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In Reply to: Re: Another tip posted by astrostar59 on December 29, 2006 at 10:08:38:
I've seen a WE amp, using NOS WE300B tubes, with impressive tube life. The owner did not know that leaving the amps on shortens tube life, so he left them on all the time. After 6 years on, the we300b tested at 80%.
However, the WE 300B was designed to have very long life, its intended service was in telephone lines on the bottom of the ocean. You can't swap tubes often two miles deep... I think the specifications there were for 17000 hours of uninterrupted service per tube. There is an amp on the bottom of the Pacific ocean from US mainland to Hawaii, with a 300B amp on the bottom every few miles. During 40+ years less than 10 tubes failed......
There's no mystery about WE300B. What makes it very expensive, is that they were factory run for 6000 hours, and then tested. If they did not test as new after 6000hrs, they were tossed. Unfortunately, some of these failed tubes circulate as WE 300B, and people ask big money for them, and the buyer is not realizing that's not a WE300B, but a failed 300B. I guess that can be one reason why different people have so different opinions about them, as you can't know if you listened to the real tube or the failed one, which, instead of being destroyed, ended up in someones warehouse.Try to do this on current production tubes, and see for how much it can be sold..... 6000hours of electric bill into a 300B is a fair sum.
BTW, that WE amp had an excellent tone, I thought I am listening to a 2A3 amp.
Follow Ups:
I would say that the tubes still tested 80% after six years BECAUSE they where not switched off.I think that swithing the gear on/off shortens the tube life.
NC,My gut feeling is that when the amp is switched on/off, the tube ages because the cathode undergoes rapid cooling / heating, damaging the cathode. The cathode is a complex structure, and its ability to release electrons can be influenced a lot by annealing it. And that's what occurs during on/off cycling.
There was one study on tube life and aging, involving hundreds of vacuum tubes, when they built Eniac that showed that slow turn-on of B+ does not influence tube life. The only evils they found was heat, that was not dissipated from the anode.
Something must be ocurring during turn-on.
When I hook a stereo70 up to a variac, and turn it on, there is an initial current draw of over an amp. Then the current draw drops to quarter amp or so, then a couple seconds later starts going up as the rectifier starts to conduct.What is that initial huge current draw? As the rectifier is not operating yet, it's not HV. The filaments are drawing that excess current, and their current draw drops to the nominal as the tubes heat up. My hunch is that this initial huge current has a hand in ageing the tubes...
My idea is to turn on the amp with a variac slowly, to prevent this rush of current. (I tried it out, and turning the st70 on with a variac prevents the initial huge current surge). I'm curious if on the long run it can save tube life..... there was recently a post saying that RCA had theater amps on which they starved the cathode during the breaks, increasing tube life. I also heard from my mentor that when not using the amp, run 30% B+ voltage instead of switching the amp off, and that will extend tube life tremendously.
Looking at the physics of cathode heating: the cathode needs to be 900-1100C hot in order to emit electrons. Warming up a thin piece of metal from room temp (20C or so) to 1000C in a matter of few seconds is an immense stress to the material. After this immense stress, comes up the high voltage. As I see, the high voltage is not that stressful on the cathode (unless it is of too high current), it stresses the plate: as the electrons bombard the anode, their motion energy transforms to heat. However, the plate is a much bigger structure than the cathode: the current density on the plate is on the order of a few percents compared to the cathode. This translates to lower stress on the anode, than on the cathode, for a given current.
The heat generated by the flow of electrons has to be dissipated. If it is not dissipated by the tube envelope, it starts building up inside the anode structure, and a significant portion of it is reflected back to the cathode, increasing its temperature. When the cathode is heated above 1100C, its deterioration speeds up, tube life declines sharply as it increases.If the B+ is decreased to 30%, then, with the same bias, the cathode current is basically shut off, plate dissipation and reflected heat (& stress) on the cathode is minimal. I would think this is better than starving the filaments on full B+, where there is a full current demand on the cathode, but the cathode is not hot enough to satisfy it. I beleive a straightforward way to kill a tube is put HV on it without filament voltage....
However, it also depends how much the filaments are starved, and what is the operating point of the tubes. If they operate in deep A, starving the filaments would probably wreck the tubes, as they have high current demand on a lukewarm cathode. If they operate in class B, there is but a trickle of quiescent current, that would not hurt the starved cathode.It's worth a try to toy around........
Janos
The problem with tube lifetime studies is that there are many ways a tube can die, and many constructions that address the various mechanisms. Here's a few bits I've picked up over the years:1) Heating and cooling the cathode (and heater if separate) causes mechanical expansion and contraction. Any weakness in the material - a thin spot, a micro-crack, whatever - is a likely source of breaking the filament during this mechanical sress. That's why light bulbs usually blow out when you turn them on.
2) The inrush current to a cold heater is very high, producing large magnetic fields which apply force to the magnetic (steel) electrodes, and to the heater which carries current and hence has a magnetic field. (Presumably AC heaters or cathodes have a continuaul source of magnetic vibration as well.)
3) The mechanical expansion/contraction can work-harden the heater/cathode material, making it brittle and easily broken. (Something like this was said to be a problem with the very first generation of Vaic 300Bs - this was quickly addressed with a more complex cathode structure.)
4) The mechanical action can also cause bits of the cathode emmissive coating to flake off, reducing the available emmissive surface and sometimes producing grid-cathode shorts. Some specialty tubes have diffusion-bonded emmssive materials to address this problem.
5) An imperfect vacuum (they are all imperfect to some degree!) will have ions, which are much heavier than electrons. Positive ions are attracted to the cathode when high voltage is applied, and are massive enough to damage the cathode material when they strike it. Once the cathode is hot and the space charge built up, it will protect the cathode. This is the theory behind delayed high voltage, and also the reason starved heaters are risky. Presumably a slightly starved heater is OK when the cathode current is sufficiently low so that a good space charge can be maintained.
Ion velocity is proportional to the voltage, so reducing the voltage helps.
Note that tungsten filaments are relatively immune to damage by ions. That's the reason they are used almost exclusively for very high voltage tubes. Thoriated tungsten is in between.
6) Reducing the voltage, and hence current, to zero can lead to "stuck" cathodes; they develop a barrier and won't turn back on when needed. This was discovered when tubes were used in early computers, and might be "off" for extended periods. Special cathode material formulations (I believe mainly high purity) were developed for computer tubes. Some of these tubes (5965, 5687 for example) have a good reuptation for audio, and it's possible this is a reason.
6) Heat is the enemy of a good vacuum - it promotes outgassing from the internal structures and the glass, and leads to tiny damages to the glass-metal seals. The getter will absorb many but not all gasses. Gasses means ions and ion bombardment.
7) The "usual" (it's in all the books) cause of a tube wearing out is the finite lifetime of the cathode emmissive material. It does diffuse away from the surface, one atom at a time, hopefully caught by the getter action. You can see this in ordinary light bulbs, which get a slight darkening as the tungsten evaporates from the fiament and condenses on the inside surface of the bulb. Eventualy the cathode emmissive surface does not have enough exposed atoms of strontium, barium, etc. to emit enough electrons. Tube testers measure peak emission to detect this progression.
8) Like light bulbs, the evaporation of the heater wire itself is non-uniform - any thin spot has a greater surface area and evaporates faster, leading to weak spots and eventual breaking of the heater wire.
Paul,Thank you for your concise answer. I learned a lot. Would it be a good idea to use inrush current limiters on the AC heaters? The DC heaters probably start up slower (in case of filtering more complex than a C only), as the caps need time to charge up.
The imperfect vacuum is very troublesome, if the tube was abused even once. It starts the water cycle, leading to the accelerated deterioration of the cathode. This goes on even when the tube is operated modestly afterwards.
The cathode emission can be improved is some cases by applying high filament voltage (8-9V instead of 6.3), without high voltage, for a minute or so. This causes the cathode to melt slightly, and the emissive material, if any is left, to migrate to the surface. Sometimes weak tubes can be reformed this way.
Another tweak is to bake the tubes. The getter needs high temperature to work efficiently. However, during normal operation, there is a charge in the plate structure, preventing the ions trapped there to be asborbed by the getter. When there is no current running through the tube, and the whole bulb is heated (in a regular oven to 250F or so), the getter can absorb those atoms / ions which would be normally trapped by the space charge.
So, it seems, that for the heaters it is best to never turn them off, and for the cathode it is best to always run some current through them, but keep the entire strcture cool and unstressed.
As an experiment, I put a switch on my Darling amp between 50% and full B+. (I have a voltage doubler supply, with a center tap on the hv transformer. Switching between 150--150 and 150-center I get full or half B+.) Fortunately, with half B+ the DC coupled amp biases up to operation. The tubes run very cool, and to my surprise, I can use the 50% point to listen to music. My hunch is that the current also drops proportionately to the voltage, so I get around 25% of the normal power, maximum 50%. That is 250-500mW compared to the regular 1W.
To my surprise, that low power still produces excellent sonics, I need the 1W setting only for very loud listening, where the low setting starts clipping. However, the lower power setting has better low-level information readout, I find myself listening to the amp at much lower levels, as I can hear the same level of detail, dynamics, and body at ridiculously low volumes. I can understand why some folks choose the sub-1W range amplifiers.I'm curious how much longer life can I get from the tubes this way. I've had 6000 hours on the tubes. The power tubes (1626) are down to 80% emission, the driver tubes (cv5311) at 99%. Since then I also fitted them with diy copper heat dissipating sinks, and a fan to blow air on the tubes. This way, the driver tubes hottest surface point is around 40C! The power tubes hottest point is also very cool, around 60C. I have an 5A variac, thinking about hooking the amp to it for gentle turn on/off. I have observed that the turn on current inrush is visible: the tube lights up bright, then as the fl starts warming, the brightness subdues slightly, and after the damper diodes start pumping the hv, the tube brightens again.
With the stereo 70 the turn on inrush current is audible on the Russian EL34s: I can hear a metallic clinging sound on turn on. Must be the tubes internal structures resonating in the strong magnetic field. Seems that this one needs the variac for turn on.......
And I can agree with it.One question, why would reducing B+ to 30% be better than just switching it down completely? I don't quite get that.
It might not be better for the tubesa at all... however, 30% hv has hardly any plate dissipation, but is already sufficient to polarize the power supply caps, and to keep the amp warm. After the hv is switched off for extended period, the power supply section and signal ection cools down completely, and switching back the hv the amp can start warming up from scratch. With 30% hv the amp is always warmed up.Based on the physics of the cathode, 30% is be better than no hv, if the tube is left with that for extended periods. When we switch off the HV completely, the electrode cloud on the surface of the cathode wants to escape (it always wants to escape, when it is heated). If they are not allowed to do so, it will slightly change the structure of the cathode, releasing the built-up internal tension, altering its electron-emission capability.
What makes it very expensive, is that they were factory run for 6000 hours, and then tested. If they did not test as new after 6000hrs, they were tossed. Unfortunately, some of these failed tubes circulate as WE 300B, and people ask big money for them, and the buyer is not realizing that's not a WE300B, but a failed 300B.This is a nice story. Do you believe this is still the practice?
Hi Geoff,I don't know whether for current production they do burn in the tubes for 6000 hrs. Mind, that's two years of continuous use of the tube!
I was reading about the 300B in a book written by a WE engineer, on the history of WE, about 2-3 yrs ago. It has loads of data on the WE tubes, and tons of background information. Anyone remember the writer and title? AES used to sell that book, but they do not have it anymore.
Good luck,
Ummm ... sorry, but 2 years times 365.25 days/year times 24 hours/day is 17532 hours. Do the math! :^)Western Electric made tubes for underwater telephone repeaters, but the 300B was AFAIK designed for theater use. (Hey, I got a bunch of Astaire/Rogers DVDs for Christmas - RKO and WE!!) Lifetime estimates were 20,000 hours if the plate-cathode voltage was less than 400v and dissipation less than 32 watts. At rated maximum 40 watts dissipation, WE projected 10,000 hours life. Since they "wrote the book" on reliability, I'd be surprised if any other make could live up to that, and a little bit startled if even the current production does so - I assume they do everything that was documented, but that's probably not everything that was known...
I seem to remember that the original production process burned them in for more like 200 hours(?); I also seem to remember they rejected 75% of the production - and smashed them so they could never be sold. There was a picture in a WE publication of the mechanical shock test, swinging a tube on a 3-foot cord into a hard plate while powered up - very scary! Hard to imagine that anyone destroys that many these days. But technology is better now; many say that the new production has much better vacuum (10 times better?) for instance.
Maybe Gordon will post here? He probably has more experience than anyone else with original and new WE 300Bs.
"I assume they do everything that was documented, but that's probably not everything that was known..."- not so....
When the 'new' 300B's were first made, they were made for several years in the same plant in Kansas City used by WE, and almost all of the workers were ones who retired when Lucent closed it down. The "tricks of the trade" were preserved and passed on. Also, don't forget that WE did a LOT of gov't contracting in many areas where very linear tubes were essential.
Agreed, the 300B was not generally or ever used for undersea cable repeaters AFAIK. The primary tube for that was the 175HQ. 300B was designed for a variety of uses. When they designed tubes like these they designed for characteristics, not necessarily for end-uses. WE used them for series-pass power supply tubes (the horror!) and even used triodes as diodes in many of their theater amps.The WE book in question is by Bernie Magers, who's still around but I think the book is out of print. Much of the production info in there from an internal WE document which is available with some searching.
rejecting 75% of production run tubes seems insane to me. Maybe when they were prototyping but once a line was up and running the waste ratio could not have been nearly that high.
There is piles of factual lore about WE out there for the taking, it's not that hard to come by accurate info. It bugs me when lots of BS is posited as 'fact' when in reality much of it is myth and repeated nonsense. Not pointing fingers to anyone in particular here but if one is going to post this sort of info (anyone) let's try a little harder to get it right. Esp. since all of AA is archived and indexed by the likes of google, etc.
Ed,I'm glad you posted your corrections. I've read some facts about the 300B, and apparently my memory failed me with some of them. Someone has mentioned to me that there was a 300B amp at the end of the repeater line when he went down....
!
Addicted to Tubes
many say that the new production has much better vacuum (10 times better?) for instance.
Paul,Thank you for correcting me. I hoped someone knows the specifics accurately. 6000hrs-2yrs conversion is quite embarassing.... I was watching Black Adder in the meanwhile ;). Did the trick.
I was told that someone who went down to check the end-of-the-chain amp of one of the repeater lines saw there WE 300Bs. But as far as I know, 300B was used only between continental US - Hawaii. The other lines use other WE tubes.
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