HiFi Hacks

Valve based amplifiers have a reputation among the Hi Fi community of producing a very pleasant ‘smooth’ sound. They have different sonic characteristics to modern transistor based designs, typically the distortion from a valve based amplifier will be primarily of even order, which sound to the human ear rather more natural and musical than the harsh, un-natural odd order harmonics produced by transistors. They also have a reputation of being very expensive to buy. This may be true of the modern valve amplifier, however refurbishing an old valve amplifier is an easy way to experiment with the sound of valve amps without making the major investment of buying a new one.

It is also a satisfying task that promises the reward of listening to a DIY amplifier without needing the extreme amount of knowledge required for designing an amp from scratch. This page outlines the process of refurbishing an old valve Hi Fi amplifier, giving examples from the author’s experience.

Old amplifiers are getting more difficult to find. There was a period in the 80’s where it was generally accepted that transistor based amplifiers were superior to their valve based counterparts in most every way, and as a consequence valve amplifiers were being sold cheap or even thrown away. Some valve amplifiers can still be found however, the cheapest ones in garage or jumble sales. If you are looking to find an old valve amplifier for restoration into Hi Fi use, then it is important to get one worth restoring. Most amps I have seen out there are not going to make an easy project, at least not for hifi. Here is what to look for if you want to find a Hi Fi restoration project:

The amps to lookout for are those which are dedicated audio amplifiers. There is plenty of stuff out there which has valves in it but isn't an amplifier. There are a lot of radio sets, which have a lot of valves in them but a worthless amplifier stage, and there are a lot of mono/guitar amplifiers, given that there were no stereo sets until the end of the valve amp era this is no surprise. The valves to spot are the final stage output valves; once you know these you will have an idea as to the power of the amplifier. Some of the common codes to look out for are EL84, EL34, 5U4, ECC83, KT88, 6V6, 6SN7, there are plenty others.

A stereo audio amplifier is a lucky find, as are two identical mono blocks, as I found – I managed to get 2 identical monitor amplifiers from a Ham Radio flea market, with 2 EL84 tubes in push pull configuration, with a EF86 and ECC83 front end. All good news for Hi Fi. Apart from the valves, the transformers are the other major part that makes up the amplifier. There are usually 2 transformers on the amplifier, the power transformer and the audio output transformer. The power transformer's job is simply to change the line voltage to whatever voltages are needed by the circuits on the amp. This is usually a couple of 6.3V windings for the valve heaters, a high voltage winding (300-400 volts) and sometimes a 5 volt winding if the amplifier has a valve rectifier. Example rectifier tubes: 5U4, 5Y3. Some amplifiers had early silicon diodes and used them as rectifiers instead. The only thing you will have to look out for with power transformers is that they actually work, i.e. have no faulty windings. Unfortunately this cannot be tested visually, so unless the amp can be turned on an proved running, you may just have to take your chances.

The role of the audio output transformer is to match the impedance of the speaker to the valves. The valves are high voltage, low current devices, hence they have a high impedance, usually a few thousand ohms. The output transformer matches this to the 8 ohm speakers. The key to Hi Fi as far as audio output transformers go is size. In general, the larger the transformer the better the sound. This is because the low end frequency response of a transformer is directly related to the amount of magnetic flux in the core, and to accommodate this magnetic field takes iron. Of course modern audio output transformers can be purchased, which would probably be better thean the old one in every way, however they are very expensive, and might not fit etc. I think there is a line between restoring an amplifier and making a brand new one at great expense, and the audio transformers sit right on that line. Also note that electric guitar amplifier transformers are NOT suitable. Since the lowest note an electric guitar can play is 83Hz, these transformers take advantage of this, and do not reproduce frequencies lower than this very well. Guitar amp transformers are not known for their high audio quality throughout the upper frequencies either... Capacitors in the amplifier are one area where great improvements can be made (In fact some companies offer a ‘recapping’ service to rejuvenate valve based guitar amplifiers). The capacitors in really old amps were awful when new, let alone after decades of use and storage. The Electrolytic capacitors, which were used almost exclusively for values over 1uF and always for the main power supply, are made by rolling up thin foil plates, with liquid electrolyte between them. This electrolyte dries out over time, reducing capacity and in some cases causing the foil plates to ‘short’ together. In some cases the capacitor can be ‘reformed’ which heals the cap, but it will never be the same as new. In general it is good to replace all capacitors older than 10 years old. Luckily, compared to capacitors of the 60’s, today’s electrolytic capacitors are better in every way, one of the more notable being size.

Other parts are not as critical as they can all be replaced with superior modern equivalents. In fact usually their replacement is mandatory, as the capacitors dry out, the sockets get tarnished, and the bakelite parts get brittle with age etc. Things like power sockets and switches generally want to be replaced anyway as a matter of safety.

The mono amps that I find still had valves and actually worked still, once I installed modern power plugs. However the sound was awful, as the main capacitors had dried up over the years. There was a lot of hiss, and crackle as the capacitors reformed. They also stunk due to the years of dust baked onto the hot valves! However, it was a good start and I commenced the task of refurbishing the amps.

Opening the amplifiers up revealed a mess of old greasy capacitors and resistors, most of which had unintelligible labels due the build up of grime or else being burnt brown. I traced the schematic as best I could, this was made somewhat easier by searching the internet for similar schematics. Just by searching for amplifiers with the same line up of tubes, I managed to get a couple of schematics which helped greatly. Those components which I could not read I clipped out and measured with a meter.

Once I knew what parts I needed, I started ordering modern replacements. The amps I had were quite modern, being made in the 60’s, 60’s (There were date codes on the Electolytic capacitor), but even in that time electronic components have come a LONG way. The resistors I ordered were metal film parts, far superior in temperature stability and precision than the carbon compound parts removed. They new resistors were also quite smaller. The new main capacitors were about the same size, but double the capacity, and the signal path capacitors were so much smaller it's not even funny. With the size savings form the other components, I was able to fit in Polypropylene and polystyrene capacitors for those points in the signal path where it mattered the most. The wiring was also replaced; the old brittle insulated wire replaced with modern Teflon insulated copper wire (observing the insulation voltage rating of course). The tags on the transformers were the hardest to solder to, I’m not sure what type of solder was used when the amp was new, or if the age and thermal cycling had affected it, but the solder was almost impossible to melt, I had to get a huge 30W soldering iron just to melt it. Needless to say that solder was removed and replaced with new stuff. The valve sockets we tarnished, and cheap ones anyway, so were replaced with new ceramic ones with gold plated pin contacts.

The valves themselves although of high quality (they were Sylvania and Telefunken) were tested on an Avo valve tester and measured pretty old so I decided to replace them all. After much research on the net I decided upon JJ Tesla tubes, these seem suited to Hi Fi and are a reasonable compromise for cost. I purchased the replacement valves from Ned Carlson's website, but you can also buy them from the Tube Store, and various other places on the net.

After all the restoration the amps sounded great, and not much hum either, barely detectable when you put your ear to the woofer, so quite a good result.