The Best Stereo Valve Amp In The World

There are few greater follies in the world of electronics than that of an electronic engineering student who has just discovered the world of hi-fi audio. I was once that electronic engineering student and here follows a tale of one of my follies. One that incidentally taught me a lot about my craft, and I am thankful to say at least did not cost me much money.

Construction more suited to 1962 than 1992.
Construction more suited to 1962 than 1992.

It must have been some time in the winter of 1991/92, and being immersed in student radio and sound-and-light I was party to an intense hi-fi arms race among the similarly afflicted. Some of my friends had rich parents or jobs on the side and could thus afford shiny amplifiers and the like, but I had neither of those and an elderly Mini to support. My only option therefore was to get creative and build my own. And since the ultimate object of audio desire a quarter century ago was a valve (tube) amp, that was what I decided to tackle.

Nowadays, building a valve amp is a surprisingly straightforward process, as there are many online suppliers who will sell you a kit of parts from the other side of the world. Transformer manufacturers produce readily available products for your HT supply and your audio output matching, so to a certain extent your choice of amp is simply a case of picking your preferred circuit and assembling it. Back then however the world of electronics had extricated itself from the world of valves a couple of decades earlier, so getting your hands on the components was something of a challenge. I cut out the power supply by using a scrap Dymar Electronics instrument enclosure which had built-in HT and heater rails ready to go, but the choice of transformers and high-voltage capacitors was something of a challenge.

Pulling the amplifier out of storage in 2017, I’m going in blind. I remember roughly what I did, but the details have been obscured by decades of other concerns. So in an odd meeting with my barely-adult self, it’s time to take a look at what I made. Where did I get it right, and just how badly did I get it wrong?

Lovingly hand-drawn from life, missing the PSU components.
Lovingly hand-drawn from life, missing the PSU components.

The amp itself sits in the removable portion of the Dymar chassis, I can’t remember what the dead instrument was, but Dymar produced a range of instruments as modules for a backplane. The front panel is a piece of sheet steel I cut myself, and is still painted in British Leyland Champagne Beige, the colour of that elderly Mini. It has a volume control, a DIN input socket which must have seemed cool to only me in 1992, and a Post Office Telephones terminal block for the speakers. Inside the chassis the amp is mounted on a piece of aluminium sheet, on top a pair of PCL86 triode/pentode valves, a pair of output transformers and a supply smoothing capacitor, and underneath all the smaller components on tag strips. Though I say it myself, it’s a tidier job than I remember.

1969's hot new device, already obsolete by 1980.
1969’s hot new device, already obsolete by 1980.

The circuit is simple enough, a single-ended Class A audio amplifier that I lifted along with the PCL86 and the original output transformers, from a commonly available (at the time) scrap ITT TV set. These triode/pentodes were the integrated amplifier device of their day, as ubiquitous as an LM386 in later decades, containing a triode as preamplifier and a power output pentode, and capable of delivering a few watts of audio at reasonable quality with very few external components. They were also dirt cheap, the “P” signifying a 300mA series heater chain as used in TV sets that was considerably less desirable than the “E” versions which had the standard 6.3V heaters. Not a problem for me, as the Dymar PSU had a 12V rail that could happily give almost the 300mA each to a couple of PCL86s.

My choice of parts must have been limited to those my university’s RS trade counter had in stock that had the required working voltage, and are a mixed bag that you wouldn’t remotely class as audio grade. There are a couple of enormous 450V 33μF electrolytics, and 250VAC Class Y 0.1μF polymer capacitors intended for use in power supply filters. I seem to have followed the idea of using a small and a large capacitor in parallel, probably for some youthful hi-fi mumbo-jumbo idea about frequency response. Otherwise the resistors look like carbon film components, something that probably made more sense to me in the early 1990s than it does now.

On top of the chassis, the original transformers taken from scrap TV sets turned out to be of such low quality that they tended to “sing” at any kind of volume, so I shelled out on a pair of the only valve audio output transformers I could find at the time, something that must have been a relic of a bygone era in the RS catalogue. The original valves were a pair of PCL86s from old TVs, but I replaced them with a “matched” pair of brand new PCL86s. I remember these cost me 50p (about 90¢ in ’92) each at a radio rally, and were made in Yugoslavia with a date code of January 1980. The new valves didn’t make any difference, but they made me feel better.

How did this amplifier perform, and what did I learn from it?

Under the hood, and it's all a bit messy.
Under the hood, and it’s all a bit messy.

In the first instance, it performed 110%, because I had a valve amp and nobody else did. The air of mystique surrounding this rarest of audio devices neatly sidestepped the fact that it wasn’t the best of valve amps, but that didn’t matter. Being a class A amplifier with new components, it came to the party with the lowest theoretical distortion it could have had due to its circuit topology. Another area of shameless bragging rights for my younger self, but in reality all it meant was that it got hot.

The sound at first power-on was crisp and sibilant, but with an obvious frequency response problem, it was bass-to-mid heavy, and not in a good way. Here was my first learning opportunity, I had just received an object lesson in real audio transformers not behaving like theoretical audio transformers. It had an impressive impulse response though, square waves came through it beautifully square on my battered old ‘scope.

I could only go so far listening to a hi-fi that might have been a little fi but certainly wasn’t hi. My attention turned to that frequency response problem, and since we’d just been through the series of lectures that dealt with negative feedback I considered myself an expert in such matters who could fix it with ease. I cured the frequency response hump with a feedback resistor from output to input, playing around with values until I lit upon 330K as about right.

The Best Stereo Valve Amp In The World. Yeah, right.

Here was my second learning experience. I’d made a pretty reasonable amplifier as it happens, and it sounded rather good through my junk-shop Wharfedale Linton speakers with cheap Maplin bass drivers. I could indulge my then-held taste in tedious rock music, and pretend that I’d reached a state of hi-fi Higher Being. But of course, I hadn’t. I’d got my flat frequency response, but I’d shot my phase response to hell, and thus my impulse response had all the timing of a British Rail local stopping service. The ‘scope showed square waves would eventually get there, but oh boy did they take their time. The sound had an indefinable wooliness to it, it was clear as a bell but the sibilance had gone. I came away knowing more about the complex and unexpected effects of audio circuitry than I ever expected to, and with an amp that still had some bragging  rights, but not as the audio genius I had hoped I might be.

The amplifier saw me through my days as a student, and into my first couple of years in the wider world. Eventually the capacitor failed in the Dymar PSU, and I bought a Cambridge Audio amp that has served me ever since. The valve amp has sat forlornly on the shelf, a reminder of a past glory that maybe one day I’ll resuscitate. Perhaps I’ll give it a DSP board programmed to cure its faults. Fortunately I have other projects from my student days that have better stood the test of time.

So. There’s my youthful folly, and what I learned from it. How about you, are there any projects from your past that seemed a much better idea at the time than they do now?

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