Standby switch?

[Boogieman]

What is the purpose and function of the standby switch and when should it be used?

[John Phillips]

Purpose... to extend tube life, by:

Allowing them to warm up fully before high voltages are applied to the internal elements. Applying high voltages to cold tubes isn't good for them (called 'cathode stripping').

Shutting off the tube current during breaks, without needing to shut the amp down entirely (which is even worse). Running the tubes fully simply wears them out, slowly, just as playing them does.

Allow them to cool down more slowly in stages when switching the amp off (marginal, but if you have the option you might as well). Thermal stresses are not good for tubes either.

And:

To simply mute the amp when changing guitars etc, without having to turn down the volume and possibly accidentally change the setting when turning back up.


When to use it:

When powering up. Turn the power on with the standby off (mute). Let the amp warm up for at least thirty seconds and preferably longer, before turning on the standby (play).

For any break where the amp is going to be silent for more than a few minutes, but not moved. Possibly for quite long periods - it's far better to avoid cooling/heating cycles by not turning the amp off and back on again - this stresses both the tubes and other parts of the amp.

When turning off - not really important, but I always still do. Put the amp on standby, wait until it's stopped making sound and/or 'cooling' noises, then shut off the power.


I'm not certain where the cut-off point for leaving an amp on standby as opposed to shutting right off would be. I certainly wouldn't shut off if I knew I was going to be playing again within an hour or so, and I wouldn't leave it on standby overnight... but in between it's pretty much equal either way probably.

There is some potential harm to tubes from leaving them on standby for really long periods (called 'cathode poisoning'). But over the sort of time periods where a standby switch is normally used, it's less bad than either shutting them off fully or running them continuously.

[Boogieman]

Thank you John for taking the time to give me such a complete answer.
Best wishes,
Al

[fyrwyr]

Yes John is a very helpful guy around here, thanks for all your valuable input, much appreciated !:BEER

[Den]

If there was a prize for the most helpful ... and knowledgable member on TGP ... John would get my vote!

[zoooombiex]

Originally Posted by Den

If there was a prize for the most helpful ... and knowledgable member on TGP ... John would get my vote!

+1

This may be a related question ... if an amp is acting like it has a bad tube and you need to replace them one at a time to figure out which is the culprit, what is the best way to go about doing this with minimal stress to the amp? I have one that is acting up, but none of the tubes are microphonic or visibly damaged. I'd hate to flip it on for a few minutes, then off and on again over and over. Is it better to do the on/off really quick, so things don't have a chance to cool down much, or is there not really any good option? Thanks!

[unklmickey]

Originally Posted by zoooombiex

...Is it better to do the on/off really quick, so things don't have a chance to cool down much,...

no, don't do that.

the only thing that will accomplish is to hurry you.

take your time, make sure you get the tubes in and out straight, no bent pins, no breakage, etc.

and as always, turn it on with standby off for about a minute, before turning the standby switch on.


if you have a full set of spares, you could be methodical.

for instance, if your amp has 8 tubes replace 4 of them. (keep the pulled tubes in order)

if the problem goes away, you know the problem was with one of them.

put 2 of the pulled tubes back in their original sockets.

if the problem comes back, you know it is one of those 2.

etc.

[57paf]

John:
I really have appreciated your very informative posts.
I've observed that some small amps, such as the forthcoming Vox Heritage Series AC15H1TV, have no standby switches. How much would tube life be extended if a standby switch was added?
Also where would you add it? Would simply cutting one lead from the EZ81 rectifier to the main circuit (opposite side from the power transformer) be sufficient, or do you have to get a double pole switch to cut both leads?
Thanks.

[John Phillips]

How much it will extend tube life depends on your pattern of use.

If you typically turn the amp on, play for a while, then turn it off again for the day, it probably won't make a significant difference (although it still avoids stressing the tubes when cold).

If you typically turn it on, play some, stop for a while, play some more, stop for a while etc, it can make a big difference. Suppose you play a gig where the soundcheck is at 6 to 6.30, you play a set between 9 and 10, another between 10.30 and 11.30, and another between 12 and 1. Without a standby switch, the tubes get seven hours wear. With a standby switch, they only get three and a half. So if that's what you mostly do with the amp, the tubes will last up to twice as long with the switch than without.

The best place to cut the circuit is actually after the first filter cap and before the connection to the OT, so the first filter remains charged with the standby set to off - this avoids stressing the rectifier tube by having to suddenly charge up an empty cap when the switch is flipped. A single-pole switch is all you need.

[WailinGuy]

The best place to cut the circuit is actually after the first filter cap and before the connection to the OT, so the first filter remains charged with the standby set to off - this avoids stressing the rectifier tube by having to suddenly charge up an empty cap when the switch is flipped. A single-pole switch is all you need.

The method you described is the typical Fender blackface standby wiring. The disadvantage of doing it this way is that you get that loud POP when you turn the standby switch on. The pop is caused by the sudden inrush of current from the fully charged 1st filter cap (which also has a significantly higher than the normal operating voltage on it, since there is no load on the HT supply during standby mode) to the rest of the power supply rails, as well as to the output tubes. But a lot of us Fender players have gotten used to this noise - that familiar "pop" along with the wash of reverb noise (if it's a reverb amp) can be very reassuring, letting us know that the amp is ready to deliver the goods when we pick that first note.

Another way of wiring the standby circuit (which doesn't cause the pop noise) is seen in a lot of Fender tweed amps, where the standby switch (a SPST) is placed between the rectifier and the first filter cap. Also, there's a .05 or .1 film cap that goes between the hot side of the standby switch and ground. I've never been quite sure what the purpose of that cap is, but my guess is that it is to reduce stress on the rectifier tube when the standby switch is turned on. The rectifier still has to charge a mostly empty 1st filter cap when the standby is flipped, but at least that smaller value cap (which is already charged) takes care of the very leading edge of the current inrush, where most of the stress would take place. At least that's my best guess.