Demystifying Tube Amps---The Standby Switch

[bobyoung53]

No more tube amps? Say it ain’t so!

You can hear the sound of the room in each and every STAX record – deep and powerful in the bass but raw and almost noisy in the mids.
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OT but In late 50's Thelonious Monk first stereo records you can hear the room like you are standing in it, there is usually more bass in the same tune in mono though but they recorded those records using two machines, a mono and a stereo using room mics, unbelievable sound. I love the sound from the late 50's, early 60's records, also the hifi records from the 50's sound so immediate, nothing sounds like that today. I also have a couple of early 60's mono Warner Brothers, Everly brothers albums that floored me the first time I heard them the sound is so good.

 

[okcrum]

I thought you already explained that?

My understanding was that cathode stripping actually occurs in the tubes we use, but it is so insignificant that it's not really something to worry about. The articles I have read do suggest that it's a bad idea to leave an amp in standby for hours at time, but the 30 seconds or so it takes for a rectifier to start conducting is not an issue. Per the articles, even setting an amp to standby for a 20 minutes break is supposedly nothing to worry about.

But didn't you assert that cathode stripping cannot even occur because the operating voltages in guitar amps are not sufficient?

Still cathode poisoning is a concern we have mentioned, although I don't know how significant it is.

Vintage SVTs do not apply any DC voltages to the tubes before the standby is set to operate, so perhaps cathode poisoning is a concern here. Only the heater transformer is energized in standby mode. The standby switch is on the primary of the main PT, so none of the DC voltages develop until the amp is taken out of standby. So is it a problem to leave a SVT in standby for a long time?

How about amps that vary max power levels by lifting the cathode of some of the power tubes from ground. For example, the Trace Elliot V8 has a half power switch. In half power mode, a switch lifts the cathode on half of the output tubes. There is pair a 47k resistors bypassing the two poles of the switch. Maybe the resistors are intended to produce just enough tube current to prevent cathode poisoning.

For practical purposes the tubes would be biased off, but there will be enough current to power a LED. Each pair of tubes in the V8 is fused at their cathodes. The fuses are bypassed with a (different) 47K resistor in series with a red 3mm LED. So if the fuse blows, the LED lights.

I was partly addressing the issues raised in that Peavey article Jimmy cited.

It's not the voltage per se that does the stripping, it's the electric field, if the field gradient gets too large. So element spacings matter, too.

You're going to get cathode poisoning if you get too many free ions in the tube, regardless of whether there's a space charge around the cathode. The (positive) ions are going to migrate to the most negative electrode. That's the point of the getter, to Link Removed those atoms before they get ionized by the electron stream. Once the getter is saturated, then I'd expect to see the cathode start to deteriorate. You can use the tube itself as a hot filament ionization gauge to measure the relative qualtity of vacuum (and, indirectly, the amount of junk atoms) in the tube. I think the very best of the old commercial tube testers did something like this testing for gassy tubes. We used that technique to see if we'd blasted any material off the anodes of those real big tubes after high current pulses, and make sure the getters were still working, since you can't just look at them.

 

[okcrum]

Not a tube owner, but, wow, 175 posts in a thread about a standby switch ! I am impressed!

Yeah, a better thread title might have been "Debunking common vacuum tube myths" or something.

 

[Wasnex]

It's not the voltage per se that does the stripping, it's the electric field, if the field gradient gets too large. So element spacings matter, too.

Understood that it's field strength that does the stripping, but AFAIK field strength is related to the voltage differential and distance between the elements. Basically [electric field, "E"] is [voltage, "V"] divided by [distance, "d"]; the resultant formula is: E=V/d

I didn't read all of the Peavey white paper, but I got the feeling it was not consistent with what you are saying.

I believe the article I remember reading that claimed cathode stripping does occur in common guitar amp tubes was presented like a research paper using statistical methods. As I mentioned previously, the level of stripping was so low that it was basically inconsequential unless the amps were repeatedly left in standby for hours, or perhaps days at a time.

Although I actually think it's best to wait 3-4 minutes before taking an amp out of standby, to be honest, I am usually too impatient to wait with most of my amps. This should give you some idea how important I think it is to follow even my own advice on the subject. Most of the time I follow the common recommendation of waiting about 30 seconds. I do try to avoid leaving my amps in standby for hours at a time though. Although I have had accidents on occasion when I got distracted after putting an amp in standby.

I do know that a lot of people claim that tubes sound best when they are really, really hot. I think people come to some bad conclusions based on this idea. First it temps people to bias way too hot, which IMHO tends to sound mushy and unfocused in many amps, plus it puts the amp and tubes at much more risk. Also, I don't think leaving an amp in standby is ever going to bring the tubes to full operating temperature. IMHO the reason tubes sound best after they have reached full operating temperature is likely because that is where they were biased...at least that is where they should be biased IMHO. Basically you allow the tubes to reach operating temperature, so they will stabilize and hold the bias you set them at.

 

[bobyoung53]

Understood that it's field strength that does the stripping, but AFAIK field strength is related to the voltage differential and distance between the elements. Basically [electric field, "E"] is [voltage, "V"] divided by [distance, "d"]; the resultant formula is: E=V/d

I didn't read all of the Peavey white paper, but I got the feeling it was not consistent with what you are saying.

I believe the article I remember reading that claimed cathode stripping does occur in common guitar amp tubes was presented like a research paper using statistical methods. As I mentioned previously, the level of stripping was so low that it was basically inconsequential unless the amps were repeatedly left in standby for hours, or perhaps days at a time.

Although I actually think it's best to wait 3-4 minutes before taking an amp out of standby, to be honest, I am usually too impatient to wait with most of my amps. This should give you some idea how important I think it is to follow even my own advice on the subject. Most of the time I follow the common recommendation of waiting about 30 seconds. I do try to avoid leaving my amps in standby for hours at a time though. Although I have had accidents on occasion when I got distracted after putting an amp in standby.

I do know that a lot of people claim that tubes sound best when they are really, really hot. I think people come to some bad conclusions based on this idea. First it temps people to bias way too hot, which IMHO tends to sound mushy and unfocused in many amps, plus it puts the amp and tubes at much more risk. Also, I don't think leaving an amp in standby is ever going to bring the tubes to full operating temperature. IMHO the reason tubes sound best after they have reached full operating temperature is likely because that is where they were biased...at least that is where they should be biased IMHO. Basically you allow the tubes to reach operating temperature, so they will stabilize and hold the bias you set them at.

I also have heard that it's very uncommon for it to happen in a guitar amp, cathode stripping was a problem for AM radio stations when they used to shut down all night at sunset but leave the transmitters in standby night after night, eventually the power tubes would deteriorate from cathode stripping.

 

[okcrum]

With stripping it's really the overall geometry, since pointy things tend to have large electric field gradientts around them. Think lightning rods or static dischargers. So stripping, when it happens, starts around points and edges. I suspect the levels of stripping commonly seen in guitar amps are so low the getter takes care of the odd oxide molecule here and there. The data you've seen sounds like it's no problem in normal operation. It makes sense that the bond between the oxide emitting layer and the cathode base structure (usually a nickel tube) would weaken from extreme thermal stress.

It always makes me smile when someone claims factor X is the secret to an amp's sound. I agree it's not about temperature, it's about reaching a stable operating point, after the amp is fully powered up.

 

[Mark Reccord]

If there are, the amp is designed wrong.

This comes down to how we define "designed wrong" I expect. I mean, yes it's absolutely possible to design an amp that has no standby or soft start and will operate reliably with no momentary overvoltages anywhere. What I was getting at there is that plenty of amps with standby switches were designed around people using them so downstream circuits might not be designed to handle startup conditions if users ignored the standb switch. Whether that's "wrong" or not is a matter of philosophy and the potential penalties for operator error.

Wow, that could have cost you $15 Unless of course it was an old collectable tube...and that would have been a severe wallet ouch.

As for the tube warmup myth or fact, I have yet to damage a tube in that amp by not waiting, but I've only done it a couple times because I get paranoid.

Nope, I don't use expensive NOS tubes for prototyping. I'm not a moron.

Nah, the contention is that it's not necessary. Neither is television. You can choose to avail yourself of either if you want, and make a cogent argument either way.

There is no standby function on the most common tube equipment of all: the microwave oven. So it goes both ways.

I would agree that a standby switch isn't necessary assuming the rest of the design takes that into account. As for microwaves, I'd guess that the surrounding circuitry is designed to operate like that and how long does it take to get the cathode of the magnetron up to operating temperature? It can't be very long, like less than a second. Right? Not a snarky question at all, I'm genuinely interested. Also fascinated by the other things you've posted about your career in this thread. I work in a world (aerospace/avionics) where things need to be 100% reliable under harsh conditions so my design process is pretty stringent compared to that in the consumer world. Haven't had much experience with radar but I am responsible for a legacy system that produces dangerous energy levels and that is always "interesting" to work on. 20kV isn't something to be trifled with..

 

[Mark Reccord]

I've spent a lot of time on the road but i don't think I've ever cooked anything on an engine block...
I can remember being poor enough that all we could afford between six of us was a couple cans of potted meat and a loaf of bread to make sandwiches. We stole mustard packets from the hot dog thing at the gas station where we bought the other stuff.

 

[catalin gramada]

I've spent a lot of time on the road but i don't think I've ever cooked anything on an engine block...
I can remember being poor enough that all we could afford between six of us was a couple cans of potted meat and a loaf of bread to make sandwiches. We stole mustard packets from the hot dog thing at the gas station where we bought the other stuff.

All concerns regards stb sw using comes once with SS rectifier start to be used. A lot of tube operation text was writed in a time when SS rectifies didn't exist,so no associated concers was taken in consideration. With a tube rectifier a stb sw seems to not be necessary , not for start up faults, not for circuit reliability. But from a start up point a tube rectifier is a inrush current limiter, (which is a good think to use for reliability of any supply circuit), a classic stb sw is not. It is easy to implement an inrush current limiter using even a stb sw to commute manual, or a relay controled. Both will assure a soft start control as time the key is a power limiter resistor which will be in circuit at start and bypassed by switch ( manual or relay) in "on" position.

 

[okcrum]

This comes down to how we define "designed wrong" I expect. I mean, yes it's absolutely possible to design an amp that has no standby or soft start and will operate reliably with no momentary overvoltages anywhere. What I was getting at there is that plenty of amps with standby switches were designed around people using them so downstream circuits might not be designed to handle startup conditions if users ignored the standb switch. Whether that's "wrong" or not is a matter of philosophy and the potential penalties for operator error.

I tend to define wrong in relation to design if it's not physicist proof, a standard somewhat beyond idiot proof. While we could test units to destruction in some cases for R&D, the systems we delivered for research use by the physicists had to be 100% safe under all operating and fault conditions, since all the relevant safety standards still applied. The physicists were often the operators. Hence the term.

I would agree that a standby switch isn't necessary assuming the rest of the design takes that into account. As for microwaves, I'd guess that the surrounding circuitry is designed to operate like that and how long does it take to get the cathode of the magnetron up to operating temperature? It can't be very long, like less than a second. Right? Not a snarky question at all, I'm genuinely interested. Also fascinated by the other things you've posted about your career in this thread. I work in a world (aerospace/avionics) where things need to be 100% reliable under harsh conditions so my design process is pretty stringent compared to that in the consumer world. Haven't had much experience with radar but I am responsible for a legacy system that produces dangerous energy levels and that is always "interesting" to work on. 20kV isn't something to be trifled with..

It's pretty easy to tell the heater warmup time on a microwave oven, because the tube cooling fan will slow slightly as soon as it starts drawing anode current. The one in my kitchen takes 2-3 seconds. I'd posit the best consumer gear (Genz Benz, for example) is designed by people with hi-rel experience.

Once you get into realms with higher energies, things like grounding sticks and keyed interlock systems become normal components, as you know. Even simple things like a small signal passive filter get interesting (and large) when one of the design requirements is that it safely withstand 25 kv fault conditions at the input coax N connectors. Passive = no fiber optic links, either.

 

[okcrum]

All concerns regards stb sw using comes once with SS rectifier start to be used. A lot of tube operation text was writed in a time when SS rectifies didn't exist,so no associated concers was taken in consideration. With a tube rectifier a stb sw seems to not be necessary , not for start up faults, not for circuit reliability. But from a start up point a tube rectifier is a inrush current limiter, (which is a good think to use for reliability of any supply circuit), a classic stb sw is not. It is easy to implement an inrush current limiter using even a stb sw to commute manual, or a relay controled. Both will assure a soft start control as time the key is a power limiter resistor which will be in circuit at start and bypassed by switch ( manual or relay) in "on" position.

Amps had standby switches for awhile before before SS rectifiers were practical. One example is the 1957 Fender Twin Reverb. While the tube rectifier will limit charge current to the filter cap bank as it warms (and the standby switch is in the open position), it is itself still subject to immediately applied plate voltage, as I noted above. FWIW, I have a 1941 radio receiver with a standby switch. It is labelled Standby/Receive. It's not there for current inrush or tube life. In those days you flipped the switch to standby before transmitting, so you didn't burn out the receiver input stages.

 

[okcrum]

I also have heard that it's very uncommon for it to happen in a guitar amp, cathode stripping was a problem for AM radio stations when they used to shut down all night at sunset but leave the transmitters in standby night after night, eventually the power tubes would deteriorate from cathode stripping.

That's not cathode stripping, because there's no cathode in those power tubes. They use thoriated tungsten filaments. That's simply the filaments wearing out, and becoming exhausted of emitting material.

 

[catalin gramada]

Amps had standby switches for awhile before before SS rectifiers were practical. One example is the 1957 Fender Twin Reverb. While the tube rectifier will limit charge current to the filter cap bank as it warms (and the standby switch is in the open position), it is itself still subject to immediately applied plate voltage, as I noted above. FWIW, I have a 1941 radio receiver with a standby switch. It is labelled Standby/Receive. It's not there for current inrush or tube life. In those days you flipped the switch to standby before transmitting, so you didn't burn out the receiver input stages.

It is also an other benefit using an inrush current limiter: you can use a more calibrated fusible fuse supply protection. Usually tube circuits use oversized slow blow models to not blow instantly at the start up. With a limiter into circuit we can choose a fast fuse more close as rating to nominal max current demands by circuit. That mean more effective protection.

 

[JimmyM]

Nope, I don't use expensive NOS tubes for prototyping. I'm not a moron.

Heehee! No, you certainly are not a moron!

On another topic, as far as 12ax7's go, I'm pretty well done with NOS. Got a handful of them, and when they're gone, they're gone. Except for 6SL7's...most new ones still suck in B-15's so I'm either stuck with them or new Tung Sols, which I hear are really good but about as expensive as NOS.

 

[bobyoung53]

That's not cathode stripping, because there's no cathode in those power tubes. They use thoriated tungsten filaments. That's simply the filaments wearing out, and becoming exhausted of emitting material.

The filament is the cathode in those tubes but you're right most large transmitter tubes are thoriated tungsten tubes but not all, I don't remember where I read that cathode stripping was a problem first seen in big transmitter finals. If I find I'll post it.

 

[BogeyBass]

I want a band called Cathode Stripper

It's so funny though, watching guys freak out over standby switching.

Best thing do is unplug and replug your bass and pedals with amp on lol.

Pssst zzhhh ssttt pop pssst zzhhh pop pop.

What??

Pssst zzxhhhh pop

Standby ??? What's stand zzhhh click