standby switch

[9am53]

Hi, I looked for info about wiring a standby switch in my amp, but couldnt find anything helpfull. All I know is that a standby switch helps prolong valve life...could anyone quickly explain what it actually does and how to wire one in?

[Joel]

I've seen no one present solid evidence of that, other than anecdotal.
As far as I can tell from my reading over the years, the only instances where actual "cathode stripping" occurs is in pulse applications and very high current tubes such as the 6080 - and even then only when they are in circuits that run them all-out, at 125mA.

What I do know, for a fact, is that stand-by switches have a nearly 100% failure rate over a unit's lifetime (20 years), and usually much sooner than that. Not only is this a well-known issue, but I've experienced it myself on two seperate guitar amps.

Joel

[9am53]

ok, so why should (or shouldnt ) I put a standby switch in, what are they for?

[anatech]

Hi 9am53,
A standby switch delays the high voltage until the heaters are up to temperature. It also reduces the surge voltage across the supply caps. It's looked on as good design practice.

Your choice, you can use a timer circuit to do this automatically for you using a relay. The contacts are subject to the charging surge from the filter caps which may burn them over time.

-Chris

[Joel]

They were put into guitar amps in the 60's, as a way to keep the amp "ready", but not running. If you ever come across a Fender Deluxe Reverb that has a stand-by switch that doesn't arc-over (with a very loud pop), let me know.

The funny thing about the "inrush current" trope is that the rectifier on amps like the Deluxe Reverb was itself a "slow startup". The 5AR4 takes about 11 seconds to come up to full conduction for the B+.

IMO, you should not use a stand-by switch.

Joel

[dhaen]

.
Nearly all switches are rated for AC voltage. During zero crossing the arc extinguishes, which gives the designer a good idea how long the arc will last.
With DC the arc continues until the distance is too great. This distance can be considerable if there is any reactance in the circuit - which is inevitable.....

[anatech]

The standby switch only arcs when you open the circuit. Musicians will put the amp in and out of standby instead of turning it off. A stereo amp is used differently and so a timed relay is a good choice.

I can't imagine yet another switch to throw at home. The wife annoyance factor would really peak!

I have repaired tons of guitar amps with burned out standby switches, some more than once over the years. It's the way they are used and that is better than leaving them on between sessions.

-Chris

[9am53]

the amp in question is a Peavey Classic 20, I most often use it as a practice amp, but sometimes I practice with the band with it. I take it that since I dont turn it on and off repeatedly I dont really need the standby option?

[anatech]

Hi 9am53,
I'd put it in, the amp stays stock. A switch every what, 10 yrs? Not real expensive if you ask me.

-Chris

[Enzo]

That C20 doesn't need a standby switch in my view, though it won't hurt it. You can put one in the way Peavey does it on their other amps by breaking the power supply between the plates and screens. of course an easier way would be to put the switch in the HV transformer wires, that way the board remains intact.

But that amp is going to wear tubes out before the idle time between sets gets to it. In other words I don't see it affecting tube life. The little EL84 is not a robust tube and I expect to replace them twice a year if you gig regular. Having the tubes right in the baackblast of the speaker will shake them loose before cathode life is an issue. Just turn it on, play it and turn it off when you are done.

A switch is electrically simple, but it does have to go somewhere, and there is limited panel real estate on those. Unless you want to stick it underneath somewhere. And now you have a hole in your amp.