First glass: building a DIY valve amplifier from scratch

  • Karel Mars' valve amp assembly kit
  • Close up of top of valve base, showing retaining clip.
  • Sturdy binding posts are used for the speaker taps.
  • Note orientation of power supply capacitors. The lugs are positioned for an easier layout for connecting cables.
  • Transformers (in this case, one of the outputs) pass their leads through the chassis via rubber grommets.
  • Main hardware laid out. The mains transformer is on the left, the two output transformers centre and right closer to the speaker taps.
  • Start of day 3; note the daisy chain linking valve bases.
  • Point-to-point layout necessitates some bending of component leadouts to allow for suitable positioning and heat dissipation. It also looks neater.
  • Late on Saturday night, testing starts. Credit: Anthony Doman
  • Mars starts to slot in the valves. Credit: Anthony Doman
Date:12 December 2013 Author: Anthony Doman Tags:, , , ,

Some time after midnight on Sunday, we heard music. It was ambient New Age (not my choice) and distorted. But it was music nevertheless.

A tired “It lives!” escaped my lips. Dr Frankenstein couldn’t have been prouder.

Not that the matter was in any doubt. I’d just finished – finally – building a valve amplifier under the watchful eye of its designer, Karel Mars.

I’d driven screws, tightened nuts and snipped tag strips until I squinted. The reek of solder fumes, despite my best efforts, seemed ingrained in my nostrils. My left thumb was cramping, and my right thumb was aching. My neck hurt. I even bled a little.

But I felt great. It lived!

WHY VALVE, AND WHY DIY?
Mars, the subject of a DIY amplifier designer profile in PM a few months back, is the kind of man who’d have been featured in the Whole Earth Catalogue. A big part of his worldview is to avoid waste: he repurposes and reuses. I spent plenty of time rummaging around in his oddments box locating machine screws for the build. We fired up the finished amp using a variac rescued from a dump. Even the salad greens we enjoyed with our lunch were re-sprouted in his garden.

Most importantly, Mars is a purist when it comes to music and amplification that uses those glowing glass envelopes known variously as valves or tubes. We were only just perfecting
valves when solid state came in – so there’s still plenty of untapped potential there, he believes.

Besides being a go-to man for restoring, refurbishing and repairing vintage valve gear, he designs new amps: domestic units for home and traveling kit used by pro musicians on their gigs.

The design I built is a new one from Mars that traces its heritage back to the classic Mullard 5-10 of 1954. That “push pull” design used a pair of Mullard’s then new EL34 pentodes, one to handle each half of the waveform. The original’s 10 watts rated output may look puny next to today’s high-wattage monsters, but it was plenty back in the days of efficient horn speakers. In fact, even today that output is adequate unless you have extraordinarily power-greedy speakers. For most sane listening, 5 watts provides ample volume. Party animals, look elsewhere.

There’s nothing quite like the feeling of satisfaction, he says, when you can settle back to listen and, glancing across at the faint red glow, say, “Hey, I did that”.

THE PRINCIPLES
The Mars amplifier is a stereo version of that mono Mullard original. It is an integrated amp, which is to say it combines pre-amplifier and amplifier in one unit. The Mullard original provided plenty of options – tone controls and different inputs for the sources of the day, including vinyl. The unit we built provides just a volume control and a single input matched to outputs from iPods and laptops. (Options with more inputs are
available.)

Modern solid state power supply rectification is used because it’s simpler and cheaper, but a valve equivalent is possible.

Mars’ layout uses the time-honoured point-to-point technique. Valve designs generally have a lower component count than solid state ones, which makes them suited to this kind of layout. They also run a lot hotter, so the lack of a printed circuit board aids cooling.

That said, it still helps to plan the layout with care, as you would with a PCB, to avoid having a rat’s nest of wiring afterwards. Besides, it’s more aesthetically pleasing. What difference does it make, though, considering it’s all hidden away inside a black box? Well, there’s a functional benefit in having matched, shorter signal paths. It also makes fault-finding easier. In short, just do it.

Mars has also used a kind of virtual star earth arrangement. Leads are soldered to solid copper bus bars to provide a fairly central earth return path.

Finally, he has incorporated a minimum of negative feedback to avoid excessive interference with the amp’s designed sonic qualities.

THE JOURNEY
Amplifier-builders of yore would have had to be quite handy with more than just a soldering iron. They were required to wield screwdrivers, spanner, pliers, cutters, and possibly drill, hacksaw and file. The kit takes most of the drudgery out of building the amp.

You can buy the parts for this project yourself; schematics and component lists are available from www.marsamps.co.za

Potentially, that could work out cheaper. But getting the complete kit of parts from Mars means you don’t have the worry of sourcing sometimes hard-to-get items. Plus, you get transformers wound to specification and a pre-drilled, painted chassis.

Mars provides kits and a full set of instructions for a DIY home build at a cost of R6 900. That price also buys you online backup and guidance. However, for R2 000 extra you can get the man himself to guide you through the build. The process takes place over the course of two days either at his Paarl home or in other centres by arrangement (the latter being group sessions to make them viable). At the conclusion, you have a working amp.

Because of scheduling conflicts, and possibly my soldering technique, PM’s guided build rolled over into a third day. Instead of the allotted 16 hours, I used up something like 24. Most of that was devoted to my efforts, though towards the end Mars needed to do some tweaking to component values.

As my guide and mentor, I had a laidback Mars slipping in and out of the workshop to cast the occasional beady eye over my work. (And the less occasional “that looks good”.) In the background, my favourite kind of jazz playing.

What could possibly go wrong?

Rewind the clock a few hours. “Oh, you’ve soldered those two to the wrong lug. That should be 6 and 9. I’ll got out and make us some coffee while you re-do that.” Sigh.

Thankfully, there were few of those. But my soldering technique certainly got a brutal workout.

TIPS AND TRICKS
Here’s a comment I made several times: “Soldering to lugs is not for sissies.”

My previous experience with soldering was confined to solid-state equipment. That procedure generally involved a short, sharp application of heat to get the juices flowing – without vaporising the often delicate semicondutors. In valve amps, the components tend to be a whole lot more robust and can take what may seem excessive heat.

Mars prefers to use a temperature of 300 degrees. This usually means you will have a longer dwell time on your solder joint. It’s good practice to tin both ends to be joined; heat each tip thoroughly, then feed in the solder so that the resin penetrates cable or flows over component lead-outs to clean the surface. The result, when making the joint, is that it all flows nice and easy.

One, less robust part of the assembly is the valve seats. When I first started soldering these, I bent the lugs outwards and soldered leads from the centre outwards. This seemed sensible from a safety point of view, but given that some lugs take up to three items, things can become crowded.

Our pictures show a recommended component layout. Besides being optimised for easier construction and generally pleasing look, the way it’s been set out here does have a bearing on the sound quality in regard to matters such as hum rejection.

LISTENING
This little amp happily accepts the output from an iPod – and that’s exactly what I used. It was the littlest iPod – in fact, the Nano.

Hooked up to my 30-year-old pair of Technics SB30 bookshelf miniatures, the Mars simply sings.

There are one or two things that need attending to. That slight scratchiness on the volume pot, for instance. Speaking of the volume, much more important is the question of gain. At the moment, there’s too much: only the first bit of travel of the knob is usable before the amplifier is driven into overload. That should be a straightforward fix, though.

This combination of utterly contemporary and seductive retro is a docking station with style that makes off-the-shelf units look oh so dull. Okay, so you’ll sacrifice some utility, not the least of which is wireless connectivity and remote control. But my guess is that you’ll trade all that for the knowledge that, besides looking über-cool, it represents your very own blood, sweat and tears. That’s a special feeling.

Video: Karel Mars describes his valve amplifier design
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