A Valve Pre-amplifier for 78s.
The musings of a newcomer (of mature years) to valve audio.
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.)
14th November 2016. It seems curious, seeing that we grew up with valves (vacuum tubes), that until a couple of weeks ago, we had never, ever, home-brewed a piece of valve equipment for playing 78 rpm records. It simply had not occurred to us to do so. Had it, there would immediately have been a craze for such things. It finally came about because we had already made a couple of solid-state phono pre-amplifiers which greatly improved the reproduction of our 78 rpm discs. They did this, because they offered a choice of equalisations (EQs) that were appropriate to 78s – as opposed to the near-universal RIAA equalisation which has been built-in to all modern amplifiers (and most if not all commercial pre-amps) for several decades. This RIAA EQ, of course, is the one required for playing microgroove LPs and EPs, 78s having been a thing of the past since the mid-1960s. However, if you play decent 78s with RIAA, you are ‘throwing the baby out with the bath-water’; they will sound dull, muddy & insipid. (For much more on this, see phonopreamplifier.htm )
So we idly googled about, with the vague idea of possibly lashing up a valve phono pre-amp for 78s. Naturally, we had long been aware that valves had been making a come-back for the last 20 years or more. In fact, it would be more true to say that valves had never really gone away; rock bands always favoured valve amplifiers, & we ourselves used Leak and Quad valve amplifiers in the 1970s – second-hand, they were quite cheap then, believe it or not. But we never got truly addicted to them. I think the reason for that was the loudspeakers we had then – only one at a time of course; we only played 78s in mono. – were never really very good. In fact, they were ruddy awful, so we never really heard the quality possible. It’s a general rule, that most people’s speakers are the weakest link in their audio chain. In any event, we spent hours googling & reading on-line articles, blogs & forums, & became bedazzled by the vast array of pre- and power amplifiers available both commercially and as home-brew projects.
Eventually, we hit on a simple circuit that appealed to us because it used two early 1940s valves. The 6SN7 double-triode came out in 1941. It is, in fact, two 6J5 (1936) triodes in one envelope. It was used for a variety of purposes, perhaps most familiar in audio applications as a phase splitter in push-pull power amplifiers. Radio amateurs have even used it in a ‘novelty’ singe-valve low power Morse transceiver!
Here's the circuit we chose. I don’t think it’s copyright; indeed, the one above is a modified version of an earlier version by somebody else. Besides the 6SN7, there is a 6SL7, another double triode contemporary with the 6SN7, with higher gain, though less power capability. The 6SL7 triodes are used in parallel. Of course, I’m not a technical boffin, so I assume this was done to ‘share the load’ of the first stage of amplification, thereby reducing noise. The incoming signal is from a moving magnet pick-up cartridge – I use Shure SC35s – and these only have an output of 2 or 3 millivolts, so a lot of amplification is needed. The components ringed in red provide an equalisation. If they were removed, the circuit would still function, but all frequencies would be amplified equally. The first ‘inverted L’ capacitor and resistor constitute a high-pass filter. As its name implies, this emphasises upper frequencies at the expense of lower ones. The second ‘inverted L’ resistor and capacitor is the opposite: a low-pass filter which favours lower frequencies. Together, these give us an RIAA EQ.
Prodnose: Wait a minute! I thought you said the RIAA EQ was bad for playing 78 rpm discs?
Field: Yes, I did.
P: Then, for heaven’s sake, why are you using it?
F: (i) Because I couldn’t immediately find a circuit for a valve pre-amp with 78 EQs; (ii) because I like 1940s valves, and above all (iii) in my garden shed there was a box of old valves which contained several each of 6SL7s and 6SN7s! I thought I would just build it, then sort out the EQs later.
P: H’mm! I see. Well, I suppose that’s all right then. You may proceed.
F: Thank you, Prodnose.
It is a feature of most modern valve phono pre-amps that they tend to use very high voltages (and to be sure, low currents). I know there is a reason for this, but I’m blowed if I can tell you what it is. Anyway, the attraction of this circuit won the day. We had the valves; most of the other components were in our modest stock, and above all, we had readily available, a high voltage Power Supply Unit. This was left over from an ambitious but unsuccessful amateur radio project of a few years ago. This PSU would give out 420V DC off-load – more than enough. All we had to do was disinter it from the loft, check it over, and MOST IMPORTANT – not stick our fingers in it while it was switched on, and not for some time after it was switched off. See the notice at the top of this page!
In the amateur radio project, we used three modules, because a morse transmitter usually has three basic sections. We chose 8" x 5" (20.3cm x 12.7cm) panels of aluminium as the top plates. Front and rear panels 2.5" (6.35cm) deep were attached using 3/4" aluminium angle. The sides were wooden inserts, added last. The great advantage of this, was that most of the wiring could be carried out, unobstructed, on the underside of the top panel. Soldering on an ‘open’ board is far easier than trying to work into a 2.5" deep box, as would have been the case using a traditional box chassis. So this system was also employed for the phono pre-amp prototype, with the exception that the top plate was single-sided printed circuit board rather than aluminium. Components meet on small strips or squared of PCB superglued to the main board. Ground connections can conveniently be made anywhere on the ground plane of copper, whereas aluminium (which of course will not take electronic solder) would require the fixing of solder tags and other anchoring points. (It is true that centralised ground points are often prescribed, but this was a First Prototype. We just wanted to see – and hear! – how it worked. Other sophistications, such as running the valve heaters on D.C., might also come later.) We left a large space between the two valve holders because we could there play around with various equalisation networks.
Here is the completed First Prototype. The front knob controls the level of the output; the rear knob adjusts a makeshift high-pass filter. The milliammeter on the front panel is actually a relic of the P.A. stage of the morse transmitter, and not really necessary. Still, it does serve a useful purpose, because we are still a bit short of high working voltage capacitors, and one or two of those in this unit are being worked rather hard. If they failed, or even began to fail, the current in one or both valves would rise, and we would notice that on the meter. Or at least, I think we would. 8^) And in any case, it looks sort of cute. Functionality is the main thing of course; but for years now there has been a tendency to glamourise valves – they must always be visible, like a kind of icon, faintly glowing in the dark, as they work their subtle magic! And, to be fair, they keep cooler in the open air. Although these two valves only draw a total of 12 mA, the 6SN7, drawing the bulk of it, does get quite hot. The meter is shunted to read about 17 Ma full scale.
The back panel is self-explanatory. The use of a flying lead from the power supply terminating in 4mm plugs is dangerous, disgraceful & much to be deplored; but then again, there’s nobody else here but me. And it’s only a rough prototype anyway. If we ever actually make a considered final valve phono pre-amp, it will be done better. You’ll notice that I have ‘floated’ the 6.3 V for the valve heaters – the two green sockets go directly to the 6.3 V winding on the mains transformer. Yet on the circuit above, one side of the heaters is grounded. I’m pretty sure this means that in the output triode there is a potential of rather over 100 V between the heater itself, and the cathode in which it resides. I thought this was a thing generally to be avoided. It’s true that the standard U.S. practice was to ground the heaters, whereas here in the U.K. heaters were always floated & fed with a twisted pair of wires – at least in audio amplifiers. But what the heck – it’s a prototype, as I keep telling you.
Here's the front of the P.S.U. This was little changed from the morse transmitter, apart from being checked over, and the wiring tidied up a little. Actually, rather a lot; our home brewing standards have evidently risen over the last few years. Also, we greatly increased the value of the electrolytic ‘smoothing’ and reservoir capacitors. These are now a 47 µF 450 V wkg, through the choke at the front, into a 220 µF 450 V wkg reservoir capacitor. Idiosyncratically, we had used a green neon rather than a red one to show when the unit was on, and followed this up by wiring the on-off switch in the American fashion, with ON being up, and OFF being down. I always thought it was a good example of American laconism, that should a catastrophe befall an operator, and they slumped down across their apparatus, it was better that in doing so, they would turn something OFF, rather than turn something ON. Of course, the point could endlessly be argued. 8^) The rectifier is a 5Z4, but the VR 150-30 is slightly spurious; that was to feed a stable 150 V to the VFO of the transmitter. But it has been left in, as otherwise the unloaded output voltage of the P.S.U. is rather higher than necessary. The neon stabiliser brings it down to a reasonable value. Also, it looks nice in the dark, and would impress the uninitiated – though I, myself, attach but little importance to such things. 8^) The transformer and choke were rescued from a piece of scrap apparatus bought an amateur radio rally some years ago, but I can’t remember what it was.
The back of the P.S.U. simply has an IEC socket for the mains, and a Jones socket to connect to the pre-amp.
And here is the thing up and running. It goes into an auxiliary input on one of my old NADs. Curiously, even though the unshielded pre-amp is sitting on top of the NAD’s mains transformer, there is no noticeable hum – not enough to worry about for a preliminary evaluation, at least. This has been going on for a couple of days now, but naturally must continue for a much-extended time period before any conclusions are reached. In particular, our makeshift EQ network needs upgrading. But it is difficult to pluck forth such a network from one circuit, and transplant it into another – the impedances for a start, would be all over the place. But it does sound very nice, and we are enjoying playing various 78s to test, it, some of which have not been off the shelf for years. ‘What a marvellous record that is!’ we keep on saying. Perhaps this is a Principal Benefit of experimenting with audio amplification? It draws us from complacency. Perhaps the old dictum ‘A Change is as Good as a Rest’ applies strongly here?
In conclusion, we have recently found a three-valve circuit that incorporates switchable turnover frequencies for the bottom end, and a variety of slopes for the upper. The Western Electric & the later Blumlein EQs generally have a ‘flat’ top end; but variation in the top is very useful for some early 78s which can be very strident. We are urgently gathering in components; several ceramic valve-holders and two ECC82s arrived this morning; happily, we already have a couple of ECC83s, so ‘watch this space.’
Gosh – isn’t life exciting!