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Thread: How long do tubes really last?

  1. #1
    Noodle of Reality Steve Conner's Avatar
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    How long do tubes really last?

    I've always thought of power tubes as having a lifespan of a few thousand hours, similar to lightbulbs. Then I saw this:

    http://lists.contesting.com/_amps/2003-06/msg00151.html

    This radio ham claims to have over 40,000 hours on a 4CX250B tube, and it's still good!

    This raises a lot of questions, such as:


    Do you really need to retube your amp every couple of years?

    Do modern tubes have a pathetic lifespan compared to NOS?

    Do glass power tubes have a short life compared to ceramic RF tubes like the 4CX250B?

    Do musical instrument amps abuse the tubes so badly that they go to an early grave?

    Are there easy design changes we could make to said amps that would make the tubes last 5-10 times longer without compromising tone?


    I've got a big collection of used tubes that I've pulled from scrap equipment, and I rarely see a bad one. I think I saw one GEC KT66 with a damaged grid that caused a red spot on the plate, an EL37 that had thermal runaway, and a pair of metal US Army 6L6s that were gassy. Every other tube I've ever pulled has been full of life and sounded good. I have some Sovtek tubes too, but I've not put enough hours on them to draw any conclusions.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  2. #2
    Senior Member Old Tele man's Avatar
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    ...check the MIL-STD RELIABILITY Handbook for vacuum tubes, there're equations and tube-life estimation data...yes, the MIL/NOS tubes lasted quite awhile!

    ...RDH states: with power tubes, "end-of-life" when gm falls below 70% of original spec.
    ...and the Devil said: "...yes, but it's a DRY heat!"

  3. #3
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    Environment that the tubes are in, the amount of abuse they get affect longevity...in most cases (semi regular use, realistic bias, not run "balls to the wall" all the time) they seem to last more that 24 months. The thing is you get accustomed to how they sound and don't realise small amounts of degredation...it's a bit like looking in the mirror and thinking, "Who's that fat grey haired bloke?"...it creeps up on you.

    Unless you have a bunch of the same brand of tubes (new, used, just functioning) how can you rate their tone...yes that old used GEC KT66 probably sounds great, but how about compared to a NOS tube? Tubes will continue to "function" well after they have stopped sounding like they did in their prime.

    A lot of harp players I know don't run their power tubes as hot current-wise as guitar players, 4-5yrs of regular gigging (2-3 nights a week) is no problem for current prod 6L6.

    NOS vs modern is probably more relative to brand than age, I've had St Petersburg Svets let go at dissipations they should handle easily (but still recommend them in less arduous situations because they sound good), the venerabe Sovtek 5881 just trundles on & on (no help to you if you don't like the tone though!).

    Guitar amp tubes are relatively cheap, accessible - changing them every couple of years is not going to break the bank on your average 4xEL84, 2x6V6 or 2x6L6 amp. Having said that, despite my nagging, I don't know anyone who does regularly retube for reasons of reliability, some do it out of curiosity though. Most wait til their amp barely functions/has trouble finishing a set/starts screaming & humming at them before they bring them to me.

    "Do musical instrument amps abuse the tubes so badly that they go to an early grave?" Some do, some don't. There's probably millions of little SE amps with the original tube in them. Can't be many vintage AC30's with original tubes?

    "Are there easy design changes we could make to said amps that would make the tubes last 5-10 times longer without compromising tone?" - equal reaction for every action and all that, but humour me - if I told you that running your Super Reverb at 15mA per tube with Sovtek 5881s would make them last 10 or more years would you...or would you fit the tubes you liked the sound of best? If biasing hot gave you a magical tone but meant that tubes only lasted 12 months what would you do?

    Perhaps note on a bias pot min & max setting for the tubes you have & set by ear within those perameters...you *may* possibly find that your favourite sound may not be at max dissipation limits? I think though in most cases, guitarists will find the hotter end more pleasing.

    Amps are musical instruments, they're ONLY purpose is to make a pleasing sound and be reliable enough for you to be comfortable knowing that they'll make it through the night, night after night for a period that you are happy with.

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    I've heard some anecdotes about certain tubes lasting very long such as the GEC KT66 and Mullard GZ34. I don't think the new ones are worth as much compared to the old ones, though of course sometimes you get duds with the old ones sometimes. From a quality control perspective today's stuff doesn't make sense if the tubes need to be sorted after they leave the factory. The old ones had etched or printed codes that said when and where they were made so they could track problems down. Built at a better time for tubes. More, bigger markets, more serious and continuous tube production, more testing on some of the MIL, industrial, special purpose old stock you can buy today, more competition, etc. As an example of a superior old tube, Bendix 6106 ($15-30 or so?). Sort of a super 5Y3GT rectifier tube. If you saw one (special glass, ceramic spacers instead of mica, ceramic cathode sleeve?, etc.) I don't see how anyone would see a modern version as a better value.

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    "I've heard some anecdotes about certain tubes lasting very long such as the GEC KT66 and Mullard GZ34." With respect, I have to say thast I have heard anecdotes and had practical experience to the contrary regarding the Mullard GZ34. I am not alone. Even the boutique builders were mainly using the Chinese Ruby a few years ago, before the JJ & Reflector came along.

    I think generalisations based on "new" & "old" are really too broad to have any validity today, especially after some stocks have been picked over to the point that it's just the ink on the tube that sells it. Undoubtedly, back in the heyday there were tubes of a consistency and quality that surpasses much (in some cases all, in others not) of what is available today...and many commonly used amp tubes (such as 5Y3 that you mention - what production amp came with a Bendix 6106 by the way?) are still so readily available, at a reasonable price, that there is no real reason to pursue a modern equivalent...if indeed there is such a thing? (The high voltage Sovtek/Reflector "5Y3" clearly isn't, reports on the Reflector 5751 are not favourable either, but then I only like the GE out of the NOS choices).

    I have thrown away countless noisy/short lived, military grade, NOS 12A#7 tubes (a friend of mine has dozens of Brimar 12AX7 NIB - ALL useless)...in the end, even after screening by a reputable dealer, cost vs reject rate/lack of longevity means that I'm generally better off buying current production, unless specifically requested otherwise. If the odd current prod needs replacing a few weeks down the line, then it costs less than when the same thing happens with NOS (which it does). Admittedly, for all I have just said, I do succumb every now and then to bit of NOS bait.

    I'm sure that we can bat anecdotes back and forth regarding "this NOS tube" and "that current production", but for a lot of folks it's just more practical to use the better current production choices. It makes no more sense to pursue a tube simply because it is NOS than it does to buy a crappy sounding tube because it is current production.

    Whilst there are amp manufacturers (hundred+ amps per year) who offer NOS upgrades, are there ANY that can rely on NOS exclusively? Shouldn't be a problem for smaller scale production/keen hobbyists.

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    yes I guess it would depend on your circumstances whether old stock is a viable choice and sure they aren't all good or were so and some of the new ones seem okay (Svet EL34, JJ) but (while there seems to be some improvement and promise) the manufacturing stance as a whole seems fundamentally compromised in the sense that there seem to be no standards. As I mentioned with the etched codes, how do they track down defects? How much do they care if they don't have such a system? Can you get a datasheet with genuine specs? I see things like old famous trademarks being bought up and "Mullard" put on "EL34s" which don't seem to have that much to do with the original, etc. Personally I could care less about the brand. The performance should matter more than whether it's Russian or Chinese, or whatever brand it has right? But maybe all of that is inevitable because markets are too small, labor costs too high for skilled manual labor in the "West", not enough competition, etc.

    such as 5Y3 that you mention - what production amp came with a Bendix 6106 by the way?
    the point there is that a civilian couldn't buy a "super tube" like that but now you can for what must be a whole lot less than it would cost if they were made today. Another example: you can buy those Russian "5881"--original MIL USSR versions--sometimes quite inexpensively. Tubes that because of what they were going to be used for that had extra quality control, testing for reliability, improved designs, parts, chemistry, etc. which new production probably will not go through.

    Whilst there are amp manufacturers (hundred+ amps per year) who offer NOS upgrades, are there ANY that can rely on NOS exclusively? Shouldn't be a problem for smaller scale production/keen hobbyists.
    yes, not really viable for mass production and more popular stuff gets harder and harder to find, but for individuals, hobbyist DIYers, etc. there seems to be plenty of old stock as whole (but smaller for popular types) to play with. Good used works also. Alternate choices. Same thing with different branding, no brand. Same thing but different package (12AX7 = two 6AV6), or basing, pin outs, "overlooked" tubes. Of couse price may vary A LOT depending exactly where you buy(individuals, dealers, ebay, etc.) plus it makes a BIG difference if you have some knowledge (if you can tell who made them by looking at them regardless of branding, tell the difference between older and newer, etc.).

    But back to the original post, I do agree that it would be good to come with practices and/or methods that help extend the life of tubes. Soft start, not dissipating more than you need to, shock mounting, and whatever else this involves, etc.

  7. #7
    Senior Member Enzo's Avatar
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    Was that 4CX tube running in a CW rig in class C?

    Life of a tube is a slippery concept. I just replaced the quad of Tesla EL34s in a MArshall last night. AMp "worked" but sounded terrible. DUll and lifeless, but it worked. A fresh set of tubes and it sounds great. In fact the bias setting coincidentally was already right on, even.

    When the military specs 70% or Gm or something for worn out, they are thinking field radios, not screaming guitar solos. They don't care if the "tone" is not quite what it once was. or if there is a little noise in the audio. or microphonic. Does the radio communicate or not?

    I used to work with an organ repairman, who went on service calls to old ladies' homes to service their old Lowery organs - or whatever brand. Some of those things had a module for each of the 12 notes of the scale, each module with an oscillator tube and a few divider tubes. Could be a hyndreed tubes in one, literally. All those tube had to do was oscillate. Didn't much matter how weak they got. They could be down in the yelow in the tube tester and still work fine. WOuld have sounded terrible in a guitar amp.

    I think "tube life" is about as specific as something like "NOS tubes are better (or worse)."

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    Hi Dai,

    I agree with just about everything you post here. However...

    "Can you get a datasheet with genuine specs?" JJ have data sheets for all their tubes at their website, perhaps not as in depth as some of the classic tube data sheets. I'm not sure what a "genuine spec" is. Most classic amp designs completely ignore limiting values of manufacturers specs anyway.

    Indeed, the practice of resurrecting well known brands (in name & cosmetics mainly) should really be recognised for the marketing ploy that it is. That's not to say that there is anything wrong with these tubes in their own right, it's just even if they were made by the same company, after years of non-production, by a different workforce...they probably wouldn't compare directly with the originals. Move the manufacturing operation halfway round the world and source different materials....?

    The name gives the consumer a comfort level and the manufacturers (not just tubes either...speakers, whole amps, cars...you name it, the practice is everywhere) realise this. Whilst I don't care much for the ethos, it does mean that people are paying attention to our market and evolving the product range...which is a better situation than say 20yrs ago, with respect to current production tubes.

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    my point basically is if they are forthcoming with genuine specifications about their tubes as you see with old manufacturer datasheets ("GE6550", "Tung-Sol 6550", etc.) or manf. datasheets for transistors, ICs, parts with dimensions, max. this and that, etc. Will they say, "this 'KT88' of ours is the same thing as our 6550 in a different bottle"?, or this "12AX7" is a re-based tube which doesn't have exactly the same characteristics of a "12AX7", or "this 5AR4/GZ34 may blow up if you try to use it as such", or "this is BS designed to maximize profit" and so on. I guess the situation is getting better but it seems a bit of a wild west situation. But again perhaps it is about what to expect given the situation. So (given the situation) in some cases finding old stock may give better sound, higher performance, more reliability, longer life, and better value.

    Most classic amp designs completely ignore limiting values of manufacturers specs anyway.
    You mean like screen and plate, cathode to heater voltages? Maybe the tubes survived because they were better than spec and rated conservatively. Maybe the philosophy was different and they wanted to make good products that lasted (and they could because of the different circumstances) and the ideal tube for someone else is something that fails immediately after the warranty runs out...lol.

  10. #10
    Noodle of Reality Steve Conner's Avatar
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    Quote Originally Posted by MWJB View Post
    if I told you that running your Super Reverb at 15mA per tube with Sovtek 5881s would make them last 10 or more years would you...
    The thought of it makes me feel ill... point taken

    As far as I know that 4CX250B was in a linear amp that spent most of its time idling at a small plate current.

    FWIW, I've been using a pair of the New Sensor "Tung-Sol 6550s" for a while and like them. Even though they look like the standard Russian 6550 guts in a coke bottle, they are nicely made with heavy glass, lots of thick mica spacers, and triple getters, and I like the tone. (Biased as hot as the power supply can stand of course... :P) I compared them against a pair of used-but-good GEC KT88s, and I think they hold up well. I'm tempted to get a set of six if I ever get my 300W bass amp project to work.

    I've always used shock mounts in my home-made amps, too, and observed the screen dissipation ratings for EL34s.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    I wish current tube manufacturers would offer accurate and detailed
    datasheets for their tubes. I find it hard to believe that they don't,
    how hard could it be for them to do so ? Is it perhaps because their
    tubes are so inconsistent that they wouldn't meet the specs most of
    the time ?

    You'd think that someone would have taken the time to measure some
    tubes and make their own datasheets. Why hasn't anyone done this ?

    Paul P

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    it seems some people have. A tech on alt.guitar.amps apparently did trace curves or whatever and from what I gather found say a Sovtek 6550 equivalent to a US 6L6GC (must have made appropriate mods such as supplying extra filament amps, etc.). What I'm referring to above w/the re-based 12AX7, the Sovtek "12AX7WA" internals actually a near-equivalent with bit lower gain (90), internal shield between sections (not connected to anything I think in the Sovtek). Perhaps if they were more honest and said "this is different and isn't exactly a direct sub but is a decent tube and can be made to work better in 12AX7 circuits with the following modifications blah blah blah...", then that would seem more acceptable than "better than Telefunken!" hype and BS-filled advertising. Or with Groove Tubes and their "made in USA" (mostly Chinese made?) reissue GE6CA7s or whatever those were supposed to be. Again, maybe it was actually quite a technological feat to make the original tubes. They had the whole history of inventing and using them for electronics for years(WWII--all tubes), no sub(pre-transistor) to use or develop, lots of effort, money, continuous development and refinement, same people working for years and years, etc. I just think of situations like buying a single French Mazda 12AX7 from the 1950s which I didn't get to cherry pick or anything, taking it home and trying it, and it has no problems with microphonics, sounds good. Kind of makes you think WTF!?

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    Senior Member Enzo's Avatar
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    One reason, if not the main reason, part manufacturers release data sheets is for engineers to use in designs. They want an engineer to design THEIR parts into new consumer products. There is in th overall scheme of things not much new product design using tubes. We think tube amps are important, but to the electronics industry we are a fleck of dust on the annual report.

    As to the tube specs themselves,they never represented the absolute max for the tube. The specs were there for table radio designs - don't design your product with over this or that voltage if you expect it to perform on the table top for 10 years reliably.

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    We think tube amps are important, but to the electronics industry we are a fleck of dust on the annual report.
    yes that's probably something a lot of musicians don't realize. I do see people occasionally (basically) opining, "if someone could make a kick ass tube, they would be rich" but I would guess the reality is the opposite and if they spent the money to really do it to the old standards they might end up poor real quick.

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    Noodle of Reality Steve Conner's Avatar
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    It's true. I bet that even in the musical instrument amp business, most of the money comes from shifting solid-state products.

    I used to look down at Marshall's Valvestate amps. They have a nasty cheap feel, it just doesn't seem right when you pick up a Marshall head box and it feels as if it's empty, and then you take the back off and it actually is empty apart from a puny transformer and a few silicon cooties!

    But then I realised they are actually very clever and are delivering exactly what the majority of the market wants. Something affordable that kind of sounds vaguely like a Marshall, can rock out in the garage and has a headphone socket so you can practice without waking up the whole block.

    I think of tube amps more like hot rods or choppers. They're a kind of culty minority thing, and that's part of what makes them cool.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    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...

    Quote Originally Posted by Steve Conner View Post
    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.

  17. #17
    Old Timer km6xz's Avatar
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    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.
    Last edited by km6xz; 02-13-2011 at 07:11 PM.

  18. #18
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    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?
    John R. Frondelli
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    "Mediocre is the new 'Good' "

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    Quote Originally Posted by jrfrond View Post
    ...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.
    In this forum everyone is entitled to my opinion.

  20. #20
    Senior Member Enzo's Avatar
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    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.
    Education is what you're left with after you have forgotten what you have learned.

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    I curse them to H... eh notin'.
    In this forum everyone is entitled to my opinion.

  22. #22
    Old Timer km6xz's Avatar
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    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.

  23. #23
    Noodle of Reality Steve Conner's Avatar
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    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.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  24. #24
    Old Timer km6xz's Avatar
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    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.

  25. #25
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    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.
    In this forum everyone is entitled to my opinion.

  26. #26
    Noodle of Reality Steve Conner's Avatar
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    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.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  27. #27
    Old Timer km6xz's Avatar
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    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.

  28. #28
    Old Timer km6xz's Avatar
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    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.

  29. #29
    Senior Member trobbins's Avatar
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    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
    Last edited by trobbins; 03-03-2011 at 10:17 AM.

  30. #30
    Supporting Member tubeswell's Avatar
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    Quote Originally Posted by km6xz View Post
    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?
    Building a better world (one tube amp at a time)

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