Used power tubes

[Mick Bailey]

Probably like everyone else I save reasonable 'pulls' for testing, so have a suitcase full of them taped up in sets. Often there's a reason for removing them - sometimes one of four may have shorted or gone soft, so I get a set of three. More often than not they've had a decade or more of use and never been changed. I still keep them if they were working ok in the original amp, but not if the bases are charred, they rattle, are discoloured or tapping them sideways drops white powder on the inside of the envelope (emissive material).

Sometimes I get lucky and a customer will bring me a fairly new amp for a tube 'upgrade' or swap Mesa 6L6s for EL34s.

I've bought old used tubes but never paid much for them. I had a large number of boxed original KT66 that appeared to be brand new but when I tested them they all had identical faults - the heaters were OC. I did get my money back on those. I also recently bought a box of 50 807s and only a third are useable, but those are too poorly matched to get more than 2 pairs out of them.

Only buy them if you can afford to throw them out if they're no good.

[Gtr_tech]

I built a little output tube testing/matching deal just for going thru used/unknown tubes. It is basically a SE output stage that I can either dummy load for checking tube characteristics, or run a speaker from for checking microphonics. Plate is brought out to banana jacks for monitoring current, and banana jacks for bias voltage input/monitoring and plate voltage in/check. Power input provided by a bench supply. Its nothing super elaborate, but it works fine for getting a general idea of plate I vs plate V and bias V. And with a speaker hooked in I can flick a tube around to check for noise issues. Signal input as well for calculating amplification factor if I have extra time to burn. Very useful for matching up pairs of orphaned output tubes for later non critical use which would have otherwise been tossed out. Reduce, reuse, recycle right? Most often they will end up in my own gear until they are no longer useful.

[fredcapo]

For consideration, I had a discussion with my uncle last night...he is in his 90's and was an engineer with Dumont back in the '50's. Still very sharp. He said that heating the cathode without drawing plate current - which sounds like standby mode in guitar amps - can effectively renew or extend the emission capability of a tube...

[Gtr_tech]

AFAIK cathode coatings can get damaged from leaving a pwr tube in stby for long periods of time (cathode poisoning). In the days of CRT TVs, you could "renew" the cathode coating by running it for a short time on elevated heater voltage via CRT rejuvinator. It was in the "clean/balance" procedure.

I don't think anything like that applies in our present situation of current mfgr tubes. The cathode coating isn't right to begin with....which is why we see reduced service life.

[fredcapo]

Yes but we are talking about vintage USA made power tubes... Still need some rocket science on this topic.

[km6xz]

I think you might have confused what your uncle said. Running a tube with hot heaters but no current flow for a long time is not good for them. As Gtr-Tech said, running at elevated voltage, by 10% or so can rejuventate them. CRT's suffering from low cathode emission could be returned to normal emission for a while using a common "CRT Booster", a small step up transformer that increased the heater voltage by about 10% that simply plugged into the back of the tube. Directly heated cathode could not be run in standby mode without shortening tube live so large power tubes, if standby was needed, was do so by switching to a lower "warming" voltage and if standby was needed for an hour or so, the heaters needed to be turned off entirely. Directly heated cathodes however, had very short warm up time, almost instant on.

[fredcapo]

I do recall the CRT boosters. I had another uncle that likely funded his kids college education installing those...

Well, google to the rescue again. I found the article below. I think this must be the process my uncle was referring. Perhaps it could yield some benefit...

Rejuvenation of Vacuum Tubes

[km6xz]

That 1978 article has some errors and is primarily targeting old tubes, not the relatively modern indirectly heated cathode designs of the 30s, 40s and 50s. His statement that grid shorts are due to oxide flaking is in error. Oxide is a very good insulator, if it wasn't we would have no ICs or modern electronics or electrolytic capacitors.
It is interesting how he says failures were not found due to mechanical fault, only low emission. New tubes die primarily by becoming mechanically unstable.

[fredcapo]

I have many 1920's tubes and they appear to be built quite ruggedly...and guessing there was substantial mass to the internal elements. In contrast, so many new production tubes appear to have crooked internal elements and if you shake you can hear a rattle... They don't make 'em like they used to...

[Mick Bailey]

I have many 1920's tubes and they appear to be built quite ruggedly...and guessing there was substantial mass to the internal elements. In contrast, so many new production tubes appear to have crooked internal elements and if you shake you can hear a rattle... They don't make 'em like they used to...

They certainly don't make 'em like they used to, at least not audio tubes intended for guitar amp use. We're like the starving peasants being thrown scraps and then gratefully devouring them. What passes for 'acceptable' these days wouldn't have got a look in in the 1950s. These companies that peddle this dross would have gone bust long ago, because the general public was exposed indirectly to tube quality and longevity through consumer goods, industrial and medical equipment. Nowadays manufacturers and suppliers comfortably know that there are a myriad of getouts to effectively kill off any complaint long before they cough up on the 90 day warranty (or whatever). So we shift endlessly between suppliers and manufacturers. I recently claimed for a pair of TAD-6L6-STR from my supplier - guess what "you must have fitted them incorrectly. They were tested at the factory and would never have gone out of the door if they were faulty. We also tested them" That's after buying over £2000 worth of tubes from them. They made me return them at my expense "So we can send them back to Germany for testing". There was a pinhole bubble in the envelope of one and the gettering had turned white around it. How the hell did I fit them incorrectly to do that? How much wasted time, return postage, telephone calls and e-mails does it take to get a tube exchanged?

Any tube manufacturer who supplied reliable, well-made tubes with a no-quibble guarantee ought to clean up. The market is awash with tubes that last just long enough to achieve the escape velocity needed to clear the warranty period.

In this business it's all about creaming the people who would otherwise represent your product, throwing obstacles in your way and generally treating everyone like idiots or fools. We're only as good as the products we recommend and supply. If my customers only expected their amp to last a few weeks before it became so microphonic it could amplify their voice with nothing plugged in, that would be fine. But that isn't what they expect.

You want to claim under our 90 day warranty? Certainly: Prove beyond all doubt there was a manufacturing defect, calculate in your head the cube root of Pi to 1,735 decimal places and quote it back to us, prove the existence of God and life on other planets, then send them back at your expense for us to examine. We'll then replace at our discretion at some indeterminate point in the future.

A tube manufacturer doesn't care what we think about their products or how well they perform, or what the warranty is worth. What will you do in a market where there's no place better to turn to? If I stop buying a particular brand of tube it will have no impact whatsoever. I would dearly love to be an agent for a company that would back me up, just like in every other industry I've been involved with.

I call this the 'battered wife syndrome' - no matter how much of a beating we get, we keep coming back for more. We love it.

[km6xz]

I guess I was lucky for having a good distributor in the same county when I had my larger shop. A defective new tube? Toss it in a box and in a couple weeks the owner of the distributor would come by and log the bad tubes and replace or credit for them, no questions asked. That was Magic Parts which is the owner of Ruby Tubes. We had very few tubes fail initial burn-in but I suspect it is worse now. I get bulk tubes from China that I resell under my own brand, mostly 12AX7s but I also buy from the New Sensor office right here in St Petersburg, mostly EH because that is what customers want. I have terrible out of box failure rate for the EH 12AX7 but the power tubes:6550, 6L6 and EL34 have been surprisingly solid for me, after never liking them, they are doing a lot to improve my opinion of them. But why are the 12AX7s so bad? Dead, big differences between triodes, loose elements, high noise and some just have both triodes having the same very low gain. I have a pile of them here and I do not replace that many tubes. A Randall RH50 came in an hour ago with JJ 6L6's, new, and one tube has an intermittent heater. Swapping the tubes causes the problem to remain with the tube, not the socket.

[Mick Bailey]

I wouldn't mind the failure rate quite so much if the tubes were light bulb-type prices and the expense could be ignored. I've been thinking for some time to buy in bulk at a decent per-unit cost and invest in test equipment. I had a reasonably good run with Sovtek 12AX7WA but the last ones I've had have been way too noisy. The answer? pay nearly double for re-tested matched triode halves, low noise, low microphonic tubes. Yeah, pay double for what I should have got already.

At least if I test them and guarantee them, I contain the problem to myself and remove the angst of the whole situation. The single biggest problem for me, all day long, is tube quality and reliability. Poor tubes means equipment failing early, coming back, and disgruntled customers. The failure rate is way too high to be a sustainable business model as it stands.

[fredcapo]

Interesting... My exposure to new product is limited so these revelations are valuable. But just yesterday I installed a new EH 12AX7 at V1 in a JTM60 and it sounded quite harsh at high gain. I stuck in an old Sylvania and it seemed to clean up the harshness... Typical or not, I still favor my stash of pulled 50 ish year old tubes over new product.

[km6xz]

Why was the amp harsh sounding with the EH? Was the gain far different. Did you measure the distortion of that stage? When something works but not as expected, taking a few minutes to determine why can be quite useful in the future to you. A tube by itself does not have much impact on response or distortion, it all depends on the characteristics of the circuit as a whole. I might be that that tube is fine but breaking up early because a difference in plate Z, if so that same tube might sound better in a different circuit. Just having different gain does not make it a bad tube, just not what is wanted in that particular amp. The problems I have been seeing with EH 12AX7s have been really defective...open or intermittent heaters, high microphonics, low gain, severely unbalanced gain etc.

[fredcapo]

I did not perform any of the tests you suggested. I recently repaired the amp and was showing the amp to a player who cranked it up beyond the level of my incomplete testing. The sound became harsh and clipped to my ears, not unlike the sound I recalled when I once yanked all of the USA tubes from a head, and retubed with Chinese tubes. I often hear the term "musical" used when describing the attribute of some electronic apparatus. The amp became more musical after I swapped the tube. But, This is not a fair assessment of the tube (I used the word "seemingly" in the OP). I am steep on the learning curve in this area, and will take your advice, which I value, and attempt to quantify the issue.