By Andrew Westcott
Many years ago I acquired an old vintage valve (tube) amplifier - a Concert Master according to the front panel. Due to the need to turn everything out in order to move house it has been 'rediscovered', and as I'm considerably older now I have far more interest in this kind of thing. After a lot of searching about on the internet, I could not find a single reference to this amplifier anywhere, so I have decided to put some pictures up on the web in the hope that someone might stumble across them and tell me something about it. My e-mail address is at the bottom of the page.
The output devices fitted are a pair of PP5/400 triodes, equivalent to the P27/500 which according to the chassis labelling is what were originally fitted, which in turn is equivalent to the PX25. There are 6 smaller triodes fitted, three 6C5 and a 6J5 which are equivalents, and two 6SF5, which are high gain triodes. These six smaller valves are constructed within steel cans as can be seen in the photos, the can being connected to chassis earth to act as an effective screen. There is also a 5U4G rectifier fitted which is correct according to the inscription next to the socket.
To the rear of the amplifier are the three iron-cored wound components - the speaker transformer, the mains transformer and the choke in that order - note the chrome strips fitted around the cores to prettify things a bit. There is also a three-position mains voltage selector - 200/210 volts, 220/230 volts and 240/250 volts, plus a 750mA fuse fitted between the HT supply's centre tap and the chassis. The value seems a little high all things considered, but it's possible that this is one of the changes made over the decades and the original fuse might have been of a lower rating such as 500mA, something I'll install when I get my hands on one.
The above photo shows the right-hand side of the amplifier where the inputs and outputs are located, some of the jack sockets evidently not being original. As can be seen we have the usual 3 speaker outputs to aid matching, plus some inputs and a dedicated powered socket for a separate tuner. Note also the microphone input - unusual if this was made for domestic use. I'm unsure why all that tacky 70s labelling was applied, as the sockets were adequately marked in ink anyway.
The above photo shows the underside of this Concert Master with the base plate removed. One interesting design point being the removable box housing the controls, enabling remote control of the amplifier. Two front latches are turned and the entire box can then be unplugged and removed, and reconnected using a longer multi-core cable. The octal plug and socket for this unit can be seen to the left of it. Irritatingly, I used to have this cable and the user manual, both of which have been lost in various house moves.
The above photo shows a closer view of the components. I believe some of the capacitors, particularly the blue 450V 8µF ones, are not original although considering that this amplifier could be approaching 70 years in age, this is hardly surprising. I have powered the unit up carefully using plenty of protective series resistance in the high tension supply and it does function, although 2 of the capacitors have proved to be very leaky and heat up excessively so I consider them to be unrecoverable. The capacitors in question are in the high tension power supply and sit either side of the choke, so new ones will be fitted in these positions. The other filter capacitors in the power supply chain will also be replaced along the same lines, as it's silly expecting them to function without trouble for too long.
Much as I'd love a totally original amplifier, repairs have already been done before I got it and I simply see this as part of the amplifier's history - electrolytic capacitors are known to have a limited life, especially if not used for a long time so having these replaced at some point is to be expected. I will be keeping the replaced components and documenting any changes I make, partly for my own records but also just in case I ever pass this unit on.
If you have any information on this amplifier, I'd be very grateful if you'd get in touch. My own thinking is that this may have been intended to be used in an auditorium or cinema rather than in a domestic setting, as valve data suggests that this amplifier may be capable of in excess of 20 watts output - way more than you'd expect in a private residence of the day. But who knows? If you do, let me know too!
Before I started any serious work on this amplifier, I was going to need some kind of schematic to work from and after much head-scratching and the consumption of several Pot Noodles, I believe I have the complete circuit traced out. I've drawn it on three pages as it's a little too big to fit clearly on one - the first three stages on the first diagram, the removable control unit on the second and the remaining stages, phase splitter and output stage on the third. The images here are severely reduced in size and are unreadable, so click on them to see the full-size scans of my sketches. It's always possible I've made a mistake somewhere, so if you spot anything questionable please let me know so that I can investigate it and correct things if necessary, as I'd like the information on this rare amplifier to be correct and useful to anyone else who may own one.
After initial careful tests and inspection, it became obvious that this unit is a little ill, mainly due to the leaky electrolytic capacitors in the power supply sections mentioned earlier, but also some of the old body tip spot type carbon resistors now measure far higher in resistance than they should and the interstage coupling capacitors are probably leaky too, although at this stage I had not yet determined whether this was the case.
The bass and treble controls on the control unit seem to offer sound modifications somewhat unrelated to the positions of the switches which suggests that some of the components inside are faulty, so this is an area in need of attention, but as it's purely passive within the module itself with no DC present, this can wait.
First on the list of repairs was to replace the faulty cotton-covered 2-core rubber insulated mains lead with a modern PVC 3-core one, as the rubber insulation had perished to a dangerous degree. Whilst doing this, I spotted two rubber grommets (mains lead inlet and 750mA fuse lead) which had gone hard and brittle, so these were replaced too.
Next was to remove all that unnecessary tacky 70s style labelling on the chassis which proceeded well with the help of some meths, and which revealed the same details written underneath in fine ink. I see this unit has 'serial No. 102' written in ink too. Possibly of no consequence, but it's there.
Delving inside, I examined the various sockets which had been fitted by a previous owner in holes he had drilled in the chassis - three jack sockets for the three different speaker output impedances, and an additional socket which connected to the cathode of valve 2 via a 0.1µF capacitor which according to the labelling was to provide an output for a tape recorder. The additional wiring was not particularly well done, so I decided to strip out all of the modifications as I was quite happy with the original sockets provided.
The power supply electrolytic capacitors used in this unit have become leaky with two of them becoming uncomfortably hot after a few minutes' operation so I decided to replace all with new as there's a good chance of a problem occurring later if I attempt to keep any of the originals. The capacitors fitted were 8µF devices, but I made an executive decision and replaced most with 22µF ones, except in the decoupling filter for the early stages where I threw caution to the wind and fitted some 100µF units I had in the hope of completely eliminating any vestigial traces of hum on the supply line. Over-engineering perhaps, but I'm quite happy about it as extra capacitance here can only be a good thing, and places no extra stress on the rectifier.
Photos of some swapped components:
Photo showing old and new interstage coupling capacitors
Photo showing old and new electrolytic capacitors
Once the suspect electrolytic capacitors had been replaced I was able to run the amplifier properly and start checking voltages. I was able to confirm that the 0.1µF capacitors connected to the grids of the output valves were leaky, causing the grid voltage of V7 to rise almost 5 volts above what it was without the capacitors connected. This was quite an error so I replaced all interstage coupling capacitors (which were of the same type) with new, which allowed the grids on the various valves to sit at a more realistic level. The eagle-eyed amongst you may notice black dots in varying numbers on the carbon rod resistors in the photo to the left - this was to make the chassis components quicker to identify by marking up certain resistors on their bodies with a code of dots, and doing the same on my working circuit diagram.
Whilst poking about inside I happened upon a chassis earthing point that was hovering considerably above chassis potential so evidently wasn't making a good connection to the metal of the chassis. I could have spent time removing all of the many chassis connections and ensuring they made contact, but I felt my time would be better spent by simply adding wiring to connect all such points together and tying them to the negative line. This additional wiring is easily removed if the desire should arise, but it does eliminate completely any chance of faults arising in the future from poor chassis earthing. During this job I was interested to find a pair of earthing tags which should have been soldered together, but had been overlooked at the time of building. The oversight had gone unnoticed because the two tags were touching under slight spring pressure and so had presumably not caused a problem. Quaint as this relic was, I soldered the two tags together as was originally intended to avoid the possibility of a future fault developing.
Next on my list was to deal with some of the large carbon rod 'body tip spot' type resistors, some of which are used in the power supply filtering, and some as anode loads. In all cases they have gone high by a fair amount but the worst case were the two 47 kilohm resistors used in the split-load phase splitter. The phase splitter in this amplifier, as can be seen in the appropriate diagram uses a single triode with two 47 kilohm resistors, one as anode load and one in the cathode circuit, with an additional 4.7K resistor generating the valve's bias voltage. These two 47K resistors measured 69.3 kilohms in the anode load and 64.5 kilohms in the cathode circuit, far higher than intended by the designer. I was originally going to strip out the offending components and replace with new, but I do like the appearance of those old resistors so I decided to piggy-back small higher value resistors in parallel with them to bring their combined resistance within range, as I believe the old resistors have probably stabilised in value and will change little in the future. In practice, fitting a 150K resistor across the anode load and a 180K resistor across the cathode load brought both loads to within 1% of the design value and would also reduce imbalance in the amplitude of the two phases. One interesting point is that no cathode bias resistor bypass capacitor was ever fitted here - perhaps the negative feedback was considered beneficial at this point in the circuit.
As some of the electrolytic capacitors had already proven troublesome, I had a closer look at the cathode bias bypass capacitors fitted around V1, V3 and V4. These were all of the same type - 25 volt 25µF electrolytics, and were therefore immediately suspect. I connected a current limited supply across each one in turn and slowly turned up the voltage, noting any leakage current above that drawn by the parallel resistor. Unfortunately each one showed serious leakage of several hundred milliamps at just 10 volts, and attempts to slowly reform them didn't work so new capacitors were fitted at these points, the new ones being rather different in value - 200µF in capacitance and a far lower voltage rating of 10 volts, closer to the working voltage they would experience in use, this being in the order of 2 volts. The worst enemies of electrolytic capacitors are disuse and running with insufficient polarising voltage, which was why I chose devices with a lower voltage rating than the originals.
There was now only one original electrolytic capacitor left in this amplifier, and that was the 50 volt 50µF one fitted to bypass the output devices' common cathode resistor. As before, I applied voltage across it to observe any appreciable leakage current, but in this case it seemed fine. I did, however, notice that there was no current spike when making the initial connection, caused by the charging of the capacitor. Upon further investigation it appears that the capacitor had dried out, possibly due to being fitted next to the hot cathode resistor and was therefore ineffective. I fitted a new 63 volt 470µF in this position as I happened to have one going spare. Whether this far higher value of capacitance will cause a problem is yet to be seen, but if it does it can easily be changed.
With the amplifier now operating more or less correctly, I made some measurements of the standing current being drawn by the output devices and was a little surprised. One valve was drawing about 35mA, meaning that at the measured 450v anode voltage it was dissipating about 16 watts, within limits although a bit on the hot side for an old valve. The other valve was drawing a massive 90mA, dissipating about 40 watts, way above its maximum rating. With a common cathode resistor little can be done to adjust individual valves, and if one has a tendency to draw a heavy current this exacerbates the imbalance in the system by tending to bias the other valve to draw a lighter current on account of the higher bias voltage developed across the common resistor.
This potentially damaging situation needed addressing before we had a meltdown so I looked at what remedies I could dream up. I could have changed the circuit somewhat and incorporated separate cathode resistors of differing values to try to balance things up a bit, but this would have meant some major changes on the tag board, and I didn't really want to disturb the original circuitry. Another option was to produce a power supply designed to inject a negative voltage onto the grids of the output devices to set the standing current at a reasonable value and this is what I ended up doing, shorting out the common bias resistor. This would change the output impedance characteristics of the amplifier, but as I never intend to use it for critical litening, it is of no practical concern - more important that it simply works when powered up.
The power supply used a small 48v 1.5VA transformer connected to a voltage doubler which in practice gave about 170v off-load. This was fed to a zener diode string via a limiting resistor to regulate the output at 128 volts, this stabilised output was then fed to 2 separate potentiometer and resistor chains to enable me to tap off an adjustable negative voltage of between roughly -50 volts and -115 volts relative to chassis, which was then injected into the grids via high value resistors. The entire supply is fairly small and was constructed on an offcut of circuit board which was mounted to the chassis using two pre-existing holes, so required no modifications at all, apart from soldering the grid bias wires in place. The device can be seen in the photo here, fitted to the side of the chassis behind the two large electrolytic capacitors, the two potentiometers for setting the bias voltage being visible.
In operation, the tacked-on grid bias supply works very well and I was able to adjust the grid voltage on each valve independently to set the standing current in each valve to a beautifully low 5mA. This is well into the non-linear region of the valves' characteristics and I was expecting to see and hear some crossover distortion, but I could see no evidence of it on the 'scope and all sounded fine, within the constraints of what this old amplifier is capable of, and I can only assume that there is enough negative feedback to force the output valves over the non-linear portion of their response. But by far the greatest benefit of all this was that the output devices are now dissipating little more heat than with heaters alone, meaning that when on the odd occasion that I fire up the old girl, I can be assured that those rare and expensive output valves are not being thrashed to within microns of their lives just for the sake of some background music.
With the output valves being so rare and expensive I have looked into the possibility of using something different in this position, particularly as I can now adjust the bias over a wide range to suit. I do have a pair of PP3/250 valves here which use the same heater voltage and valve base, and during a reckless moment I plugged them in and biased them up to 5mA each. They actually worked fine but when I examined the specifications and saw the maximum permitted anode voltage I quickly switched the amplifier off. Although there had been no problem and none of the valves had flashed over I was aware that my supply voltage was measuring a healthy 560 volts with the new low quiescent currents, a fair bit above the stated 300 volt maximum. I'd like to use these in place of the rather more expensive and rare originals so need to look at a way of reducing the anode voltage a bit to ensure they survive. One as-yet untried solution is to fit a pair of 100 volt zener diodes in series with the anode supplies to drop the voltage to something a bit more survivable, but I'm not sure how this will work in practice. This will probably be my next experiment, and this will at least mean that I can use an alternative valve whilst keeping the working originals somewhere safe and unused.
I also have a pair of DA30 valves, sensibly equivalent to the CV1178, and using these is also a possibility, being adequately rated for this application. I did try them briefly with the original automatic bias system but they drew such a heavy current that I quickly abandoned the idea, at least until such time as I could arrange for a more negative voltage on the grids. Now I have such a device incorporated within this Concert Master amplifier I'll give it another try and see if I can set the idle currents to a suitably low value.
One problem I experienced with my fixed bias system and one I should have foreseen, was that the quiescent current drawn by the output devices changed considerably with fluctuations in the mains voltage as is to be expected with simple triode technology. The old cathode bias system had a degree of negative feedback which opposed this tendency but has now been eliminated by being bypassed. To help mitigate this I could perhaps incorporate lower value separate cathode resistors to have a low degree of stabilisation to reduce the fluctuation but this would involve altering the existing layout too much. Another possibility would be to build on off-board regulator for the high tension supply, but this could be considered overkill. For now I think I'll tolerate the variations and do some tests to see how much variation could be expected for a certain mains voltage change. The ultimate 'cure' would be to have a matched pair of output devices, but... you 'avin a laff??!
This dear old amplifier suffers from a couple of issues which would be considered unacceptable in these days of quiet solid-state amplifiers, namely hum and noise. The white noise is more like the sound of frying bacon than true white noise, and is no doubt due to those big old carbon resistors in the anode and grid circuits generating this as the electrons meander their way through the irregularities in the resistive material. It is relatively unobtrusive in use and only really audible when the amplifier is turned up on quiet passages or with no input signal at all. In any case I consider it quite quaint, and have no intention of trying to remedy it by replacing the suspect resistors as this is all part of the amplifier's character.
The hum, however, was a bit of a problem as it was quite conspicuous. One minor source of hum is direct induction into the speaker transformer from the adjacent mains transformer, but this is at a low level and difficult to cure without physically moving the transformers apart, which of course I have no intention of doing. There was another source of 100Hz hum which was at a higher level and seemed to be injected into the amplification chain after the volume control, as the position of this made no difference. Altering the bass control, however, did make a difference, suggesting that the hum was being generated around the V3 stage. It was unlikely to be due to insufficient supply filtering as otherwise this would have affected V1 and V2, which it didn't, nicely ruling that out. The most likely remaining scenario was poor layout causing power supply ripple currents to be injected into the cathode circuit of V3, so I had a look for this.
With the base plate once again removed, I examined the layout and could see that the cathode bias resistor and its associated bypass capacitor were connected to the same point as the initial reservoir capacitor's negative lead, and these to the chassis via a length of wire. It would seem that there was enough current flowing here to cause an AC voltage to appear across the wire connecting to the chassis, and this was being injected into the cathode circuit of V3 as I suspected. Isolating the bias resistor and its capacitor from this point and connecting them directly to the chassis using V3's metal can earthing tag cured the problem. There is still some very low level hum, possibly being caused by similar layout issues around other valves, but it simply isn't enough to warrant messing with the circuitry anymore, and I'm now very happy with the way this vintage amplifier works.
I'm going to take the opportunity to have a bit of a moan about the audiofools out there - you know, the ones who believe that expensive copper speaker cables improves the sound over cheap ones of the same cross sectional area, and believe that early triodes somehow have some superior sonic ability over later valve developments, and the better transistorised amplifiers.
Much as I adore these old amplifiers with the warm glow of the valves and the big transformers, the hard fact is that they are sonically inferior to a well-designed solid state amplifier. As is well-known, the origin of this 'valves sound better' myth rests squarely with guitarists and their liking for the kind of distortion valve amplifiers produce when overdriven, far more musical than the hard clipping experienced with solid state units. Distortion is the key word here - we are talking about a distorted output, not clean sound, and this is the part that has been so sadly misunderstood by the audiofool brigade. Someone obviously once said that a valve amp made his guitar sound better, and this has been misunderstood and translated into something completely false.
The result of this is that output devices such as the PX25, PP5/400 and the P27/500 now sell for stupid amounts of money - over 500 quid each - simply because some idiots have perpetuated the myth that somehow these early valves are better than what came later, and have opened up a market for new amplifiers using the limited supply of these old valves. What this means is that genuine vintage equipment such as the amplifier featured here all too often sit on shelves incomplete and unused because the cost of replacing a failed valve has become prohibitive. Quite frankly I'm now afraid to operate this amplifier in case the heater in one of the output devices should fail, turning this old and rare piece of audio history into a rather heavy doorstop, as I wouldn't be able afford to replace one of the output devices at these obscenely inflated prices. Oh well, I suppose what people do with their money is up to them, but I think it's a crying shame as there are superior valves out there in plentiful supply, and some such as the EL34 are currently being manufactured so why use up the stock of rarities on new projects?
Any adverse comments on these last paragraphs won't even receive a response from me, so don't waste your time - you'll not change the way I feel.
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© Andrew Westcott 2003 - 2021