Matching tubes at home Q?

[Chuck H]

So I need new EL84's in a couple of amps. I went on theebay and found a "lot" of 16 "new and lightly used" 6p14p-k tubes where all test as new on a standard tester. The seller has a good rep. high sales and all his "lots" have their own individual and seemingly honest descriptions, even when less than good. So I expect to get what was described.

So now I'll need to pair them up. In the past I've just plugged tubes in one at a time and matched them for standing idle current. These are for guitar amps after all, so I didn't worry about details and I haven't had any real problems. Now I'd like to take it a step further. Sans tester (which I've often read isn't a good test anyway) here's what I propose to do. Using the same socket for each tube I'll test and mark down:

Standing idle current.
Current when conducting a fixed signal (perhaps to 60% expected output).
Actual output at that signal level.

Then I'll try to mate up a couple of quads and the others in pairs.

Since Rp is pretty much swamped by the OT I figure this should be enough. Yes/no?

Is there a better/easier method? Other considerations? Should I also test for, say, max signal and/or power before clipping?

TIA

Chuck

EDIT: and P.S.

I chose to get these NOSish tubes because they are supposed to be more vibration stable. So I'm hoping these and some damper rings will yield better results than most of the current production tubes I've tried. Which, in my combo amps, seem to go horribly microphonic in 150 to 250 hours! Pretty poor for a tube that is supposed to last for 1000 to 3000 hours depending on which version you buy. So I'm hoping to get some relief from this.

[Drewline]

I've had good luck matching up my test tubes by using the single EL84 bias probe I have by measuring all the tubes against one control tube from a matched pair & marking the current measurement of each test tube on it. Not an exact measurement but it gives me matched tubes to use for testing amps as I can pick out tubes with close values.

[octal]

I just match idle current and look at the output of the amp on a 'scope playing a sine wave. If it looks symmetrical and the top and bottom of the sine wave clip at the same output level, I'm happy. I have heard anecdotal evidence that no one even bothered with tube matching back in the day- though I wonder if tighter manufacturing tolerances back then may have made it less necessary than it is nowadays. Or maybe we've progressed further in the "cork-sniffing" direction now that tube amps represent the higher end, tone conscious segment of the market, instead of just being what everyone played because that was all there was.

[Chuck H]

Thanks gents. I've done the same. That is, match up idle current and cull poor performers. I bagged a small crate of old Sovtek standard EL84's and I'm down to my last pair. Probably a blessing in disguise because they just don't hold up in combo's. Since this small lot is coming from Ukraine I really don't know what to expect so I thought it might be good to go a step further. Not too much work once you set up.

[Chuck H]

Well... I received my tubes. Most were even darker on the plate windows than the microphonic tubes I have to toss! Two were DOA (bad form *bay seller) and two did a funny thing with a steady current rise that I've actually seen in some new tubes. I'm testing these in a push/pull circuit with one tube out and such tubes are often stable with both sockets occupied, so we'll see about those two. Otherwise the final tally is twelve out of sixteen that test well. With ten testing as I would expect from new tubes. So let's call it five and a half matched pairs (as one pair may have fewer useful hours on them). For what I paid it comes to fifteen bucks a pair for some coveted tubes. I was stunned at how non microphonic these "vibration resistant" tubes are. If they exhibit this trait for the remainder of their rated life I will surely be impressed and would recommend them to anyone with a combo amp. Though I would suggest paying the extra for actual NOS rather than a "lot" of lightly used ones. It was partly an experiment just to see how they perform used compared to the lower hour tubes I've been tossing! Kudos to Reflektor and shame on Sovtek. These older, used tubes sound just as good as new Sovteks (a little better actually) and are completely non microphonic. The fact that they remain this way after some usage is a testament to their quality. Sovtek is using the same equipment and making inferior tubes. If you have an EL84 combo that rattles your tubes to death, get some of these NOS 6p14p-K Reflektor tubes. I can't testify to longevity yet but if my intuition serves I won't be disappointed.

That's all I have for now. If I have any short term trouble I'll report back.

[Mick Bailey]

Microphonics in new tubes is driving me crazy. I can't find anyone who can supply tubes good enough for combo use - tubes will nearly always fail through microphonics before any other reason. It's pretty depressing to pay premium prices for selected tubes (such as TAD) only to tap them and they rattle straight out of the box. The problem seems to be getting worse and manufacturers aren't interested in improving quality, just selling more of the same in the knowledge that we just keep coming back for more. I've resisted buying NOS tubes 'blind', but maybe that's the road to getting something that's actually useable.

[Chuck H]

Dr.Z is making more models with 6V6's now, his EL84 amps are being shipped with vibration dampers installed and customers are still having problems. It's the worst with EL84's. I had this problem with one pair of EL34's also, new and straight out of the box. Phooey! I'd read the problem is getting worse for most octals and now I've seen it. I don't like the idea of relying on NOS either. But I can tell you that with these 6p14p-k tubes it's really nice to at least get a break from the problem.

[Ken Zuercher]

Octal, you are right about not bothering to match tubes in the old days. We went to the TV repair parts place, bought a pair of 6L6s in whatever brand they carried and put them in the amp. Bias was set by the voltage listed on the schematic or just left as it was. After all, many of us were high school kids. I did have an Ampeg B-15N bass amp that needed new output tubes so I just went and got a pair of RCA 6V6s. When I put them in, they red plated so I took them back to the store and told them that they were defective. They gave me another pair that worked fine and problem solved. Later I find out that the High Fi guys always bought matched tubes and had them carefully biased. We are definitely sniffing the cork compared to the old days. I've generally had better luck with modern tubes, especially JJs than I ever did in the "good old days"

[Chuck H]

I've generally had better luck with modern tubes, especially JJs than I ever did in the "good old days"

Welcome, and thanks for the report. We need "experience" here more than ever regarding tubes and old vs. new performance. I guess your report doesn't surprise me, but for every story like yours there's a story like mine. I tried two pair of JJ's bought from a vendor months in advance in a build last year. Both failed. One was a heater to plate short and the other was grossly mismatched after burn in and couldn't be reconciled with my single bias circuit. Because I was months from the purchase date there was no warranty to take advantage of. As often as I need a single EL34 tube I'll call this two pairs bought and tossed The Ruby (Shuguang) Chinese tubes are electrically fine, but a little microphonic. This has often been my experience with the Chinese power tubes. However, Shuguang 12ax7's, when selected for low microphonics, have proven excellent, if not as long lived as NOS tubes. I will continue to use them. A note for the audiophiles... The triode gain balance on the Shuguang tubes is freaking atrocious! Not a big deal for guitar amps, and I haven't had a problem with this, but if you are buying for a balanced, stereo system it's going to be a problem.

It's interesting to me that "back in the day" it wasn't uncommon to return an amplifier because it underperformed. I expect this was common, as it is now, but is the sort of thing that gets overlooked in the face of current circumstances. Please stick around Ken. We need the benefit of experience as we move into a new and less familiar genre of vacuum tube usage

[Mick Bailey]

I also never used to match tubes - certainly not in the 70s. Just bought what I needed from the local radio shop - I think I would have got laughed out of the place if I'd asked for a matched set. It was also common in those days to see an amp with a variety of tube brands (all the same type, though) because they got replaced when one failed and not as a set. Maybe people were short of money in those days, but a complete retube certainly wasn't as common as it is now.

I get a lot more problems with tubes than I used to. My heart sinks when I open up those boxes and tap each one, only to find the ring that spells trouble. TAD are supposed to be matched and graded, but my own findings are that they're no better than the factory-matched Shuguang tubes at 1/2 the price. They're certainly no better for microphonics, which is a killer if they're ending up in a studio amp.

JJ has some good-sounding tubes, but the mechanical qualities are abyssmal and I now won't buy their power tubes at any price. It's a shame they just won't get the quality good enough - I have a pile of 'pulls' taken from amps with just a few weeks of use. They ought to come in a Christmas-cracker style package. No need to include a joke; the tube inside is sufficient.

[Drewline]

I've had good luck using EH power tubes sourced from CE. I use a mixture of JJ & Sovtek preamp tubes. My favorite current preamp tube is the Tung Sol 12ax7 reissue followed by the Sovtek 12ax7LPS. In Mesa amps I use JJ's like they do. Usually price is more important than sound quality on the majority of equipment we see.

[Chuck H]

I really like the Sovtek 12ax7LPS. Too bad I've never had one suitable for the first position in anything due to microphonics. But the tone! (I know, Stan... Tubes don't have a tone ) And they're usually quiet (other than microphonics) and reasonably balanced. To me they sound spot on like the standard gray plate RCA's. Which is one of my favorite tubes for good gain and nice, balanced highs.

I've had many problems with the Sovtek 12ax7WA WB and WC tubes. Low gain, crackling that never stops, humming like they don't know the words, Low gain (lower than it should be even for a lower gain tube, especially for the WC's). They're utterly non microphonic though. Still not good for V1 in vintage type amps because of the poor tone, but if you can find a quiet one they work well as V1 in ultra high gainers. I bought four once just to be sure I would have one suitable tube for a project. Two were unacceptably noisy!

[Drewline]

My stock of 12ax7LPS tubes are from 5 -10 years ago & have been sorted for microphonics. They do sound similar to gray plate RCA's. The more microphonic ones work great as a PI in some amps. Never use them in a cathode follower stage.
But there is something about the Tung Sol Reissue 12ax7 that gets my attention as V1. Plus they are not microphonic in my experience.

[Ken Zuercher]

Interesting experiences all. Perhaps all brands have good and bad examples. As I mentioned earlier, I have had universally good results with JJs. I have not had as good experiences with Sovtek power tubes. I have had good results with the few Sovtek 12AX7s that I bought, all have been 12AX7WB. very low micro phonics and would work fine in the first position of a Marshall which is a tough order. I have been making amps for sale for the last 7 years and always gotten my supply of JJ tubes from the same supplier and get them matched and burned in. They have been very consistent and never failed in that time period. With the other reports, I suspect that my supplier has very carefully selected out the best and then matched and burned them in. Where the others go is a mystery. Considering the reasonable price, I am happy to continue. I have many old 12AX7s, mostly of unknown condition. Often they have too high a microphonics level for me. I do pick a few out for a particular sound and have a set saved for studio use when the amp is under the magnifying glass. Every JJ 12AX7 has the exact same sound and micro phonics characteristic so I suspect that they are carefully sorted. I was at the recent Summer NAMM show and had conversations with tube suppliers that revealed that I might be correct. OH well, thank you all for the cordial welcome to the group and I'll be around.

[km6xz]

We go through a lot of tubes here since we are doing repairs, and find that the range of parameters which are sold today just would not have been sold in 1965. If they did not fit the published spec, the tube was defective and crushed. None of the 3:1 plate current ratio of soft to hard tubes sold now as "features". That tighter QC was the reason matching was not required.
The only power tubes I depend on are out of production Svetlana's (but still have a large stash of them) for 6L6 and EL34 needs. JJ are are non-starter for use, the power tubes fail out of the box at amazing rates but the 12AX7's are OK. The EH power tubes have been pretty good for reliability so I have no problem warrantying the repairs. The EH 12AX7s have very high failure on first use and there is no warranty from New Sensor here which is possibly why they send all the junk here. I am more concerned with reliability than "sound" in a particular amp design. Using spec meeting 12AX7's in the same amp, doing A/B tests normalized for gain, no one is going to be able to pick one from another at any odds better than chance. Preamp tubes are not sold by range but they need to be now since a "good" tube can have a gain of 30 or 90 and still be sold in the same boxes. Those WILL sound different, as expected.

It is interesting to note that my stash of Svetlana 6L6 are not matched and I do not match them because they are very close. Knowing the history of these companies and many of the rebranders, they were likely the only tube maker with the QC and expertise to continue on making tubes of 1965 quality, given their 111 year old track record of making tubes and light bulbs. Everything else is pretty amateur. The engineers and process engineers who worked at RCA or Svetlana are all long retired and most are long dead so in a few years a hundred years of refinement in tube manufacturing will disappear. All we will have are flimflam salesmen.