David Lamkins picked up his first guitar a long time ago. As best he can recall the year was 1967: the year of the Summer of Love. Four decades later David has conjured up an amalgam of folk, rock and jazz solo guitar music for the occasional intimate Portland audience.
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location: Portland, OR USA

Facets: amplifiers, physics, technology, tubes, @musings info

How to use a standby switch

There's a lot of information on the `net and in manufacturers' literature about how to use the standby switch on your tube amp. Not surprisingly, there are a lot of different procedures. In the words of at least one frustrated player, "there's not a lot of concensus among the experts."

Most of the experts cite the "cathode stripping" boogieman to convince guitarists to religiously follow a standby switch ritual. Their notion is that applying high voltage to the circuit before the cathode is fully up to working temperature will physically strip the delicate coating from the cathode and prematurely age the tube. You'll be left with a tube that doesn't work and has a bunch of white fluff (the stripped cathode coating) floating around inside the glass envelope. Expert opionions vary regarding how long you should wait before turning your standby switch to the "play" position; I've seen times ranging from thirty seconds to five minutes. And some experts even recommend that you put your amp on standby for some period of time to give your tubes a chance to "cool off" before turning off the power.

Let's look first at the range of "wait" times cited by the experts. Heaters come up to temperature in about 15 seconds. You can find out by powering-up your amp without using the standby switch. (I'll bet that makes you nervous, doesn't it? Read on...) Even if there was something to the threat of cathode stripping, the threat would be eliminated as soon as the cathodes came up to temperature and the tubes started functioning. So what's up with the recommendations to wait sixty seconds, or two minutes, or ... five whole minutes? If fifteen seconds is good, then three hundred seconds must be really, really good?

Cathode stripping is real. (Sit down. I haven't misled you.) It happens at very high voltages and currents. If you go back to the manuals and engineering texts of the `50s and `60s you'll discover that no one ever wrote about cathode stripping with respect to "receiving tubes" (as compared to "transmitting tubes"). "Small" tubes like the 12A?7, 6L6, 6V6, EL84, EL34, EF86, ... are receiving tubes. With the exception of a few tube amps having very high (~700V) plate voltages (like the SVT and a few high-powered Marshalls), you don't really need a standby switch except as a convenience for muting the amp; I'd rather just pull the cord from the input jack or turn down the volume control on the guitar. There is no way that cathode stripping can occur in preamp tubes, even in an SVT; the available energy is limited to a very low value by resistors in the plate circuits.

OK, so what about that bad tube you found with tiny bits of white fluff rattling around inside the glass bulb? Yes, that can happen. It's a result of poor bonding of the cathode coating combined with temperature cycling and vibration. It's not caused by cathode stripping.

Then you have to wonder about those amps that don't have a standby switch. The experts will tell you two things about that:

  1. You should probably have a qualified tech add one just to be on the safe side. Hey, techs have bills to pay just like the rest of us...
  2. Some amps don't need standby switches because they have "controlled warmup" vacuum tube rectifiers which don't reach their working temperature until all the other tubes are ready to go. Nice try, but not quite right or consistent with other expert advice... Rectifiers start working before they come up to full temperature, so there's some DC on relatively cold tubes throughout the amp. And given that the rectifier heats up about as fast as the other tubes, what becomes of the advice to hold off operating power until the tubes have warmed up for some number of minutes? Hmmm?

Standby switches introduce their own problems. They're not rated to break DC at the voltages present in any tube amp, let alone the bigger ones. (Check the ratings on a switch some day... The DC rating is always much lower than the AC rating.) When you break high DC voltages, the switch arcs and will eventually fail. It's a lot tougher to swap in a new standby switch than a new tube...

Switches are most prone to arcing as they open a circuit. In the case of the standby switch this happens when you switch the amp to standby. However, switch contacts "bounce" when you close a circuit; arcing may also happen on each bounce before the contacts finally come to rest. Arcing in this case is much less severe because there's less stored energy (see below) in the circuit as you're switching the amp off standby.

Arcing depends somewhat upon the circuit design. In general, though, standby switches are exposed to DC voltages well in excess of their design center. The reason it's so hard to make a mechanical high-voltage DC switch is because an arc is formed as the contacts open. The arc tends to continue so long as there's enough voltage to maintain it as the contacts separate. In circuits where there's a big inductor (like a filter choke) attached to the switch, the collapsing magnetic field (created by breaking the circuit) creates a high voltage that's sufficient to cause arcing. The duration of the arc is related to the amount of energy stored in the inductor.

A switch can and does arc with AC across it. However, the duration of the arc is limited because the AC voltage goes to zero 120 times per second (100 times per second in Europe). As the voltage approaches zero it very quickly becomes too low to sustain an arc, and once the arc is quenched it tends not to recur because the contacts have moved far enough apart that the voltage can't break down the air between the contacts. This is the reason that a switch has a higher rating for AC voltages than for DC.

Here's one more thing for you to ponder: "cathode poisoning". Normally the heated cathode throws off a cloud of electrons that get immediately pulled away from the cathode by the electrostatic field created by the plate voltage. When your amp is on standby you don't have any voltage on the plate, and that electron cloud hugs the cathode. Some of those electrons bump into the cathode coating and cause an electrochemical reaction which reduces the efficiency of the cathode coating which in turn reduces the useful life of your tubes. Unlike cathode stripping, cathode poisoning is not dependent upon how high the plate voltages are; when the amp is on standby there is no plate voltage. So now you have one more thing to worry about: If you put your amp on standby, how long before cathode poisoning becomes an issue? Is it cumulative?

You could lose sleep over things like this. Just keep in mind that no item of consumer electronics gear (and for that matter it applies to the small sampling of industrial electronics gear I've seen) made in the heyday of vacuum-tube electronics had a standby switch, except for guitar amps (which don't really need one) and ham radio transmitters (which really must have one). When you want to use the gear, you turn it on. When you're done, you turn it off. No rituals needed...

Frankly, with the exception of the arcing issue (which can cause an amp to become scratchy, poppy and to lose power because of carbon build-up due to arcing on the standby switch contacts), standby switches on most guitar amps (remember what I wrote about amps with very high plate voltages...) are harmless.

By all means use the standby switch if it makes you feel good. We all need rituals. But don't lose sleep over having powered your amp on (or off) the "wrong" way. It should be quite obvious from this discussion that guitar players follow a lot of different standby-switch rituals and no one's really suffering from having used any of the techniques described...

I leave my standby switches in "play"; they never move.

In deference to prevailing wisdom, though, I use the standby switch when cycling power on other people's amps; otherwise they get a bit pissy...

April 18 2006 22:49:37 GMT