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    Building, troubleshooting and testing of these amplifiers should only be
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standby switch

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"All I know is that a standby switch helps prolong valve life..."

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
 
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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
 
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
 
Why standby switches blow

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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.....
 
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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
 
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.
 
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I think I missed something here. The amp does not currently have a standby switch? If that's the case, leave it alone.

A standby is only used really if you are turning the amp on and off a lot. Amps with big output tubes and a SS rectifier need them the most to provide a delay for the heaters to warm up.

-Chris
 
I was a bit confused with this same topic (and still kind of am).

I bought a Vox AC30CC2 a few months ago, and had a problem with it about a week into usage. I have always been told that if its got a stand-by switch, use it. And so i was, until the day that i turned the amp on, and heard a quick buzzing sound, saw a flash of blue light, and then a loud popping sound (not from the speakers), as the amp went comletely dead. No lights, no sound, no nothing.

Naturally, with a brand new amp, i was very concerned, and extremely confused. After extensive research online, with the Vox site, and others, i found that it is actually bad for the amp to use the stand-by switch.

Turns out that the problem was the rectifier tube burned out, taking every single fuse with it. The way i understood it, the amp was poorly designed as far as the power ramping was concerned. I don't really know the exact science of the issue, but in the long run, it ended up costing me $200 in tube replacements (retubed the entire amp), and a few more in fuses.

Needless to say, i don't use the stand-by switch anymore.
And if your amp doesn't have one, it more than likely doesn't need it.
 
The Peavey Classic20 is a SERIOUSLY bad candidate for a "traditional" standby switch - why? - because it has a concertina phase splitter. When HV is applied with heaters up to temperature you get a big positive spike to the grid of the output tube on the anode side of the splitter (while the coupling cap charges up). That produces a BIG current surge through that tube. This is from experience - I had a push pull EL84 (It was a Morgan Jones Bevois Valley) which had separate cathode resistors and bypass caps. It kept blowing up the cathode bypass cap on that side - until I deleted the standby switch, never had a problem thereafter.

If you really want a standby function - look at switching the screen voltages to the EL84s instead. Use a MOSFET to do the switching with a standby switch switching low voltage and current to an opto coupler on the MOSFET gate.

Cheers,
Ian
 
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Can we get some things clear?

1. Running normal amplifier valves with HT/B+ but cold heaters does them no harm, provided there are no funny DC couplings to put HT on g1.
2. Running normal amplifier valves with hot heaters but no HT will gradually degrade them, due to the development of a cathode interface layer.
3. Running normal amplifier valves with full HT and reduced heater may gradually degrade them due to ionic bombardment of the cathode as there is no protective space charge.
4. Suddenly applying full HT to a hot amplifier will cause big surges in various places and is asking for trouble.
5. Normal 'switch-on from cold' surges should be handled automatically. It is weak design to do otherwise.
6. There are no normal 'switch-off from hot' surges. Just switch off!

So what do you do if you want to reduce electricity bills and valve usage, but want to use the amp again shortly? If the time gap is less than a few minutes, just leave it switched on. If the time gap is more than a few minutes, just switch it off. But don't keep switching on and off every few minutes, especially if the amp has no protection against switch-on surges.

My conclusion is that there is no reason to have a stand-by switch. The most it can do is to do manually what should be done automatically. The worst it can do is shorten the life of components, including itself.
 
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