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Home > Services > Valve Amplifier Repairs

Valve Amplifier Repairs

Servicesound can offer a complete service for your amplifying needs. From servicing to restoration, from repairs to rebuilds. Please go to our 'CONTACT US' page for more details.

The Technology

Valve amplification has been around for over 80 years with most valve technology devised in the 1930's and 1940's. Most modern valve amplifiers are based on this technology. Little is new today apart from the higher specification components available and some of the outrageous prices asked for them. In Servicesound's opinion most of these esoteric parts are based on the 'Emperors new clothes' syndrome. They are very expensive and little different than a similar items at a fraction of the cost. ServiceSound use the best sounding, cost effective parts, which goes back to the design ethic of the 30's and 40's, "Forget what it says on the specification sheet! What does it sound like?"

The Sound

In Servicesound's humble opinion valve amplifiers sound much better than transistor ones, there are a few exceptions as the Sugden A21 gets pretty close.Fig.1 A pair of Shanling
amps converted for UK use.
Shanling mono blocks converted for UK use.
People listening to a good valve amp for the first time are 'blown away' by the experience. Valves have a humanity and clarity about them as if they have a 'soul'. Apart from the warmth of the sound there is also the visual aspect of lots of little glass tubes glowing away, a bit poetic perhaps but then ancient technology often is.


Some people are of the opinion that valves are unreliable. Valve amplifiers are in fact more reliable than transistor amplifiers. Most of the large airports around the world still run valve driven radar systems because of this. The USA is the world's biggest importer of valves for their radar sets. The Russian defence utility is almost completely valve based. Their fighter planes and ships are valve based. During a conflict valves are much less likely to be damaged from magnetic pulses and interference spikes. Valves should be left on all the time, 24 hours a day 365 days a year! If a valve does fail it is much less likely to harm its neighbour. Transistors on the other hand can blow up all sorts of other parts when they fail. A good quality valves should last for 10 years or more!

The valve matching myth

There seems to be a common misnomer that valves used in 'push-pull' out put stages should be matched. Having thought about this it occurred to me that a new matched pair of valves will go out of balance within a few days of use so negating the matching procedure however one should never mix new and old or different makes of paired valves. The use of decent quality valves in the first place and making sure they are operating correctly will ensure they continue to function perfectly for many years. Looking through old design books I have not found one reference to out put valve matching and that includes the design bible, the Mullard book of amplifier design.


In order for any valve to work properly they must be given the correct voltages. Biasing is the method by which a valve is held in a stable condition within its best operating limits. The famous Nagra power amps
Fig.2 A pair of Nagra power amps.
There are two methods of biasing. Grid and Cathode. Grid biasing forces a valve into a stable condition by applying a measured negative voltage to its control grid. Cathode biasing, by the use carefully chosen resistor values, allows the valve to bias itself automatically. In Servicesound's opinion Cathode biasing is much safer and sounds smoother. The drawback with Grid biasing is that there is no protection for the valve as the Cathode is taken directly to earth. If an output valve flashes over, the full HT of up to 600 volts is shorted out. Cathode biasing uses a resistor in the cathode circuit and the worst that can happen in the event of a valve failing is that resistor burns out. Another advantage is that no setting up is needed when re-valveing. With grid biasing it is necessary for the output valve bias to be set up properly otherwise there is a risk of damaging your new valves or they will not be operating correctly leading to loss of power and distortion.

Important issues with sound quality ~ Power ratings

A lot of amplifiers are optimistically rated by their manufacturers. Many claim to achieve power outputs well beyond the capabilities of the valves used. As an example, a pair of EL34 operating in class A pentode can, at the very most, produce 25 Watts RMS of power and yet many companies quote 40 watts or more.

Depending on the speakers used and their efficiency, most of the time we will only be using about 5 watts of power when listening to music. There is little point in having an amplifier that can produce vast amounts of energy that will be hardly ever used. It's a bit like having a 200MPH car and all it does is drive in the city!

The most important item in sound reproduction is the amount of distortion present at these sorts of power outputs, distortion is commonly measured as; THD or, Total Harmonic Distortion, the lower the THD the better the sound and at 10 Watts for a maximum rated 20 Watt amplifier the THD should be less than 0.1%. THD represents the combination of various corrupted signals caused by incorrect bias conditions, failing components and worn out valves.

As mentioned above, biasing is crucial to delivering the best sound and ensuring the valves operate at their best. Most Grid biased amplifiers must be manually adjusted to the voltage levels provided by the manufacturer, even then, they do not take into account the possibility of the valves, if they have been replaced, being different to those used during the amplifiers design. In Servicesound's opinion the only way to accurately set up the bias conditions is to measure the THD and adjust the bias for the lowest figure, this also has the added benefit of showing up any other sources of distortion. As an example, The Leak Stereo 20 is listed as having a THD of 0.1% at 10 watts RMS, the modifications and setting up enables this to fall to 0.05% at 8 Watts which is why there is an enormous improvement in the quality and response of the sound. This method of setting up applies to both Cathode and Grid biasing arrangements. Almost every amplifier Servicesound have attended to has been improved, sometimes drastically, by this method of setting up.

What we do

All valve amplifiers go through much the same procedure. A previously unserviced amplifier will usually need all the power supply high tension capacitors replacing, this along with replacing all the coupling and bypass capacitors will ensure a long and reliable service. Unless the output valves measure perfectly these too are replaced but quite often the smaller driver and input valves are OK especially if they are the original makes used, this also applies to pre amplifiers.

All resistors are checked for value and accuracy and any that have overheated are replaced. The chassis is cleaned inside and out, often there will be dry and or overheated soldered joints, these are remade and if any wires have become burnt with heat these are replaced with PTFE silver cable.

All the valves and their bases are cleaned and any bases or their internal contacts are replaced.

Controls, switches, connectors etc are all cleaned and those that refuse to operate perfectly are replaced. Once everything that needs to be replaced has been, the amplifi er is slowly powered up and all the new capacitors aged over a period of an hour to ensure reliability. All the various voltages are checked and anything amiss is corrected.

Those amplifiers with grid bias are set up correctly and then left to settle in for a while.

The maximum output power is measured and then the distortion level. All amplifi ers benefit from a reduction in distortion and we try to reduce it to the lowest level possible by altering one or two component values, the amplifi er is then 'soak tested' (which means leaving it on for a long time! not sticking it in the bath and turning the taps on!) and then checked again.

Many amplifiers benefit from updating by either the use of modern components or altering the circuit. Little things like replacing poor speaker terminals with decent ones do make a difference.

Leak Amplifiers

Servicesound has developed an upgrade for the LEAK ST20 and TL12 range of amplifiers. This consists of redesigning the input, driver circuit and power supply feeds resulting in an extended bass response and a faster top end. This does not in any way damage the superb sweetness of this wonderful amplifi er. In addition, the silly speaker screws are replaced with proper WBT terminal posts. The speaker impedance selector panels on the top of the transformers are replaced with perforated stainless steel grill, this is also done on the mains trasformer. The speaker tappings are now situated underneath and all the transformer wiring replaced with PTFE silver cable. All the valves are replaced with Electro Harmonix and the GZ34 rectifier valve is replaced with a plug in solid state alternative. This is an important alteration as the mains transformer is conservatively rated and gets very hot. Replacing the rectifier with a solid state version reduces the loading on the mains transformer by about 20%, this and the replacement grills ensures the mains transformer remains fairly cool.

Please enquire about the fixed price for this.


Most valve preamplifiers made before 1980 are pretty poor. They have tone controls and complicated switching arrangements, they also have complex feedback systems and poor sockets. All this contrives to ruin the signal, using these preamplifi ers with a freshly overhauled and perhaps modified power amplifier will negate most of the advantages gained by all the work done. Often the case and chassis of the original preamplifier can be used as a basis to build a new one thus keeping the looks and gaining the sound quality, Servicesound has often done this with great success.

Customer Comments

Since repairing and upgrading the power supply of my Croft pre-amp the sound has obtained a new level of authority. Thank-you Geoff, I am impressed!

Rob. Seaford, East Sussex

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