? Not being a valve guy I have never looked.

karatestu wrote: ↑Sun Mar 13, 2022 10:35 am So how much are the best output transformers ? Not being a valve guy I have never looked.

For good stuff such as Sowter you start at around £250 each for 2A3 valves so that’s £500 plus VAT plus p&p (£620) of output transformers before you start.
£330 each plus VAT plus p&p for 300B transformers which is £800 ish per pair.
https://www.sowter.co.uk/acatalog/E_A_S ... UT_15.html

You can go a lot more expensive than this with Japanese exotica from the likes of Tamura, and Hashimoto.

But Sowter is the minimum you need to be looking at to get the best from a SET amp.