the standby switch

[bob p]

> I get plenty of modern amps with failed rectifiers, especially when the 1st filter cap size has increased by the designer to reduce noise in higher gain amps.

Think of it as a learning opportunity. If a modern amp designer chooses to ignore the limiting values in the data sheets (either deliberately or out of ignorance), then his amps will fail. He has a chance to learn from his mistakes, but he has to be quick about fixing the design error before the the customers learn to avoid his amps.

I haven't followed the Vox situation or looked at the cap values, but I do seem to remember hearing that they switched from tube to SS rectification in the AC30 reissues.

[bob p]

The standby switch is the guitar amp's equivalent of an appendix.

The appendix and the spleen actually do serve useful purposes.

If you want to take a break from playing, it lets you cut off the current to the power tubes so they can cool down a little, and you're not burning through your tube life quite as fast. But with the heaters still on, the tubes don't cool down completely, so the thermal shock isn't as great when they get going again.

I agree that the standby switch has value when implemented properly. I like to use them.

It'd be interesting to know how much tube life prolongation you get out of B+ switching vs. heater switching. I'm not sure whether idling with B+ on shortens tube life all that much, or how much life extension you can really expect to get out of using standby. I'm thinking that most of the wear in a tube's has to be related to amplifying a signal (thermionic emission at the cathode), so that if the cathode isn't actively emitting electrons then it's coatings are not being consumed. In contrast, there is constant thermionic emission from the heaters that get left on. I'm thinking that if you can weed out things like screen failure, then the tube lifespan is ultimately going to be determined by the longevity of the coatings on the cathodes and/or the heaters.

This brings up another topic, which is sort of a hijack: Personally, I've never understood the idea of 50/100 watt switching using the cathode interrupter switch. Sure, it gives you power reduction, but it doesn't really reduce volume, and I don't really think it's going to do much in favor of tube lifespan. If you've got a 100-watt amp and you play it most of the time in a 50-watt setting (cathodes on one tube pair disconnected from ground by the switch), you're still leaving the heaters on all the time on those two tubes that are "switched off." I've seen amps with a 4-tube compliment that are run with 2 tubes on and 2 tubes off for most of their working lives. The heaters are on all the time. That can't help tube life. I'm thinking that 50/100 switching would make a lot more sense if it were done at the heaters. Why aren't people doing this?

[Merlinb]

It'd be interesting to know how much tube life prolongation you get out of B+ switching vs. heater switching.

The decay of a valve is primarily due to:
1: Evaporation of barium from the cathode.
2: Growth of interface resistance.

The first one carries on as long as the cathode is heated, so a conventional standby switch will have no effect on valve life this way. A switch that reduces the heater power a bit ought to reduce evaporation.

The second effect occurs when the cathode is heated but little or no anode current is drawn, which is exactly what a conventional standby switch does.

In other words, a conventional standby switch is more likely to reduce life than increase it. What you want is to reduce the heater power a bit, and reduce the anode current a bit, but not to zero. Then you might have a standby switch that is more than just a mute switch.

Personally, I've never understood the idea of 50/100 watt switching using the cathode interrupter switch. Sure, it gives you power reduction, but it doesn't really reduce volume

+1. It's kinda pointless.

[Merlinb]

Mick Bailey, educate me please. I have several 60's amps with the original tube rectifiers with standby switches. I have never had to replace a failed rectifier tube in any amp, whether my own or a bench repair.

Fender was the first and only guitar amp manufacturer to independently design in a standby switch; all others are copies of copies of copies... Fender included his standby switch to protect the PSU capacitors and possibly the cathode follower; it had nothing to do with tube life, but that fact is unknown/ignored by other derived manufacturers, and myths about tube life quickly crept in, and now guitarists expect to see one. Even the 1960s tube rectifier data sheets explicitly say NOT to use hot switching. If yours have survived longer than most, then you're just lucky.

[Steve Conner]

There is a third mechanism, escape of gas occluded in the metal. This is a function of temperature and time. I think this is significant because most power tubes end their lives by "red plating", which is thermal runaway caused by excessive gas current. It doesn't help that every guitar amp maker violates the maximum grid resistance spec. If they were more diligent about this, I think power tubes might last longer and end their lives in other ways.

So anyway, I think the standby switch helps by reducing the amount of time that the tubes spend at full operating temperature.

[Mick Bailey]

Guitar amps appear to be in minority when it comes to standby switching. I've never seen tube industrial equipment with standby switches. Even stuff that gets turned on and off regularly, whether tube or solid state rectifier.

[bob p]

thanks everyone for your insights.

1. standby. I understand Merlin's point about reducing heater power and anode current a bit to help preserve tube life in a standby situation.

I'm going to continue the hijack and try to re-focus this same line of thought on long-term power switching.

2. power switching. the current embodiments that we see in amps for power switching don't seem very well conceived. there are amps that do power switching by leaving the filaments turned-on on all of the tubes, and perform power switching by turning anode current off via a cathode interruption switch. this does nothing to help tube life, as the filaments are continuing to burn away while one set of tubes is not in use. it's not a very smart design. it seems like it was formulated as a good solution by someone who conceived that an end user would want the ability to instantaneously switch between high power and low power configurations on an amp.

i don't really see how this paradigm has any value. cutting power in half by turning off a pair of tubes won't provide much difference in volume -- that halving of power only reduces volume by 3 dB. what probably will happen though, is a subtle shift in operating point of the tubes based upon changes in Z-matching between the tubes and the speaker load as the number of tube pairs seeing the reflected load impedance is changed. i guess this could cause some changes in the distortion levels in the amp due to the shifting operating point for the remaining tubes that are switched-on, but i think that in the type of modern high gain amp that uses this sort of switching, the effects of cascaded preamp distortion are going to outweigh our ability to perceive the shift in operating point on the output tubes, which are often biased on the cold side in the commercial high-gain amps.

i don't see how power switching by cathode interruption provides any net benefit. i think it's just a way to fool customers into thinking that they're getting more than they're really getting when they buy this kind of amp. for the cost of an inexpensive switch and some wire, the vendor creates the illusion that he user is getting a valuable feature. the reality is that the "feature" is deployed to add profit margin to the amp. IMO this type of circuit helps the vendor more than the customer.

what would seem to make far more sense is heater-based power switching. while it would make sense to reduce heater power for a standby application, it would seem to make more sense to completely turn-off the heaters in the situation when you want to turn-off pairs of power tubes for a long period. such as the case where you've got an amp with 3 pairs of power tubes, and you only want to run 1 pair when you're practicing at home, and you only plan on turning on all 3 pairs when gigging. the reason for doing this isn't really to reduce the power of the amp by a few dB. the real reason for doing this is: a) to reduce the amp's consumption of wall power, b) to reduce the amp's consumption of matched tubes, and c) to reduce the amount of heat that gets pumped into the room. it doesn't make much sense to have an SVT running all of it's output tubes when you don't need that much power. it would make sense to just turn off pairs of tubes, and dynamically re-match the output load accordingly.

so this brings up the question of what's the best way to implement effective long-term power switching, where you turn pairs of tubes on/off to change the output power of an amp? the objective of doing this isn't really the commonly perceived goal of reducing volume; the real objective is to reduce power consumption, to prolong tube life, and to reduce the amount of heat that's getting dumped into the room.

I'm thinking that the answer is to completely interrupt the heater supply on the tubes that you want to power-off. if you do this, then the next consideration is the question of how Z-matching between the OT primary and secondary changes when pairs of power tubes get turned off at the heaters, but otherwise remain connected in the amp. i'm thinking that when you've got a 6x6L6 amp that you want to switch between 2x6L6 and 6x6L6 operation, the primary Z of the tube compliment is going to triple. correct me if I'm wrong on this, but I'm assuming that the 4x6L6 that are turned-off can safely be ignored when it comes to calculating optimal and the effective plate load Z. if that's the case, then it would make sense to preserve a reasonable range of impedance ratios by concurrently switch the secondary load from 4R (two 8R speakers wired in parallel) to 16R (two 8R speakers wired in series).

[R.G.]

Since we're digging into the pile of "things involved with reliability and also maybe attached to standby"...

Design for reliability is one of those things that was a requirement for some of my first assignments, so I got whapped with a lot of it from older and wiser heads in general, if you want things to last a long time:
1. don't move them around or vibrate them any more than you absolutely have to
2. don't get them any hotter than you have to
3. if you have to get them hot, minimize the cumulative time they're hot
4. don't thermal cycle them any more than you can avoid.

This all presumes you're using them within their design limits to start with, of course. If you're not doing that - well, that's what written design limits are for to start with. In my past life, choosing to use a part within its design limits or not was kind of viewed as an IQ test. So was knowing what the design limits were in the first place. The old saw is that "ugly" can be fixed, but "stupid" goes all the way to the bone.

The problem in designing for reliability is that if you're concerned about reliability at all, you're probably designing something that gets hot, gets shocked and vibrated, and may well have lots of thermal cycles. Power cycling may or may not be an issue. An artillery shell only gets one on/off cycle, but you really, really want it to be reliable.

Everything I've read from Golden Age textbooks and tech reports follows along with this. The number one ways to kill tubes are to vibrate them, especially when they're hot, get them too hot, and thermal cycle them. Oh, and there's that "design limits" thing.

We'll ignore the design limits issue, as we can't control whether the new Dreadnaught MegaBlitz amplifier, which is a clone of whatever is most hot on the internet these days but with MORE GAIN, MORE POWER and truly vintage , uses tubes correctly or not.

We can't fix the vibration issues either.

About all we can look at is heat, hours hot, and thermal cycles. As the alert reader has already guessed, hours hot and thermal cycles are in direct opposition for the same number of hours in use. Either you leave it on and avoid a thermal cycle or you turn it off, incur a thermal cycle, but save more lifetime from the heat induced decay during power on hours.

Which is better? Who knows? How do you use your amp? The real issue that's hard to quantify is this one: How many power on hours equals one thermal cycle in terms of tube life reduction? Obviously, that depends on how hot the tube runs when it is operating.

Which leads to the real longevity factor: cool the tubes in operation. A couple of the Golden Age texts I've read say that glass envelope temperature is the key factor in estimating tube life; moreover, even modest airflow over power tubes at the power densities in guitar amp output stages will keep them cool to the touch. The glass envelopes are not good conductors of heat, but by keeping them cool, you are minimizing their re-radiation of heat back into the tube elements, and so keeping the tube elements as cool as they can be, given that they can only cool by radiation into the glass and a little conducted down the leads.

This is, of course, the only longevity thing a guitar player will NOT do. Hate fans. Fans ugly. Fans nasty, noisy, probably smell bad. Fans run off groupies. Fans make guitar out of tune. Yada. Yada.

Fans are a PITA. Worse yet, they absolutely require air filters to keep dust buildup down and regular maintenance of the filters. But they will extend tube life.

It'd be interesting to know how much tube life prolongation you get out of B+ switching vs. heater switching. I'm not sure whether idling with B+ on shortens tube life all that much, or how much life extension you can really expect to get out of using standby.

There are two deleterious effects I know of. One is ion bombardment ("cathode stripping"), the other is sleeping sickness.

Ion bombardment is caused when residual gas in the tube drifts into the plate, has an electron sucked off by the voltage, and then runs hell-bent toward the cathode. If the cathode is hot and is providing enough electrons to make a space charge, the ion is neutralized before it hits. There it whams into the oxide surface and erodes/chemically eats it, lessening emission. If it's neutral, it causes much less chemical damage than if it's an ion. All the literature I've ever seen says that you need plate voltages on the order of 700V or so for cathode stripping to be an issue, because you need it to be moving fast enough to cause damage.

When you leave a tube hot, but bias it down to zero current, there is an electrochemical effect that makes the cathode develop a higher resistance on the surface. This sleeping sickness was discovered when computers used tubes and tubes would stay non-conducting for long periods of time. You can rejuvenate a tube with sleeping sickness somewhat by overheating the cathode mildly. This was done by boosting the heater voltage either temporarily or permanently. The old "picture tube rejuvenators" which most of you will not remember did this. They got more life out of a dying TV picture tube.

When Merlin says evaporation of barium and high resistance, I believe he's referring to these effects. Strictly speaking, I don't believe the barium (and strontium, mix depending on the manufacturer's secret recipe) oxides evaporated so much as being poisoned by incoming ions. However, at some temperature, you could get localized heating to spit emission-chemical ions off the cathode. If they lodged on the heater, you got VERY noisy tubes because the heater now active spat out hum electrons.

The higher resistance was sleeping sickness, I think.

I'm thinking that most of the wear in a tube's has to be related to amplifying a signal (thermionic emission at the cathode), so that if the cathode isn't actively emitting electrons then it's coatings are not being consumed. In contrast, there is constant thermionic emission from the heaters that get left on.

I think its a bit more complex. When an oxide cathode is heated, it spits out a LOT of electrons. But an electron leaving the cathode metal leaves the metal positively charged by one electron. So the electron is attracted back in. Electrons around a cathode are in a kind of sub-orbital loop, being spit off the cathode by thermal vibration, then falling back in. The hotter the cathode, the more electrons are looping out into the vacuum at any instant.

If there is no pull on them to go elsewhere, they pile up into an electron cloud around the cathode. The more electrons there, the more charge out there and the more they press electrons back into the cathode. Things balance out to where the space charge is maintained at some level for a given temperature.

If the remote plate is made positive, it pulls some of the electrons away from the cloud, and the power supply seeps some in the bottom. It is possible for some plate voltages to suck electrons out of the cloud faster than they can be spat out. The bad effect of this is noted in datasheets as plate current limits. Sucking the space charge cloud dry leaves the cathode open to ion bombardment, as well as self-heating since there is now a larger current flowing into the cathode and across the resistance of the energy barrier to jump an electron off into the vacuum.

I think that a tube idling at modest currents is not particularly bad. However, driving the tube to and beyond maximum current is BAD. Each cycle sucks a little life out of it. Not to mention that massive overdrive also makes the tube much hotter, speeding up every other bad effect.

What is a highly sought characteristic of tube amps? Power amp distortion. Ah, well, tubes are cheap.

I'm thinking that if you can weed out things like screen failure,

(largely screen current exceeding design spec and getting the screen too hot)

then the tube lifespan is ultimately going to be determined by the longevity of the coatings on the cathodes and/or the heaters.

I read a tech report from the Golden Age estimating the expected life of a small signal triode in a conservative design. It came to about 50,000 power on hours, and the estimate was for three hours per day. The subject eval was for a portable radio. That comes to about 45 years. My father had a portable radio that was tube based, powered by D cells. He listened to it a lot, almost daily. We had the thing the entire time I was growing up that I can remember. It NEVER needed a tube replaced that I can remember, and service calls for radios and TVs were a Big Deal in our house.

The bottom line is:
- keep them cool, which means not shutting them up in an oven like an AC30, and not exceeding the design current and power ratings; surges from power on and standby can easily exceed design current
- make your peace with power on hours versus thermal cycles; this will be personal

There's a lot of other things that could be done, but in general, guitar players will not do them.

[Enzo]

And that brings up context. As long as I have been in electronics, this sort of discussion has bubbled along. People look at these various little effects in terms of tube life and what not. But in my own experience, I see no reason to think any of it has much effect. Sure on paper we can infer that tube life - however we define that - might be "shortened" by some amount. OK, WHAT amount? And have we even defined tube life? I can imagine similar discussion on cars. "But the tail spoiler improves stability." Sure, if your mom drives 140mph to the store, it might keep the tail down a little better. At 35mph in traffic? Pointless distinctions.

I don't see conspiracies here. They sell 100 watt and 50 watt amps. We know there isn't much difference other than maybe headroom, but the players want what they want. making it switchable makes any amp either way. So now if someone wants a MESA 123XYZ they can't say, "Well I WOULD have bought it but I wanted the other power output." FOlks like to pontificate over whether the cathode should be lifted or the screens dropped, but I can't recall any amps that wore their center tubes out before the end ones, not really.

[Steve Conner]

I think those power reduction switches are kind of pointless. I would rather reduce power by leaving all the tubes in circuit and decreasing the screen voltage.

Bob P, if you're interested in "dynamic re-matching", check out the Fender 400PS. It had three speaker jacks, and each one had a switch contact connected to the cathodes of one pair of output tubes. So if you only plugged in one speaker, you'd only be running on two tubes, and the reflected load impedance would work out the same for one, two, or three speakers.

Your idea of switching the heater supply has issues of its own. For instance, the regulation of the heater winding isn't perfect. If some tubes get disconnected, the heater voltage to the remaining ones will rise.

[Tom Phillips]

And that brings up context. As long as I have been in electronics, this sort of discussion has bubbled along. People look at these various little effects in terms of tube life and what not. But in my own experience, I see no reason to think any of it has much effect. Sure on paper we can infer that tube life - however we define that - might be "shortened" by some amount. OK, WHAT amount...?

+1 on that.

My thoughts:
There is a tendency to spend an inordinate amount of effort on things that make little overall difference. You could add all kinds of automatic soft start and fancy standby circuity to your amp and the effect would most likely be minimal. Especially if you transport your amp to a gig in the trunk of your car where the tubes get exposed to lots of shock and vibration. That vibration could be a much more significant tube life factor than any standby scheme.

When one does a reliability analysis there are lots of things to consider. All the things that could occur do not have equal probability of happening. Furthermore, the effect of things that will most likely happen are sometimes insignificant when you figure them into the big picture.

Sometimes we just worry too much or fixate on a particular parameter. Like the situation where someone has an amp that develops a problem. He opens it up and finds that the heater voltage is 6.75 Volts. OMG that's high! Now he is worried and considering replacing the power transformer. What he doesn't consider is that the heater voltage may have been high when the amp left the factory. It has nothing to do with his current problem but the issue takes front stage and rational troubleshooting does not happen.

Tom

[Steve A.]

One more reason in favor of standby switches (if you really want one): some amps make a popping noise when you turn off the power switch, something that you might wish to avoid in a live setting. Yes, you can turn the volume control(s) down which might help but then you'd have to reset your volume levels.

One more reason against Standby switches (if you don't want one): What the hell does "On" and "Off" mean on a standby switch? When standby is "On" I would think that there would be no output from the amp but most mfgs do not agree...

Steve Ahola

P.S. I think that the choice boils down to convenience- if you want a standby switch go for it (but make sure that it is wired up intelligently.) I think that the least disruptive method would be to ground out the input grids of the power tubes. Or would it be better to disconnect them?
I think that cutting the power tube cathodes would be the second least disruptive method (if you have quad power tubes you could wire up the standby switch to cut one or both pairs of tubes.)
Those are just my own opinions...

[bob p]

check out the Fender 400PS. It had three speaker jacks, and each one had a switch contact connected to the cathodes of one pair of output tubes. So if you only plugged in one speaker, you'd only be running on two tubes, and the reflected load impedance would work out the same for one, two, or three speakers.

Interesting implementation with the cathode switching on the speaker jacks. i think Fender may have chosen that method in the 1970s because they were using speakers with a low power rating back then. today's bass speakers are rated to handle a lot more power, so it's not as if you've got to attach additional cabs to handle the power increase. today 400W speakers are common, and having a 2x15 setup would allow you to handle all of the power in a single cabinet, where only Z-matching would be necessary.

I was thinking about doing dynamic re-matching in a 2-driver cabinet where the speaker complement wouldn't ever change. My plan was to just do series parallel load switching to deal with the 2 vs. 6 tube compliment, as the Z-ratios stay fairly constant -- when reducing the number of tubes from 6 to 2 there's a 3x change on the primary side vs. a 4x change on the secondary side when you go from parallel to series wiring. 3x vs 4x. that's pretty close.

Your idea of switching the heater supply has issues of its own. For instance, the regulation of the heater winding isn't perfect. If some tubes get disconnected, the heater voltage to the remaining ones will rise.

sure, if you don't want to regulate your heater supply for low noise, then you have to load balance the heater supply when you go changing the load. so the switching paradigm has to deal with switching tubes on/off at the heaters, normalizing the load on the heater line, and load balancing the output. it doesn't sound too hard to make one switch do all of that, even if you don't want to go to the trouble of regulating the filament supply.

based upon your comments about the 400PS (and fender's implementation), am I correct in assuming that the answer to my previous question is yes? that switching off 2/3 of the output tubes results in tripling of the required load Z, and the tubes that are turned off but remain connected don't contribute in any way when it comes to load matching?

[jpfamps]

It appears that the standby switch was originated by Fender in the 50s.

Now Fender must have had a good reason for adding that at the time, as Leo didn't like frivolously wasting money on products.

As the standby appeared in short order in amps that employed the DC coupled cathode follower I have always assumed that it was to prevent excessive failure of this valve, although protecting the filter caps from the unloaded HT may also have been a further reason (although valve rectifiers will reduce the hazard here).

Use of the standby switch will also reduce the voltage stress on the coupling caps in the PI of a fixed bias amp. These are rated at 400 VDC in early Fenders, but have to tolerate the full B+ on one side and the negative bias voltage on the other if the PI valve is not conducting.

Regardless, I suspect the standby was seen as a useful feature and thus was soon employed on all larger Fender amps since.

Some manufacturers (eg Silvertone, Selmer) have simply shorted out the audio to effect a standby.

Regardless, I think it would be hard now to market an amp without a standby switch.

[loudthud]

The Vox-Thomas amps from the 60's had a standby position are they were solid state. It might have been there to reduce thumps from the speaker. There were two tube models IIRC, the Cambrige and Berkely II. I don't think they had a standby switch.