RAISING THE TONE
in part two of our feature on valves, Jamie Dickson looks at maintenance and buying old valves, with adrian emsley
Last month, we joined amp-design wizard Adrian Emsley of Orange to find out more about how the major types of power valve influence the tone and performance of your amp.Taking up the theme again, Adrian explains that as valves near the end of their useful life they can let your amp down tonally – and, ultimately, it will fail altogether. Simple knocks and bumps are likely to take their toll first, he explains: “The main thing you’ll see is mechanical loosening up of the insides of the valve. This will manifest itself as rattles and microphony and things that sound like they’re being shaken. Or pops and crackles, if you’ve got a loose connection within the tube.And in a combo, that’s probably the first thing that’s gonna happen, before the valve actually wears out.”
If no mechanical issues affect the valve first, the process of gradual wearing out will make itself known more subtly.“There’s a material on the surface of the cathode that gets depleted because it ends up on the plate over time, due to the operation of the valve,” he explains.“As it wears out, it’s going to go down in power slightly. It’ll lose a bit of output and it’s probably going to sound a little lifeless, a little muffled. It won’t quite have the chime or the sparkle that it had.”
The last category of issues with valves is more spectacular – and potentially damaging to your amp.“The other thing that can happen is the valve can fail,”he says.“There are a few ways that can happen. One of the common ones is that it loses its grid bias and then it starts drawing as much current as it can and the plates glow cherry red. That will pull from the power supply and you’ll get a 100-cycle hum, plus one of the valves will look like a little nuclear bomb’s going off in it. So, that’s a common one.The other one is that it can short and blow the fuse, which is normally better in some ways, because it happens quickly and is less likely to damage something else in the circuit.”
Adrian adds that power valves are far more prone to wear than the small, three-element or ‘triode’ valves typically found in the amp’s preamp section, controlling preamp gain, EQ and so on.“As long as they don’t go microphonic, preamp valves can last 20 years,”he says.“[This is] because they’re not making anywhere near as much power as the
· “A valve can lose its grid bias and fail… if that happens one of the valves will look like a little nuclear bomb’s going off in it” ·
output tubes are.The output tubes are the real workers of an amp, so they’re going to wear down quicker.”
Given all of the above, there’s a thriving market for replacement valves. There’s also the increasingly popular option of buying so-called New Old Stock or‘NOS’valves. The mystique surrounding old Mullards and RCA valves from the 50s and 60s, among others, has certainly pushed up prices. But are they really better than what is made today?
The answer is a qualified yes, says Adrian, provided they are still in good working condition.“They are better tubes than what’s made now, for sure. Like Mullard, General Electric preamp tubes, for example, they’re going to be, for the most part, better sounding. They’ll probably always yield a bit more gain and sound better. Something of note with New Old Stock tubes, especially preamp tubes, though, is that you should clean the silver-oxidisation off the pins. Because there was more silver in the pins back then, that’s why the pins turned black on old Mullard preamp tubes.And they’ll be intermittent in the socket as a result, unless you clean them with silver dip or something and get that oxide off.”
Are there any things you can detect visually that might warn you off buying a defective old valve, we ask?“Yeah.Where the windows are on the plates [the large vertical component inside the valve bottle, usually dull gray with small ‘window’ apertures inset], if you see charring of the glass on the inside where the windows are it means that has been heavily used. So I’d tend to avoid that. If you have any kind of valve-testing device, then take it with you to see what the valve is yielding and test it for real.And, finally,”Adrian advises, “tap the valve next to your ear to make sure nothing’s really rattly.”
So that’s the lowdown on old valves, but what about new replacements? By popular demand, we’re going to extend what was going to be a two-part feature into a third part on buying new valves the savvy way, so stay tuned for even more of Adrian’s advice next month.