Ben Duncan concludes his examination of things glowing with a few recommendations on quality, replacements and availability overseas.

UNLIKE transistors, tubes wear out steadily. There are no sudden demises as the material on the cathode which emits electrons gets used up gradually. The problem of steady wear out over thousands of hours is that there's no definite place to draw the line. The sound of the amp may change slowly as the valves 'mature', but you might not notice it.

Output valves wear out fastest, and wealthy touring bands commonly get new valves fitted up after every major gig, or at least once a week. At the other extreme, a valve can keep going for five, 10 or 15 years, particularly if your amp is semi-retired.

Provided your amp's valves don't show any signs of ill health (like glowing red or blue, or giving forth crackling noises), it's cool to run them until you feel like you've had your money's worth. But if in doubt, it's a good move to fit new valves a few days before an important gig, so they're 'bedded in'.

In their heyday, the 1940's & 50's, valves were manufactured by the million. Today, most of Europe's production lines have ceased; there's Siemens in Germany, and Philips in Holland. At home, the last manufacturer, London's M.O. valve Co, is scheduled to close down in a few months time, when its final fling is over which involves manufacturing a "life time" supply of KT88 Gold Lion valves, for a stockholder who's sure that there'll still be some HM guitarists and valve Hi-Fi amplifiers around in the late 1990's.

Valves, pardon, tubes are still manufactured in the USA, on a similarly diminished scale. But for the other half of the planet, the story's very different: a factory in China is about to begin making a classic British valve from the 50's (the KT88) while most of the world's consumption happens back in the USSR, and eastern Europe.

One thing is certain: large stocks of valves still exist, and even esoteric types last made in the 50's can still be had, if only you know where to source them. Trouble is, valves made in Eastern Europe with UK/European/US part Numbers don't match up to the high standards implicit in the number's original specs. The tide of Gorbachov-ism may render East Block goods better in the future, but for now the quality of valves marked 'foreign' (for which read 'east of Austria') or of branded quality (eg. "Vlostok brand") are for the most part shoddy, and unsatisfactory for Rock 'n' Roll.

If you can't get the spares you need from your local music shop, or you're seeking a good deal on a bulk buy, you can get valves by mail order from the suppliers listed below.

P.M. Components Ltd., (Contact Details), Contact: Mr. Watson
Colomor (Electronics) Ltd., (Contact Details)
Langrex Supplies Ltd., (Contact Details)

These and other companies regularly advertise hundreds of valves and their prices in electronic, er... organs, like 'Electronics & Wireless World' magazine. The people on the other end of the phone are also very informative, so if you're not sure which type you need, or which valve is best for a given budget, don't be afraid to ask for advice.

The problem of 'which valve?' is dealt with in figure 1, a master guide to substitution. Starting in the left-hand column, you can see the common, classic valves. These are basic, standard types. If they're made by any of the famous makers listed in the table in figure 2, they'll be of high quality, but no frills, like a Fiesta 1.1.

Moving into the second column, we have a choice of alternative numbers. These are exactly the same basic design of valve, but they're built and/or selected to a higher spec; like a Ghia or XR2 model.

The 'M' series are 'special quality' commercial versions, whereas those with the 'CV' designation are built to military specifications. When ordering, ask for 'special quality to UK specification'. These will cost more than the basic valve in column 1, but noise (hiss) levels and microphony (pinging noises) will be less, and thanks to toughened suspension, they should keep going longer in spite of the shock and vibration meted out at before, during and after every gig.

In the USA, familiar UK/European valves are made to an identical spec, but under different numbers. Looking into column 3, an American ECC83 is called 12AX7. Similarly, in the special quality column, the 'WA' suffix indicates a military specification, like our own M.O.D.'s 'CV' code. In general, you'll find tubes with US numbers are just as readily available as the UK/European types, ie. the alternatives in columns 1 & 3 and 2 & 4 are fully interchangeable.

If you're stuck in an exotic country where they disclaim all knowledge of mainstream valve numbers, try quoting the ones in brackets. These include long obsolete equivalents from Cossor and Mazda, and Japanese numbers. You wouldn't normally ask for them in the UK (they're scarce, so they'd cost more), but they might get you out of a tight spot in deepest Portugal, or wherever.

Tube Replacement Tips

Beware of inferior 6550 tubes. This US model reliably replaces the KT88 if it's made by RCA (Radio Corporation America). If it comes from another maker, it's probably unsuitable, and it's best to steer well clear of any with the suffix 'A' (ie. 6550A).

The first stop for replacement is your local music shop. Manufacturers like Marshall have wisely stockpiled thousands of 'Grade A' spares, available from their dealers. Replacement (Mesa) Boogie valves are also warmly recommended, if your own amplifier can make use of the same type numbers. If you're buying a new instrument amplifier, or even a 'new' secondhand amp, it's a good idea to buy up kits of spare valves at the same time — enough to last you a few years, if you can afford it. The reasoning is economic. Maybe the spares will be available in 1993, but on past experience, as stocks have become scarcer, prices have risen inexorably. And unlike other electronic components like batteries, valves shouldn't deteriorate when stored: the 'Use By Date' is somewhere in the 21st century!

If you want a special quality version of the KT66, you'll notice there's no CV version (the military didn't use it), but PM components have informed us that there's a limited stock of 'Yellow Spot' versions. Being specially selected, a hundred of the best creamed-off from a batch of thousands, these amount to a good thing.

If your amp's output valves glow purple or blue, replace them immediately. If you see (or hear) sparks leaping across, check your speaker connectors — and replace the damaged valves. If the problem continues, get the amp checked out. Don't be tempted to 'save' money by replacing one output tube at a time. It's OK in an emergency, or as a stop gap, but running a new tube alongside three decrepit ones is potentially damaging if carried on for any length of time, especially if you play hard.

Coolout

Power Amplifiers aren't 100% efficient; some of the juice from the wall socket gets turned into heat. In general, the higher the power, the hotter the amp's guts. Trouble is, the lifespan of tubes and other electronic components is related to temperature in a dramatic fashion: every extra 10°C halves their lifespan. So the parts in a fan cooled tranny amplifier, running at 30°C, will on average last 32 times longer than the parts in a valve instrument amplifier, cooking at 80°C. This explains why valve instrument amplifier's 'service intervals' are more frequent. Aside from the bottles, resistors and electrolytic capacitors regularly decay, or fail outright.

Because the hastening of death through excess heat applies to electronics across the board, the makers of more exotic technology (like control systems, telecoms and computers) have twisted the arms of cooling fan manufacturers: Today, 4½" fans are efficient and relatively inexpensive. If you can afford £20 to £30 to get a fan fitted to your prized instrument amp, it should soon pay for itself, by doubling the lifespan of the tubes.