These are perhaps the most risky purchase and the most expensive to put right. The ubiquitous Quad II amplifier is forever in demand and non working units sell on Ebay for what seems silly money.
Small faults on these and indeed many other valve amplifiers can cause very costly faults, failing (leaky electrically and sometimes physically) coupling capacitors can cause the output valves to draw too much current, the valves are fairly robust and will often work overdriven for some time; long enough to burn out the mains or output transformer, both very costly to replace. If an amplifier has black tar (in the case of Quad) or wax deposited on the chassis it’s a sign that the transformer has run too hot and may have or is about to fail.
Valve amplifiers hum! A lot of people accept a bit of hum as normal and live with it, and indeed some do but often it can be removed or at least reduced. The most common cause for hum is caused by earth loops, this is not the amplifier but the way its connected to its signal source or other parts of the system, this is a big subject in itself and too complex to address in a few words, but put simply, if an amplifier does not hum when it is just powered on with speakers connected, but nothing else (no inputs) and it does not hum then its not the amplifier, if it hums as soon as the input is connected then its most likely an earth loop or possibly a poor quality cable.
On a cautionary note many early amplifiers didn’t have any chassis earthing and relied on being connected to a control unit or pre-amplifier to earth the chassis. If you are using an old valve amplifier with modern front end equipment, (which is often double insulated and does not require earthing) make certain the metal chassis is earthed in some way; or you might be in for shock, and not the sort you recover from!
Buying Vintage Amplifiers
A constant stream of very appealing vintage amplifiers are available through on line sources but as ever with second hand gear it's buyer beware. It's safest to purchase something that you can see and hear working. Something that has been in regular use by the owner is much more likely to be reliable longer term than something that sat in a loft or garage for twenty years.
The seventies and eighties was the golden age for transistor amplifiers with competitors offering bigger and more powerful amplifiers to the market. The products from that era are as good if not better than modern units. More recent products have moved on in some respects in that modern semi conductors (transistors and diodes) are much more robust than they used to be so are less likely to fail. Early amplifiers can be upgraded with modern parts that will improve reliability.
Most of the problems on vintage transistor amplifiers are as the result of transistors failing, this can show itself in two ways. Firstly noise or crackle on the output which gets worse as the unit warms up, this is generally down to small transistors in the pre-amplifier stages failing, if one has failed it's often a sign that others may follow, and it's often good practice to replace them all, or at least all of the same type as the failed part.
Secondly, an amplifier that powers up, but never actually works or simply blows fuses is a sign that the power output stage has failed. Most amplifiers have speaker protection circuits that only connect the amplifier to speakers once the circuit has detected that the output is stable and will not damage the speaker, often evidenced by a relay clicking a short time after power is applied.
The power output stages in most amplifiers are ‘DC coupled’ this is a good thing as far as the sound is concerned as the amplifier will respond from DC (0Hz) to some upper limit determined by design (perhaps 100KHz), although this is normally limited to 20Hz to around 20KHz to avoid very low or very high frequencies reaching and possibly destroying the speakers.
The disadvantage of DC coupled circuits is a that a single part failing can cause a domino effect which can destroy every transistor in the output stage, large amplifiers that have numerous transistors ’strapped’ in parallel are most susceptible and expensive to repair as a single failure will at the very least take out its ‘complementary’ partner and if the fuse does not blow at this stage the entire output stage perhaps the driver stage and even the power supply circuits.
There is a belief, mostly fostered by internet forums, that by replacing all the capacitors in any piece of audio equipment that has failed, or does not sound nice will fix the fault, or make it sound nicer, or perhaps not. In older valve gear then this might be a wise precaution and its certainly worth testing every capacitor and come to that, every resistor.
In speakers with complex cross over circuits, especially those using non-polarised electrolytic capacitors then a full recap can often result in very worthwhile improvements, although tweeters loose their efficiency over time so if the top end is weak I would look here first.
The benefits of re-capping transistor amplifiers is much harder to quantify and depends so much on how the unit has been used and indeed the type of capacitor employed. If a unit has had regular light use then the chances are the majority of the capacitors will be fine, the odd failure can be expected but these can be replaced as required. If a unit has had a hard life, left switched on 24/7, or it runs hot by design, or incorrect set up then failures will be more common and re-capping might be worthwhile, simply to maintain reliability.
A unit that’s not been used for years especially if stored in a cold environment is the most likely to be problematic, there are numerous reasons for this but perhaps the most common problem is as the result of electrolytic capacitors ’deforming’ as they have to be used to maintain the original characteristics. They can sometimes be successfully ’reformed’ by applying small and gradually increasing voltages to them, often achieved by powering up a unit slowly, over many hours with a Variac (variable transformer).