Valve Surgery — Repairing Tube Instrument Amplifiers
Valve Surgery explained
Despite the antiquity of most tube instrument amp designs, the technology remains relevant today because tube amps retain the ability to generate a unique and useful set of sounds, whether you play New Wave, Funk, C+W, Reggae or the blues. At this point, it's sobering to note that the op-amps and transistors found in modern gear are only Mk. V versions of devices that first appeared twenty or more years ago. The BC107 was introduced in 1963, whilst the direct forerunner to the OP27 super-chip is uA710, introduced in 1962! Meanwhile, the refinement of tube technology continues in the 80s courtesy of American audiophiles (who mix tubes with the latest analogue devices and the most refined passive components and ferrite materials) and the engineers who maintain and upgrade thousands of back-line amps for gruelling tours and studio sessions. In fact, providing you're prepared spend enough money and put up with the bulk and weight, 80s tube technology can outshine Bipolar transistor amplifiers in almost every respect.
For this article, a basic knowledge of tubes is assumed: If you're not au fait in this region, it's worth pointing out that tube fundamentals are much easier to grasp on an intuitive, down-to-earth level than the abstractions of semiconductor theory, and furthermore, whilst the subtleties of tube amplifier performance can be just as involved as those of semiconductor amps, tube circuitry is simple, spare and much less convoluted.
The failure modes and repair techniques relevant to tube instrument amps are radically different to those relating to transistor equipment. Tube amps tend to suffer a gradual, overall deterioration rather than the localised, catastrophic failure that characterises transistor amps. Moreover, the most vital and susceptible components — the tubes — are readily checked out by comparison with known-to-be-good replacements. And from the repair angle, tube technology demands a different type of detective action that revolves around a close physical examination of components, electrical tests and iterative substitution tests: A sort of ad-lib empirical surgery.
The first and most important region of new cognizance when you begin to work on tube amps is the presence of very high voltages within the chassis even some time after the power has been disconnected. Often the awareness is developed when you grasp the wrong end of a 485 Volt HT rail, but whilst the lesson is then well heeded, not everyone survives the experience. Tube amplifiers can source lethal voltages and so some safety rules are in order:
1) Take care to support the chassis and valves firmly and safely, as a sudden movement could cause you to come into contact with live terminals.
2) Perform as much work as possible with the amp switched off, and disconnected from the mains, and don't be tempted to leave the amp on whilst working simply because it takes a long time to warm up! If the amp has a 'standby' switch, which retains only the heater power (so speeding the warm up), it's OK to make use of this providing you cover all mains connections with suitable insulation.
3) Build and use the test rigs depicted in Figures 1a & b. The first circuit is used to discharge HT reservoir capacitors in order to make an amp safe to work on after it's just been turned off. First connect the bottom side of the circuit firmly to chassis, then prod the +ve side of each HT reservoir capacitor in the amp for 3 seconds (or until the neon extinguishes) in turn. To be doubly safe, it's a good idea to use a test meter to confirm that the voltage on all the HT capacitors has fallen below 80v before commencing work.
4) If in doubt — discharge it! Then measure to be sure.
5) From time to time, it'll be necessary to work on a tube amp whilst it's switched on. Here, observe the precautions that apply in general when probing around inside live equipment, viz: (a) Keep your left hand in your back pocket at all times, (b) power the gear via a 30mA current balance ELCB if possible, (c) stand on a dry surface, preferably wooden or carpeted. The second test jig (Figure 1b) is a neon flasher to remind you of live status and prevent you from forgetting not to touch this... ouch!... bang!... Mount both test jigs in plastic boxes and equip them with high quality HT probes.
Although elaborate and logical flow diagrams exist for locating faults in tube amplifiers, these almost invariably preclude the symptoms you're beset with. Instead, we'll concentrate on techniques which involve the development of intuition and a sensitivity for physical signs of ill health in the components. Also, no specific problem will be assumed: we'll just go through the imaginary amp step by step as if it were being reconditioned. This action makes sense regardless of the nature of the fault because unless the amp is very new or has just been extensively serviced, the high temperatures inherent to tube circuits spell an overall deterioration in the components. Before starting work, you'll need to equip yourself with a data book to identify tube pinouts, eg: 'Radio Valve and Transistor Data', compiled by A. M. Ball and published by Butterworths.
First of all, with the amp switched off — and everything that might bite discharged — remove the chassis and take a look at the components on the underside. If any of these are burnt or charred, try to establish their values. If measurement and/or reading (after cleaning) doesn't yield results, you'll need to accurately establish and record the component's position in the circuit, and if possible, its function. Having done this, you can look inside another, similar amp, phone the manufacturer's technical department or look at a range of tube amp circuits to establish a typical value for the damaged component.
After replacing burnt out resistors, carefully examine the circuitry, looking for a cause, typically a short circuit caused by adjacent components coming into contact, the loss of insulation over a length of wire as a result of high temperatures, or a faulty valve. Don't switch on the amp again until you've discovered and rectified the likely cause of the burn out. Once obvious physical damage has been cleared up, switch on the amp and check if the valves light up. If not, and assuming the mains neon lights, trace back the 6.3V AC heater feed from the relevant valve socket pins to the transformer with a test meter. If the amp hums or hisses slightly once it's warmed up, it's likely that the HT voltages are more or less correct; otherwise carefully trace the HT from the transformer, via the rectifier, along the chain of reservoir/smoothing capacitors and thence to the valve pins. Be sure to use a well insulated test probe! A valve data book will specify typical HT voltages found at the anode and screen grid pins. If measurements reveal voltages greater than ±20% from the quoted value, this suggests an open or partial short circuit somewhere in the HT chain; switch off and discharge before investigating. Once the integrity of the HT and heater voltages has been established, you can try plugging in new valves.
Now take a look at the remainder of the components. Beware especially of discolouration, brittle encapsulations and perished or hardened rubber around the base of electrolytic capacitors. Use an ohmmeter to check resistors with discoloured or apparently decaying bodies. If the value deviates by more than 20% from the colour band's indication, carefully chop one lead to make the test out of circuit. If there's still a deviation, which is greater than 20%, replace the resistor, but do first make certain that you're not simply misreading a faded orange band as brown, and therefore thinking that a 100R component has 'gone high' when it measures 10k! In general, the carbon composition resistors found in older tube amps manifest a high resistance when they expire, thus a faulty 100R resistor is likely to read either slightly high — say 270R — or very high — say 1M2. Thus colour band misapprehension (eg: 100R or 10k?) should be readily apparent. Also note that Rock 'n' Roll tube amps aren't terribly fussy about precise values: resistors which are, say, 25.5% away from the nominal won't necessarily prevent the amp from working. Rather, they are best replaced because the error suggests the resistor's on the way out, and further more serious changes in resistance are likely if the dying part isn't extracted. Incidentally, to preserve magical sound qualities, resistors should be replaced with high stability parts (ie. metal film or oxide) having values within 5% or so of the original. For instance, if a 20% resistor marked 100k measures 118k and has to be replaced, make the new resistor 120k, ie: within 5% of 118k.
Capacitors with plastic, mica and paper dielectrics (viz: Values < 1uF) rarely give problems, unless broken encapsulation has allowed the ingress of moisture. Electrolytic capacitors on the other hand are a persistent pain, especially if they're old types with little tolerance to high temperatures. Because the impedances in tube circuits tend to be high, large value electrolytics aren't as prevalent as they are in transistor circuits and can be narrowed down to two key areas. Cathode bypass capacitors are typically 25 to 50uF and have relatively low voltage ratings in the region 16 to 40 volts. Not surprisingly, these are found in series with the tube cathodes (see Figure 2), and in parallel with the local feedback resistor. If the cathode bypass goes short circuit, the tube will lose its bias voltage and severe distortion and/or poor to zero output will result. Conversely, an open circuit or decrease in the cathode bypass capacitor's value will manifest as a lack of sensitivity or bass respectively. Even without these symptoms, replacement with a modern, high quality component is recommended as a matter of course, the only exception being in respect of amps of fairly recent manufacture, say later than 1978.
HT reservoir and decoupling capacitors are usually found in large, chassis mounted cans; often two or three capacitors are mounted in a single can, with a common -ve terminal. Small, axial versions hidden on the tag boards are also found (ouch!). They're all easily identifiable by their high voltage ratings (150V up to 500 Volts) and relatively small values. The majority of Rock 'n' Roll tube amps hum unashamedly, but if the hum is louder than usual, and has a definite and consistent tone, this suggests poor ripple rejection in the power supply and thus a failure in one or more of the smoothing capacitors. The usual failure mode is a reduction of capacitance leading to an open circuit; more rarely, a dying reservoir capacitor will short circuit, and destroy the rectifiers and/or blow the HT fuse. Assuming an open circuit capacitor, testing is simply a matter of connecting a new capacitor across the suspect component, then switching on and measuring or listening for a reduction in hum or a change in the nature of the hum. Problems arise when one or more sections of an electrolytic can are found to be faulty, as replacements with even vaguely similar physical and electrical specifications won't be readily available, assuming you can't obtain spares from the manufacturer. Fortunately, high voltage electrolytics per se are still readily available for TV repairs. If the voltage rating you require isn't available, aim for the next highest rating. If the value you require is greater than the highest rating available, you can wire two identical electrolytics in series (eg: 275V + 275V = 550 V) with parallel voltage sharing resistors, to double the voltage rating. Don't forget that the series connection halves the capacitance, so you'll need to double up the value of each capacitor to end up where you began (see Figure 3). Capacitance values rarely present a problem, as the values found in audio amps are generally smaller than those required for TVs, and uprating will do no harm. However, the first capacitor in the reservoir/smoothing/decoupling chain (C1 in Figure 3) shouldn't be much bigger than the original, otherwise there's a risk of the inrush current at switch on blowing the rectifier.
Physical incompatibilities can be more teething, as can encapsulated equivalents will tend to be smaller (a reflection of component miniaturisation) and often replacements with axial leads will be solely available. Here, some inspired work with a drill, nuts and bolts, capacitor clips, tie-wraps, tag boards and RTV 76 (liquid rubber adhesive) can secure a roadworthy mounting. Note that a replacement reservoir capacitor can be mounted almost anywhere on the chassis, so long as the extended positive connecting wire is separated from other cable and the location, if on the topside of the chassis, is well away from the injurious heat radiated by the output valves.
The output transformer (see Figure 2) and smoothing choke (Figure 3) are two components that rarely give problems, excepting outright failure through severe abuse. The same applies to the mains transformer, which usually expires when the 110 volt tapping is utilised to 'make the amp more powerful'... Output transformers are generally wound especially for the amp in question, so you'll usually be limited to seeking a replacement from the manufacturer or cannibalising another amp of the same model. If all else fails, it's possible to have a new transformer wound by a specialist manufacturer, but the cost is likely to be excessive for all but the most special and beloved amps. The choke is also likely to have been specially wound, but in the absence of a sensibly priced replacement, it can be substituted by a wirewound resistor (circa 25 watts, with a value in the 100R to 4k range) with little loss of performance, especially if you uprate the value of the reservoir capacitors. Transformer and choke tests can be simply carried out with an ohmeter, looking for sensibly high readings on the choke and the output transformer's primary windings. The secondary windings of the latter will normally give a reading of a few ohms. The partial failure of an output transformer is usually characterised by very low output, distortion and overheating in the output valves, which may glow red, or alternatively, blue or purple, depending on what sort of mood they're in...
The high temperature suffered by valve circuits not only kills passive components; contacts and terminals also suffer. The most significant deterioration occurs naturally enough in the region of the valve pins. Here, thick layers of oxidised metal can act as primitive rectifiers. To the extent that this effect has a bearing on the magic qualities of some tube instrument amps, there's no need to worry unduly about the effect. However, it's a good idea to clean the excess tarnish from the socket either when you change the valves, or annually, as taken to extremes the oxide layer can result in annoying intermittency or a low output. Valve pins are readily cleaned with fine emery paper, followed by a rinse in meths. The valve socket contacts are less accessible and thus more awkward. A matchstick dipped in meths works well on output valve sockets. For the remaining valves, spray the sockets with an aerosol foam cleanser. Follow this with meths, plugging and unplugging the valve to work the fluid into the socket and loosen up the surface contamination. Organic solvents have a habit of dissolving the identification printed on valve bodies, so protect the lettering beforehand with a strip of insulating tape. Don't use Sellotape, because when this is removed, the lettering will come with it! Also, mark the position of the valves on a sketch plan before removing them.
Jack sockets should remain clean, as the surface is abraded every time you plug in your instrument, but nevertheless, tarnishing can occur to the extent of causing intermittent sound, especially if the contact's springiness has wilted with age. In particular, the amp's input can fail to ground (via the springy switch contact on the socket) when the plug is withdrawn, which in turn can precipitate instability and a loud, edgy buzzing. Sockets in this condition are best replaced by quality types with gold plated contacts and beefy springs, eg. Rendar.
High temperatures also cause dry — or, more accurately — crystalline solder joints. Here an innocuous looking lump of solder can generate a whole series of red herrings, not to mention expletives and purple valve-glow... Generally, the symptom will be one of intermittency, but excessive distortion, low output and a host of other maladies can often be traced in the main to a single crystallised solder joint. Detection involves poking and tapping every joint in turn with a plastic rod. A freezer fluid can also be a great help, but do make sure that the type you intend to use can be safely sprayed onto high voltage terminals. As crystallization is the result of long term high temperatures, look for it in the proximity of components which may have attained and sustained very high temperature. The output tubes and wire-wound resistors are prime suspects. If all else fails, simply run over all the joints around the power supply and output stage with an iron and some fresh solder.
Tube amps have a reputation for poor reliability because they soak in temperatures 3 to 4 times higher than those even the most rugged transistor amps would withstand. Temperatures of 160 to 250°C are typical, especially when the amp is poorly ventilated and cruelly overdriven. Adding fan cooling will dramatically increase the reliability and should be seriously considered, especially since it will more than quadruple the longevity of (expensive) output tubes.