Help with 12ax7 tube testing

[Chevy]

Hi Guys,
Am getting super frustrated with new tubes I've been getting. I haven't been doing that much repair lately, but the last time I did, I had to send 3 lots of JJ tubes back to the vendor to get some decent ones, to make up a set of just 4 for one amp. Very frustrating when a customer is waiting for his amp.
I'm using a B&K model 606 tester. Nothing fancy, but it's seen me thru several years of steady use without any problems. It seems almost all the OLD stock (60's or 70's or in that ballpark) 12ax7 tubes I have in amps or laying around, pretty much test out strong at 80 to 90 or so out of a scale of 120... only problem with them is they seem to go microphonic eventually (not in all cases, but generally speaking). But these 2 new vendor-tested tung sol tubes registered like 20 on the dial, and then the needle dropped down from there. But they tested good enough at the vendor.

What is going on here?

I sent the 2 tung sols back to the vendor, and they very nicely went out of their way to test 2 other new ones before shipping them to me as replacements. The replacements were both at about 20 on my 606's dial! Both are microphonic in my guitar amp. One of them introduces a new noise that wasn't there before. According to my meter, they are crap tubes, but their Hickok PV7 gave them scores of 58#1, 56#2, and 62#1, 62#2. I don't know what theses scores mean, but they passed their tests. I'm certainly not out to bash any vendor, not my intention at all, as they dont' make the tubes anyways, and they are honestly trying to make me happy, but need to know why my tester and theirs don't agree at all... and what is the solution to this? Just get NOS tubes?

I just don't see how my tester can be so "wrong" on testing the new production tubes, when the old ones test so well.
Thanks much,
Chevy

[Chuck H]

I don't buy many tubes either. When I do I just buy 'em and buy extra and hope for the best. I don't use JJ preamp tubes because I got a really noisy batch once and have read that many still have trouble with them. So I use Sovtek preamp tubes. Actually "I" use NOS RCA tubes. But for the amps I build I use Sovtek. Lately I get some hummy tubes from them (bad balance on filaments???) and haven't troubled to return any because I don't buy that many tubes to make it worth my while. But I did learn a long time ago to avoid JJ's.

The Sovtek 12ax7LPS is as good sounding a tube as your likely to get, NOS or otherwise. But they're ALL microphonic. I use them for every OTHER preamp position besides the input tube. For the input tube I use the Sovtek 12ax7WA. You wouldn't want to use it in a Fender AB763 type circuit because it lacks definition in the top end. But in high gainers and cascade preamps it's just fine for the input position. In fact, the small top end deficiency is a plus most of the time. If you are building "clean" channels I would say use the Sovtek 12ax7LPS for every preamp position but input. And use a selected NOS tube for the input on such amps.

JMHE and JM2C

[Enzo]

You are relying on your tube tester, and while you found the tubes to be microphonic, other than that, when they were in th amp, did they sound like they had a gain of 20 instead of 100? What you don;t know is just what conditions your tester puts on the tubes. Tube testers rarely put real world conditions on a tube, so the tests are done at what I myself call "off the curve" conditions. In other words if the tube works down here, it wil probably work up there. And it is just possible your new tung sols don;t respond at the bottom of the curve like the old tubes, but once up the curve where they normally operate, those differences may even out. Microphonics aside. Think of it like a car motor. Any car can drive down the road at 50. SOme idle at 750RPM, some idle at 500RPM. I cannot infer from the idle speed how fast the car can go. I do know that if I set all the controls the same and connect the same testing gear to my car and other identical cars, they should all come up with similar parameters. But how far I have to push the accelerator pedal to go 50?

Voltmeters are standardized things, a volt is a volt. A tube tester makes a relative judgement. A 100v power supply needs to put out 100v to be good. But tubes? Look at the data for any tube, there is a whole family of curves. There is no single parameter that says if this tube measures 67, it is 100%. Is the transconductance of that tube being measured with 250, 300, 350, 400v on the plate? Is it conducting 3ma, 2ma, 1ma, 0,5ma? WHat amount of grid bias is applied?

In fact I'd be surprised if your tube testere gave the same numerical reading as theirs. COuld be similar, could be real different. You can;t say this tube has a gain of X, all you can say is that it has a gain of X under THESE conditions.

[Chuck H]

Just to add... For years before I was building amps I got my preamp tubes from a CA. SF bay area "salvage" place called Halted. They had boxes and boxes of used pulls and a tube tester in house. I figured out pretty fast that the tube tester they had was really only good for detecting gassy tubes and gross shorts. Actual in amp performance was beyond the capabilities of they're tube tester. So I would buy a sack of tubes (about $1.00 to $3.00 for each) and take them home to test in actual amp conditions. I scored about half the time. So it turned out to be a really good deal. Point is, most "tube testers" don't measure or grade at actual in circuit conditions. In fact IMHE most are way off the mark wth respect to actual suitability. You can trust them to tell you if a tube is gassy, shorted or grossley bad, but that's about all you can expect of them. Well, maybe matching sides. Not that the actual number is significant, but if it's the same.?. And, of course, there is no tester that detects microphonics.

[Gtr_tech]

I would build a test circuit...something along the lines of a std Fender gain stage and pwr it with an external pwr supply. That way you can check amplification factor (voltage in vs voltage out) and have a scope on the output so "see" microphony.

I did something similar for output tubes using an old organ amp chassis and single ended output transformer. I went thru 100 or so unknown output tubes and noted the characteristics to see which ones would pair up within decent tolerance.

[Chuck H]

Right. If you find the need to test a lot of tubes it's a really good idea. You can build in safety features to protect the circuit itself so grossley bad tubes won't damage it. Set up test points for taking readings. I just use my amp and real world circumstances. But I don't go through many tubes.

[Chevy]

More info on the tester

I got an email from the vendor, who was concerned about things, and he sent along this link to explain the situation a little more. RadiolaGuy.Com : Mutual Conductance vs. Emission Test

It turns out that my tube tester is using cathode emissions to test for quality, while theirs is apparently using transcondunctance to test for quality. That would explain why my readings are so far off of theirs... partially anyways.

However, a new tube should pass both tests, but for some reason the new tubes can't pass the cathode emission test on my meter. The NOS tubes can and do, with flying colors. So.... they just aren't the same.

The new tubes do "work" in my amp, and although I didn't test them for actual gain or tone or anything, they are microphonic and one introduced a fluttery type of noise to the amp. I expect they are not made to last at all, and the old premise of "change the preamp tubes only when they fail" probably just doesn't apply to new tubes anymore. They now seem suspect from the get-go... rather sad state of affairs, considering how popular tubes are these days.

Cheers,
Chevy

[gbono]

Just to add... For years before I was building amps I got my preamp tubes from a CA. SF bay area "salvage" place called Halted. They had boxes and boxes of used pulls and a tube tester in house.

They're still around as HSC - I'm in the Santa Rosa store every couple of weeks. I think the Santa Clara store still has tubes for sale.

[Chuck H]

Well go get 'em.

When I shopped there I had a list of all alternative numerical titles for 12ax7's. I scored old Bugle Boy's, Sylvanias, Hitachis and more. Even a couple of rebranded smooth plate Telefunkens. But the best in my stock were undefinable 12ax7's of a different numer designation. There has to be a dozen different incriptions for that tube.

I also have some old Coke bottle rectifiers and a pair of Tong Sol 5881's from there. All the tubes I save are in slightly used/like new condition and awaiting projects. If there are still tubes to be had you should definitely go and get them.

The price went from a buck a tube to three bucks for the more desireable ones. They have a psuedo cheat sheet at the counter that the clerks reference at checkout. One more reason why the alternative designations are good to know.

Santa Clara had a whole wall of tubes in cardboard boxes the last time I was in there. (about six years ago)