A Valve Pre-amplifier for 78s.
The musings of a newcomer (of mature years) to valve audio – part II.
WARNING: HIGH VOLTAGES, WHICH CAN BE FATAL, ARE USED IN THIS TYPE OF EQUIPMENT.
THE MAINS VOLTAGE IN THE U.K. IS OFTEN QUOTED AS 240 VOLTS – THIS CAN KILL YOU;
THE PEAK VALUE IS ACTUALLY 340 VOLTS. DO NOT WORK ON PROJECTS LIKE THIS UNLESS
YOU ALREADY HAVE PRACTICAL EXPERIENCE.
(I first built main-powered valve equipment when I was 12 years old,
under the supervision of an ex-Army professional Radio engineer.)
26th November 2016. We were much encouraged by the results of our first excursion into valve phono pre-amps – see valvepreamp1.htm – but that had only a single potentiometer to vary the overall EQ. Still, the sound was very attractive. I can’t apply all the diaphanous rhetoric that valves often attract these days, and for two reasons: (a) I have not yet enough experience using valves in audio, and (b) being quite old, I only have the old-fashioned mercifully-limited vocabulary to describe sound reproduction: e.g. ‘forward’, ‘backward’, ‘clear’, ‘muddled’, ‘strident’ &c., as opposed to the modern, often quasi-metaphysical terms that have come into use over the last thirty-odd years. (I was middle-aged even when it started! 8^) But to work. We found on-line an attractive circuit, using an ECC83 and two ECC82s. We cannot reproduce it here, as it is copyright (please don’t ask me for a link); but, if anyone has actually put up a circuit on-line, I dare say they won’t object if someone is curious enough to build it purely for their own enlightenment. We adopted, some years ago, a ‘standard module’ for work with valves in amateur radio. It consisted of an 8" x 5" (20.3cm x 12.7cm) sheet of aluminium, supported by wooden side pieces, with aluminium front & rear panels. For these projects, we have changed to single-sided fibreglass printed circuit board. It’s much easier to work with, and by fixing strips or squares of PCB in place with superglue, one can ‘build as one goes’ – a technique widely used in amateur radio prototyping, and often known as the ‘Manhattan’ system. It is also easy to solder components from valve-holders to deck anywhere. (I will probably get into ‘star ground points’ later.) So above, you see the first steps. The side panels are lightly screwed with the board upside down, so that assembly is easy – at least to start with! And this is just a prototype, of course. On the left is a power supply made in the same way. It currently has no shield in front of the 240V mains input – that will be added later, as well as one on the back, I assure you.
Yes; it started to get quite cramped towards the end. Pencil lines have been drawn on so that we do not stray into the area occupied by the wooden side pieces. It would have been better to make the panel 6" (15.25cm) wide. Next time.
The finished products. A word on the PSU. The transformer was acquired some years ago from a piece of ‘junk equipment’ bought at an amateur radio rally for £5. Can’t remember what the unit was for, but dismantling it yielded amongst other things, over twenty 4mm plug/pillar terminals of various colours (which even from my very good supplier of components would have cost more than £5), plus a large and very nice 100µA meter, and this transformer. The unit was made in Australia, and the transformer was by A&R (or AR), of Melbourne. I even found on-line a .pdf of an AR catalogue from the 1960s. However, this was a bespoke transformer made by AR. Much of it consists of a 16-0-16V 2A winding, of no use to me. It also had two 6.3V 1A windings, which were put in parallel. The HT winding was single-ended: 300V at 35Ma – more than enough for our project. Four 1N4007 diodes made a bridge. The raw DC went into a 47µF 450V capacitor, then via a 10H 50mA choke (mounted underneath) into a 220µF reservoir capacitor. I’m sorry to have to tell you that though it produces about 420V of lovely DC with a 10K 5W bleeder resistor in place, there is a persistent hum – more like a buzz, actually. At the time of writing, we’re still working on it; but if you know what’s the matter, please write and tell me! The pre-amp has three knobs. The first adjusts the turnover frequency – there are six options, including flat. Centre is output gain, and on the right is a top end switch. Position 1 is flat, and there are four other progressive cuts. (I did tell you that I only play 78 rpm records, didn’t I? But if I did want to play an LP, the horrid, nasty RIAA “Death to 78s!” EQ is of course available.)
The rear ends are self-explanatory. The record deck goes ‘IN’ – an earthing point is provided by the nearby 4BA nut, and the output goes into a line input of a trusty old NAD 3120 amp. The circuit is rated at 50dB gain, and I can well believe it. There’s plenty of poke in this little unit. Yellow is the heaters of course, floated. Alas, I only had one yellow pillar terminal, so a blue one was pressed into use. Tsk, tsk. Red & black speak for themselves.
In view of the hum/buzz problem, we had recourse to the PSU we used for our previous pre-amp. Again, the transformer and choke were rescued from a junk unit. The rectifier is a 5Z4. Alas, the voltage wasn’t quite up to scratch, even after we pulled the VR150-30 (used originally for the VFO supply of a small amateur CW transmitter). The circuit calls for a 300V supply to the ECC83, but this unit could only manage about 265V. It still works though, with very little hum – so there definitely is something seriously wrong with our other, chunkier PSU, which will furnish 290V.
And here we are, all up and running. Oddly, there’s no induced hum from the mains transformer in the NAD – directly below the pre-amp. The evaluation of any new piece of gear must naturally be a long & slow process. At first, this set-up sounded rather glassy & aggressive – but we shall see. I freely admit my upper-frequency hearing has always been very poor. Even at the age of 18 I could only hear up to about 8 or 9KHz. And a lifetime of playing in jazz bands often with VERY LOUD AND MUSICALLY INSENSITIVE DRUMMERS has knocked it down to about 6KHz. But the ‘old 78s’ still sound great to me. Or at least, E+ copies of laminated pressings from Columbia and Parlophone do. Many others are pretty good too. But that’s another story. These days, NADs (I have three) are hardly top-flight, and my speakers are only Monitor Audio MA6 Budget version. But there we go. More anon.
Y’know; I was just thinking… if this valve lark persists, it might possibly lead to home brewing a valve Power Amplifier. But no. No. NO-NO-NO-NO!!!!