How long do tubes really last?

[Rob Gee]

This is at Steve.

I've recently started gigging and touring a lot more often than I had when I first started collecting gear. I always have played music, I went to music school for 4 years. As a kid from 8th grade on that's all I wanted to do. But, I played mostly jazz and studied trumpet for my music performance degree. I found myself into underground music quite early on, yet while a couple of my friends might have had one or two lucky chances to see Tool, Metallica, Blink 182, or Linkin Park, I was going to at least 1 local show a week.

Local shows around my area might have cost 5-10 dollars, had 30 to 100 people, and took place in rented VFW halls or basements in Boston... You bet I heard some awful amps too, I didn't know it at the time though, and I couldn't have been more happy (ignorant) about guitar tone for the most part.

I picked up the guitar as a freshmen in college at about 19. I bought a used PRS for my first guitar and about 2 months later an incredibly sweet '81 JCM800 2204, both for excellent prices... Go figure- it only took until the first amp I owned to have something to brag about to everyone. From that moment on I had an objective outlook on guitar tone. I knew that I had a setup with more potential than most of my favorite bands did live, and I saw that first hand in rooms.

I was the first of all of my friends and acquaintances to own anything even remotely "collectible", and honestly never got a chance to be "in awe" like most players were their first time playing a cranked Marshall AVT half stack.

Sorry for the long winded-ness. Long story short...

But then I realised they are actually very clever and are delivering exactly what the majority of the market wants. Something affordable

I've grown to realize the truth in your statement.

I can appreciate that these big name companies are putting out decent/durable beginner gear. With a solid state amp as a main amp, you can likely throw it from the kitchen all the way down the basement stairs before band practice, then play for an hour or whatever, and you might not even flinch as it hits the cement. That's some genius craftsmanship and design.

Plus, who wouldn't want to be able to drop your precious amp out the bedroom window into the grass instead of hauling it down a set of stairs.

[km6xz]

RF tubes are a serious business still, they have to stand up to the standards of a technical field, broadcast, RF industrial heating, and other high power uses. A 4CX20,000 had better maintain its full rated performance of 20,000 watts of anode dissipation for 5,000 hours continuous duty, "key down" or it would be a scandal and cost the manufacture plenty. There is a regular demand for the tubes so there is a support industry for them.
My 4CX1600B's in my class AB linear amplifier never dropped in gain or output, nor increase in IMD in the 15 years I ran that monster amp in RF service.
Guitar amps have two major issues with keeping tubes from being happy for as long as the tubes were expected to last.
1.) being that they are run in modes that were never intended, with higher current, frequent grid current flow and impedance mismatches, and subject to extremes of vibration. In the past that would have called for W versions of tubes that were for mobile operation. Now, a combo amp is like running the tubes on a shaker table and we wonder why grids distort their shape or get loose. No TV set or radio in the golden age of tubes had that sort of mistreatment considered normal.
2.) The second point that limits life is the tubes are used in circuits designed for a different tube than the one stuck in it. I've seen guitar amps here with 6п3с tubes built in the 1960s with the original tubes in them and still working fine. The reason is that they were not stressed and they were in an amp designed to use 6п3с tubes. Do they sound great? They sound clean, but that is not what people want now, they want a sound that is created when a tube is being squeezed to the last possible electron. It would be as if we complained about the short life of song birds when the sound we sought was produced only when the birds was being strangled and turning blue. Sonic tastes changes, not physics or tubes.

You CAN make a tube or solid state amp that sounds anyway you want and be reliable. Just learn what the transfer function would be needed to produce that sonic signature you wish and design the circuit with it as a design goal.
I've experimented with small 1-3 watt tube reference amps in the feedback system of large highly linear, fast solid state power amps and have gotten sound that mimic class old tube amps...only big and bullet proof. A 500 watt Champ is certainly possible, I've done it, but I see most experimenters working with tweaking 1950s and 60s designs, using 2011 components with unknown characteristics. That is something like painting a still life blindfolded and than stressing over the fact that what you thought was red, caused the apple in the still life to be blue. Spend a little time plotting curves for whatever tubes you have and wind transformers accordingly and the project would not be so hit-or-miss as they seem to be now.

AF tubes for guitar geeks is not a serious market and little or no original research has been done for 40 years. Tubes don't get better, just fancier names and more colorful logos, despite the world of materials research has created metals, ceramics and polymers that would greatly improve new tubes. We know that because they ARE used in metal/ceramic RF tubes now. Do you really think that there is a single engineer and vacuum tube analysis lab in all of New Sensor? If you talk to them you get marketing buzz works and B.S. But why not, they make their money on the dreams of people, not the reality. By changing the glass envelope of a old Soviet era consumer tube they can charge 2-3 times as much as for their standard tube with the same internals. That is the reality and we just have to make the best of it. But that is most of the fun, through away the spec sheets, none of these are the tubes we think they are. If you do your own curves you will find that most are pretty decent if the sweet spot of linearity is picked after doing real curves for the tube in hand, and forgetting the label and the 6550 or EL34 spec sheet. For example, the Soviet era 6п3с-е is really a nice well built conservative tube that is very useful....but it is not a 6L6 or 5881 that the rebranders sell it as. If operated as intended it is tough and quite nice. If you have been around any of the rebranders, you will know they have test jigs that only test for cathode current for rating, dozens of tubes at a time. Ruby I know has a microphonic machine that uses 20 or so little brass hammers tripped by a small low speed motor and cam that someone made. It works but hardly scientific, yet they do the BEST in the industry in testing and ranking tubes mainly because they are not good salesmen and have not created unrealistic expectations. That also means that they are less known by users unless they use major brand's amp's stock tubes. The others have even less testing. GT's famous computerized matching system is entirely marketing BS.
When the Chinese GZ34 was being developed, there were no analyzers involved except what I had on my bench( a tube curve tracer) because the company making it brought samples, each with small changes in spacing etc for analyzing. That process was slow but a very close performance equal to the Mullard GZ34 was settled on based on one NOS GZ34 reference and the original spec sheets. As far as I know that is the only newish tube that had curves and measurements done on it before release and I did them because no one in the tube company knew how or why to do it.

[jrfrond]

Regardless of where they are made, I am finding that modern power tubes just do NOT last in guitar amps for any measurable length of time, while I still get vintage amps with original GE's, Tung-Sol's, RCA's, etc. that still test good (on the Maxi-Matcher, which I love), aren't microphonic and sound better than any of the Eastern Bloc or Chinese imports (which I will not even touch).

Most of the issues I have seen in new tubes are screen grid collapse and heater-cathode shorts. Not much you can do about the latter, but I have contemplated the former, and for hard-use applications, particularly rehearsal studios, I am now biasing power tubes slightly on the cold side, doubling screen grid resistor values to limit screen current and adding transient suppression diodes (R3000) on each side of the OT primary. So far, this seems to be working well, with no palpable sonic or feel difference in the amps involved.

NOS, particularly with preamp tubes, does NOT always mean better, but concerning power and rectifier tubes, I think it gives you a fighting chance with reliability, and a bit of a sonic edge where power tubes are concerned.

Stan, what exactly IS the primary application of the 6P3C tube in Russia? Who figured that it might make a good sub for the 6L6/5881?

[überfuzz]

...I am now biasing power tubes slightly on the cold side...

Interesting. The forum pope might issue a papal bull of excommunication now, but... I often bias by ear to run the tubes as cold as auditive possible. The idea is that it might squeeze more life out of the tubes.

[Enzo]

And you all have discovered the Peavey design philosphy. Folks like to bitch about how cold stock PV amps are biased, but the company will flat out tell you it is for reliability.

[überfuzz]

I curse them to H... eh notin'.

[km6xz]

Hi John
The 6п3с was a popular tube used in radios, hi-fi, TV's, communications equipment and just about any function you needed a super duty 6V6. It is right in between a 6V6 and 6L6GC in power and anode potential but like a 6L6 in gird and screen current. The 6п3с-е was a super duty version of the 6п3с and used for everything from p.a. amps, guitar amps, VR, modulators. This latter tube is sold as a 6L6GC or 5881 by re-branders so you are using them now. They are still being made by two plants, one now controlled by Mike Mathews. There were millions made for industry and military applications so there were hundreds of thousands or more surplus after the Soviet era ended. The USSR was an early adopter of transistors and IC of unique domestic designs so tube product slowed down from about 20 active plants making them to 4-5 by 1991 when the Russian Federation came into existence. A lot of hobbyists used either in home made guitar and hi-fi amp, which was a thriving hobby in the 60s-90's until cheap imports because available. It is a country with a great many skilled engineers, as a higher percentage of overall population than any country. There are 27 engineering universities dedicated to electronics and CS in this one city alone, of a total of 510 universities, colleges, institutes, academies and conservatories within the city limits. Throw a snow ball on the sidewalk and you will likely hit an engineer, scientist, doctor, accountant or conservatory trained singer, dancer or musician. I know only one person, an artist, who does not have a college degree. You have seen my photo album of my friends, all young intimidatingly smart well educated girls who just happen to be beautiful and fit. So there is a hobby of electronics design, amateur radio and hi-fi using novel circuits that used parts that were available, nothing imported. All I've seen used lower plate voltages than guitar amps in the west. 250-350V B+ was common for either of these tubes. As a result of running them safely, as were the western uses of the 6L6, meant the tubes lasted forever. Tubes now are usually run just below their "I.S.D.V.A.O.S" rating... Instantaneous Self Destruction Via Abusive Over-Stressing rating. The problem is that users only like the sound of a tube that is about ready to explode. There are lots of ways of obtaining that sound trait that does not involve clubbing poor tubes to death in the process.
Guitar amp tubes are not as bad by nature as users and techs claim, most of the sound character traits they claim come from easy to explain reasons of differences in gain and stress. When people sit around talking about tube sounds as a critic would discuss art, with subjective terms involving analogies( "I tried xxx but they were darker sounding in the mids and had constricted air" or "shimmering upper mids but overly aggressive low mids" etc) I usually turn and walk away. It is like going to a New Age health convention of fools trying to fool each other....for a buck. The test bench reveals no traits that can cause such a change. Double blind tests reveal no such differences. Theory does not account for any such differences. Most differences that are heard are simply gain related. If an amp is normalized between tube swaps, the claimed differences disappear. Gain changes perceived frequency response, as well as gain affects measured FR or the level of cut or boost at a particular knob setting. Normalize the amp and changes in tone disappear. A move of a treble pot a couple degrees might be needed to normalize or lowering or boosting a gain setting by 4-5 degrees of pot rotation but once done the listener can't hear any difference. A tube that is claimed to have poor high frequency response of similar design....if there is a difference, it is NOT the tube. Even the worst 12AX7 to come from any factory has a gain-bandwidth product of 60Mhz. No working 12AX7 has measurable decrease in transconductance in the audible spectrum. A circuit/tube combination might but not if the tubes tested are of the same general spec. Noise is another issue swamped by gain differences. The noise spectrum can be affected by structure and materials used however but most of the noise is a predictable type that follows the same spectral distribution.

Tubes last a long time if run as designed. No tube in current or past production labeled 6L6xx was designed for, or indented for operating conditions found in modern guitar amps. There are lots of tubes that would laugh at those elevated currents but since they are then running well within design parameters, would be rejected by the guitar community as being too boring.
To show what some people who are taking a more scientific approach to guitar amp sounds, here is a guy right here in St Petersburg who designs solid state analogs of well known tube amps and releases the pc board layouts and plans to the hobby builder community:
??????? ????????
When he gets simple transistor circuits to have the same spectral distribution and overload characteristics as the tube amp, it SOUNDS the same even if a micro power version. If you can get the same sound, why do it by killing otherwise perfectly good tubes in the process? The difference in designing sound with tubes contrasting solid state is that the devices are operated within sound engineerings safe regions only with solid state.

[Steve Conner]

Stan, do you know the KMG guy? Have you tried any of his amps? I've been having some interesting discussions with him over on ssguitar.com. I really like his stuff.

I realised what you said 10 years ago when I built my first guitar amp. I fired it up to full power and watched the screens in the EL34s glow almost white hot, and it wasn't even running them as hot as a real Marshall would. I experimented with screen current limiting, but the main effect was just to cut me off mid-solo.

I fully agree that the way forward involves being a bit nicer to the poor tubes, especially if using original ones. Well, except for those 6p3se tubes which seem incredibly tough and can be bought by the crate on EBay.

[km6xz]

Hi Steve
No, but I figure I will run into him. I know where he works and have seen his pc boards used in home brew projects that worked right off.
I still have some tube test gear decades old that have original tubes that still meets spec. But roaded musician gear is not in the same league in regards to stress. The mechanical stress alone would be enough to cause rapid deterioration, like operating on a shaker table. Add the overdriven state being the norm, and wildly varying load mismatch handling quite a few octaves in bandwidth, mis-adjustment being the normal substitute for preventative maintenance, and blind tinkering all add up to a wonder that guitar amp tubes last as long as they do.
I've measured the grid and screen current with current loop probes out of curiosity on a gain boosted JCM800 one time when played maxed out and I was surprised the abuse the screens and control grids took without melting.

One of the problems is that guitar amp tubes are not driven hard enough at steady state to have the getter action of the anodes reabsorb gases. RF power tubes are often rejuvenated by red platting for a controlled extended period with the anode coating being a getter that only works at higher than operating temperatures. As a result of that, careful maintenance of large RF tubes can be extended far beyond the period they are spec'd for. Small audio power tubes run close to destruct level most of their operating lives but not quite hot enough to reabsorb gas. The shortens their life as gain drops, they are replaced instead of rejuvenated. Other than the cost of transformers I wonder why people have not moved towards mid-power tubes in pairs instead of expensive quads of too high priced low power tubes? 4 6550s pull more heater current than a couple 812s to 572B's, which are more rugged, not much larger per tube and are relatively cheap, besides having higher plate dissipation. Maybe the plate caps scare people off. I've built power amps with exterior anode metal ceramic tubes that are quite small yet have 5-10 times the plate dissipation of any of the popular PO audio tubes. If you do not mind triodes, that Svetlana 3CX300 was a sweet little tube, tiny for 300 watts of CCS rated plate dissipation and easy to match plate Z in AB mode. Hammond has off the shelf transformers that would generate 300-350 watts of no stress, last-as-long-as-you-do reliability. All in the size of a doorknob.

Transformers or lack of them or the money to have them custom wound is one of the reasons I like the new developments in Class D and the hi-performance controllers out now. Each month the distortion gets lower and the power density increases. 400 watts of power in a 1 lb amp and 1 lb power supply, that runs cool is pretty exciting, science fiction sort of advances. I am focusing my attention to low level sound character generation to drive boosters, it opens so many possibilities when the sound character is separated from the power producing section. Need 50 watts, no problem...here is a 2 lb booster. Need 400 watts, no problem plug your sound generator into 1 or more generic current boosters for a modular system, which all sound like the 100mw signal source processor that creates your tone, even if it is tube.

[überfuzz]

Off topic: Interesting stuff, I can't see what you would need a 300 350 W stage in a guitar amp. Maybe a bass amplifier. Folks in this forum will probably not start buying these amps, but they might find your schemes interesting. Since this forum is a non commercial forum I guess you'd be happy to post them.

About the ear bias I was talking about. The other day I happened to catch a program on telly. It was about the human senses. Amongst other things they compared the ear with some sophisticated probing equipment. It turns out that the ear is holding it's own. Further, each time I've double checked the bias after ear biasing, current and voltages have been just dandy. Well not really low in all cases. Some Fender amps has shown middle to hot biasing after ear probing.

[Steve Conner]

Well, the human ear is great at detecting something or other, the hard part is figuring out what the heck it is in objective terms.

I proposed the current booster idea back in the 90s. I think I reinvented Kevin O'Connor's super scaler. But it wasn't until last year that I figured out how to make a simple, reliable one. I built my new hybrid amp as a test of the concept, and it worked great.

I always thought radio hams had the right idea. If they want more power they don't build a bigger rig, they just strap a linear amp onto their existing one. Same idea as this low-powered tube amp driving a booster thing.

The problem with larger tubes is that they generate their power through increased voltage. A 572b can't pump that much more current than a 6550. So your amp needs a high B+ and a very special output transformer. That's why amp makers used lots of smaller tubes in parallel: to make the OT cheaper.

K O'c's London Power had a bass amp powered by 4cx250bs. I always liked that, but they don't make it any more. I'm going to guess it didn't sell because it was too unorthodox.

I had one of those 400w Class-d modules, and got it sounding quite good with a preamp that used JFETs and diode clipping. But I blew it up somehow.

[km6xz]

Players "need" 500 watts for the same reason they "need" 560 HP SUV's. They don't, but they want them and claim wildly illogical reasons why they "need" them.
I, personally, have never heard a 50 watt amp that did what a 20 watt amp could do for a song but I have a different perspective, that of an audience member.

There is one reason cited that on the surface sounds reasonable, that their amp just is not loud enough to hear over the drummer. In reality it points to a lot of problems, sound volume of the guitar is not one of them.

I am not building amps for a living, and wouldn't even if someone could afford it. I like messing with designs and the connection between perception and engineering. When aspiring recording engineers applied for custodian jobs at the studio, the first question often was "what classes do I need to take to work in the control room?" My answer usually was the last thing they would want to hear "psychology(human perception), cultural anthropology, history and art appreciation". To their great disappointment, they were expecting "Recording master techniques" or such. I countered that the tech stuff was easy, anyone could get that learning some buzz words, but that would only allow someone to be a journeyman recordists, possibly recording HS jazz bands or teaching recording, be was never enough to create a connection through the sound medium to someone's emotions and paint aural pictures in brains. That is what a few engineers and producers who are earning a good living can do, while the other 9,999,999 just talk about it on forums and argue gear, and never do a project that anyone does or should care about.
I encourage others to experiment with more solid states and sound processing, both are at their infancy and will spawn all the advances, while playing with tubes is sort of like hod-rodders tinkering with their 56 Chevy, a fun past time for self and family but not getting anyone anywhere. No one outside of the cult will care a bit.
There are a lot of advantages to dealing with solid state, it is safer, cheaper, smaller, more versatile, more scalable, less dependent on technical limitations, and more on designer imagination and more.

[km6xz]

The 811 style might have been a bad example because of the 1500 volts or so but the 3CX300s loafing to last 10-15 years of continuous service matches great to an off the shelf Hammond 1650T for a bullet proof 300 watts at reasonable plate voltages. There are thousands of cheap 4CX250's of all the different versions out their surplus since they were used for so many years in communications gear and RF heating applications. It is just as easy to build larger tubes that transistors are not competitive with yet so the tube manufacturers have abandoned the sub 500 watt exterior anode tube market to focus on the area of demand....high power.

As current boosters, it makes a lot of sense to come up with an interface standard so modules can be mixed and matched. Most modern AM broadcast transmitters have hot switched power modules, dozens or even hundreds. When one fails the transmitter stays on the air, the bad module can be unplugged and a replacement shoved into the rack. 50 watt power modules that can be paralleled for audio application could be developed the same way, to allow sound generation to be a separate function than the load driver portion, from different companies or mixed and matched. I have heard about the attempts by some to create a open source alternative for hardware compared to software. Small easy to build kits or as booster modules could be very cheap and scalable. The focus than could be on sound and tone in the front end which would be a different device.

For the home builder, good HV transformers are still available on the surplus market but audio output transformers never were in abundance for high power. Modulation transformers do not match speaker type Z's. Maybe a massively series configured speaker array would allow use of such middle Z secondary transformers. The small efficient solid state direction makes the most sense and is doable. The big tube rig would appeal to those people who otherwise would be building monster trucks with road grader tires and blown Allison 16 cylinder engines, an ego trip of extremes.

[trobbins]

Out of theoretical interest only, if your aim was to alter amplifier conditions to significantly extend valve life, and you could accept any 'performance' change that those conditions caused, then it looks like heater power is the key characteristic to focus on - given that no mechanical or other failure mechanisms take out a tube by chance. This is my take of a submarine cable intensive investigation that went on in the 1950's (http://dalmura.com.au/projects/Submarine%20cable.pdf). The data suggests that 20 yr continuous operation is achievable.

Mind you this is all hyperthetical as failure modes are many and various and every application has a different set of issues.

Ciao, Tim

[tubeswell]

Hi John
The 6п3с was a popular tube used in radios, hi-fi, TV's, communications equipment and just about any function you needed a super duty 6V6. It is right in between a 6V6 and 6L6GC in power and anode potential but like a 6L6 in gird and screen current. The 6п3с-е was a super duty version of the 6п3с

That is very interesting. I have a few 6P3S and 6P3Se and come to think of it, the plates look similar to JJ6V6S (if I may say so). I tried running a pair of 6P3S at about 450 on the plates as an experiment last year but they didn't seem to last, whereas the 6P3Se seem to handle it no problem. The datasheets for both types say they are a 20 Watt tube. But maybe I should try running the 6P3S at more like 6V6S dissipation but into something like a 4k reflected load?