Tube Rectified Amps: Standby via Switch on Rectifier Heater - Good Idea?

[Steve Conner]

FWIW, I think the original purpose of the delayed warmup rectifier was not to protect the other tubes from cathode stripping, it was to allow the use of electrolytic caps with lower voltage ratings. If the rectifier came up last, they would never see the full unloaded B+.

Really thinking about it, I don't know what in heck the standby switch is for. I didn't put one on my last build, and I haven't missed it.

[R.G.]

Good point Steve, and one I hadn't considered.

Of course, this doesn't help much if someone removes the power tubes, and therefore 90+% of the loading from the power supply, or all of the tubes. But it's certainly a consideration that might have made sense back then.

[R.G.]

Tube rectifiers (at least the kind we use) do NOT like hot switching. It even says so on the data sheets, if you find the right ones. In other words, they don't like being pre-warmed before connecting them to the reservoir cap.

That's part of the thinking that went into my current clamp circuit. Force the peak current in the tube rectifier to be limited somewhat below the damaging current, and you should be able to do pretty much anything you want, even if they are pre-warmed.

This option was not available as a practical matter to designers in the Golden Age. They simply could not spend the money for tubes to do the current clamping - or relays, time delays, etc.; it was expensive enough to put in the tubes that were needed as a minimum, let alone a layer of protection devices.

I suspect that if one could question a skilled designer from that era, the reply might include some thing like "well, that's why the rectifiers are in sockets. If you blow one flipping it on and off, you can just replace the rectifier tube." And that may make as much sense as protection circuits when all you have is tubes and relays.

[Enzo]

The "problem" is now that it I think it would be hard to market a valve guitar amp without a standby switch due to "conventional wisdom" that it is needed to extend valve live.

yes, of course, there are guys who get a Champ and right away want to add a standby switch. But marketing and circuit analysis are two separate topics. If it sells more amps, then the cost of a pointless switch is the cost of making sales.

Lots of smaller amps still have no standby switch though.

[Gingertube]

2 stories of my experiences with standby switches

I did put a standby switch in my last amp build and had GRIEF as a result.
The power supply was a voltage doubler with SS diodes so the standby switch went after the double capacitor "stack". The next contributor was that I used push pull EL34 with a common cathode bias resistor and a 100uF bypass cap. When switching from standby to operate both output tubes conducted large current until the cathode resistor bypass cap had charged up to the idle bias voltage. I was blowing HV fuses every couple of days.
A chat with the guy it was built for revealed that what he really wanted was a mute he could use in breaks - so I left the switch in but rewired it as a grid 1 mute instead. He was happy as a dog with 2 tails, well a lot happier than when it was blowing HV fuses every couple of days anyway.

The reading I did last year when I designed an 845 Single Ended Triode Amp running +1250V on the anode suggested that around 1000V minimum was required for "cathode stripping" to occur. I did put a standby relay (electronic timer controlled) in that amp but that was mainly to make sure the 845 was ready to conduct when I applied the HV AC to the rectifier so that my stack of three (3) 450V rated filter capacitors (1350V total rating) was never subjected to the unloaded voltage of 1480V. That is, I put it in for the reason Steve suggested above.
Cheers,
Ian

[Diablo]

So can I conclude from this discussion that there really is no advantage to having a standby switch in a guitar amp, from the standpoint of improving the lifespan of either the rectifier tube or filter caps. And that if my amp has a standby switch, I'd be better off not to use it, or even safer to unwire it from the circuit. I assume this is also true if the amp has a solid state rectifier?

[Enzo]

STandby switches are there to silence the amp without turning it off. Your set break is over, you can flip the switch and play, no need to wait for tubes to warm back up. In many amps this is done by removing high voltage from some or all of the tubes, but in others it is done by lifting the power tube cathodes, and in others yet by shorting opposing grids together.

They are there on many amps because people expect them.

I see no reason not to use it if it does something you want. For example, if I walk off stage at the end of a set, across the dance floor, over to the end of the bar, and order a beer, and at that point my guitar starts feeding back loudly, I would just as leave have a standby switch. And certainly no point in removing it. If you don;t want to use it, just leave it turned on.

SOlid state rectifiers don;t really change the function of the amp, at least in this regard. They have high surge ratings - the common 1N4007 is a 1 amp diode, but has a surge rating of 30 amps. SO it is not in danger from filter cap charging rush. SO anything you might have done to protect a rectifier tube, no longer is necessary.

[R.G.]

I'm sure someone will help me with whatever I miss, but I put it down as the advantages of a standby switch are questionable in the way we use amplifiers today. A lot depends on what you want a standby to do. If it's mute the amp, there are safer ways. If it's to increase tube life, it may not help much, and there are certain situations where it may damage tubes, primarily rectifiers, and/or filter caps, depending on how it's implemented. One could make a case for a standby being used to let the amp cool down, I guess. There are certain amps which seem to have real technical difficulties with the way the factory wired the standby and/or the rest of the circuit, AC30 and certain Marshalls among them.

The inrush surge problems are much less an issue with solid state rectifiers, as far as the rectifiers go. Standby with tube rectifiers or solid state can damage filter caps if they're subject to repeated surges.

I think that standby switches are a relic of an era with different needs, one of which may have been tube life extension. The real question is - what do you want a standby for? I think that silent warmup and/or muting is what I'd want. Or, I guess, do you want a standby at all?

[Dave H]

I only use the standby switch to mute the amp between sets. If the amp doesn’t have a switch I pull the jack half way out of the amp’s input to ground it.

[NateS]

Or to pull the power tube screens to a much lower, or even negative voltage

Now there is a bright idea.


I wondered about this recently, but couldn't find anything about just how low the screens can go. I was looking for an 'as late as possible' mute that didn't involve scads of mosfets (since I'm cathode biased) or running antennas a long way away from the rest of the tube circuitry to the controller. Unfortunately, I can't really use it myself, because I want to mute while determining my bias points.

[R.G.]

Unfortunately, I can't really use it myself, because I want to mute while determining my bias points.

Yeah, that's a problem. You could short the speaker connections. The tubes won't care much, especially if you also mute the signal going in so they're not being driven. You probably want to do that anyway.

I ...um... may have tried something similar.

[Tage]

I don't get the whole standby to mute thing, I'm certain most guitarists play with a pedal tuner, like a TU-2 and that is an excelent mute switch.

[NateS]

I don't get the whole standby to mute thing, I'm certain most guitarists play with a pedal tuner, like a TU-2 and that is an excelent mute switch.

A tuner is the only pedal I currently use (sorry RG). Most of the slick more modern ideas for what standby can actually accomplish have been discussed already here. But if nothing else, my front panel had an extra hole that I needed to do something meaningful with, given that it's labelled 'STANDBY' and is grouped with the power switch and indicator lamp hole. Not exactly the place for 'turbo boost' or 'move the tone stack location between after the first stage or just prior to the phase inverter', which requires entirely way too much front panel space.

[R.G.]

I don't get the whole standby to mute thing, I'm certain most guitarists play with a pedal tuner, like a TU-2 and that is an excelent mute switch.

You're taking too short a view. Historically, the standby was put there to do *something*. That was at a time a half-century or more ago when tuner pedals had not even been conceived as a idea. Fast forward to today, and guitarists have many muting options, but probably would pay more or be more likely to buy a tube amp with a standby as opposed to one without a standby switch. It's expected.

Once they get the amp, they then start wondering how to use that switch.

The question is more one of "It's there, how do we use it, or should we use it at all?" and not "Do you need a standby switch to do X?"

[R.G.]

But if nothing else, my front panel had an extra hole that I needed to do something meaningful with, given that it's labelled 'STANDBY' and is grouped with the power switch and indicator lamp hole.

A good thing would be to get one of the smoke generator cartridges that model railroad enthusiasts use and have the standby switch enable a five second puff of smoke when flipped.