Acceptable variances for "matched" power tubes?

[Steve B]

I received an order of tubes that were supposed to be matched, yet they were slightly different.

There is a 6 milliamp difference on a pair of JJ 6V6's & a 4 milliamp difference on a pair of Winged C 6L6's.

A previous pair of JJ 6V6's I bought from another dealer were within a tenth of a milliamp to each other.

Am I being too picky?

Thanks,
Steve

[Tom Phillips]

IMHO your experience is not unusual. I have received "matched" tube sets from some vendors that I can only imagine were matched with a dart board. Others were always withing a couple of ma. I would consider anything within a couple of milliamps to be a very close match. Many peolpe will say that within 5 ma or even more is acceptable. I reality we should be talking about % rather than an absolute current. A couple of things to consider are: 1) The readings will probably drift as the tubes age. They may get closer together or drift apart. 2) A set of tubes may be closely matched under one set of operating conditions (Plate voltage and grid voltage) and less well matched under another set of operating conditions.

If the tubes were mine I'd bias them up to the average setting and listen to how they sound. They will probably be fine. If they really bug you I'd contact the vendor. If they make it right...cool. If they blow you off, then I wouldn't use them again.

Regards,
Tom

[Regis]

No, you are not being too picky. Hell yes I've seen this before, with all kinds of different brands of power tubes. I've talked about this before; a matched set of tubes with 4-5 ma difference is not a matched set of tubes, I don't give a damn what you say. People talk all the time about microphonic 12ax7's, but I've never seen one. I experienced my first microphonic preamp tube recently when I refurbished my Kay 720 and went through 3 5879's before I got one that worked.

However, virtually every set of matched power tubes I've seen in the last 5 or 6 years have had that 4-5 ma difference, and as far as I'm concerned that's not matched. This isn't to say that they won't sound good, but don't tell me they're matched when they aren't.

[Ray Ivers]

Regis,

I realize you probably won't give a damn what I say but IMO it's quite possible to have a truly matched tube set in a musically-important way (not merely in idling-heat-output) that nonetheless shows significant variances in DC current flow at idle between tubes. Unless you're using a toroidal OT - in which case a bias-servo circuit is in order IMO - 5mA or less DC mismatch won't matter at all IME. In fact, I've seen sets with much greater idle mismatches that put out very similar full-power currents, and sounded great; I'd trade DC current-matching at full power for DC current-matching at idle any day of the week, but I don't know of any tube seller that does this (and what is "full power"? or "idle", for that matter?). Idle DC-matching is really quick and easy to do, and it gives a number that can be readily checked with a meter - but usually it doesn't last, and unless you've got a serious mismatch (which IMO is important mainly for indicating large differences in baseline tube transconductances, which can make a real difference) it really isn't a matter of life and death tone-wise; my $.02, YMMV.

AC matching - now that's a different story. Getting a set of tubes that makes music in an identical fashion is very much a worthwhile goal IMO, but hardly anyone does it because a) it's difficult to do without a curve tracer, which involves money, possibly a computer w/attendant skill set, etc. b) you really need a huge quantity of 'raw' tubes to end up with a worthwhile number of AC-matched sets, and c) it doesn't always result in a closely-DC-idle-matched set, which can greatly increase the WTF!-factor on resale, especially when top dollar was paid. AFAIK the only big tube producer who AC-matches is Groove Tubes.

If you feel more comfortable going with tightly-DC-matched tube sets, that's great - it surely can't hurt, and at the very least it shows that some TLC was put into the selection of your tubes which IMO shows integrity on the part of the seller. I realize you posted that unmatched sets might still sound good, but I get the feeling that you'd much prefer a DC-matched set, and I'd hate to see you pass up some possibly great-sounded tubes (or having a bad feeling about a tube vendor) because they weren't within a certain arbitrary number of DC mA of each other at one particular Vp/-Vg. And as idle current (along with practically every other aspect of tube DC & audio performance) is dependant on interelectrode spacings - which are really pretty small, and can change during shipping - it's quite possible a perfectly-DC-matched set will arrive at your door unmatched.

I haven't really had too many "crusher" 12AX7's (ones that were so microphonic they made thunderous booming/rattling noises, or rang like a gong, and IMO had to be destroyed) but man have I encountered tons more that were borderline. IME high-gain combos are the worst for showing up loose 12AX7's (or beating good ones into pulp), followed by high-gain amps sitting on top of high-efficiency multi-speaker cabs. And of course all the other microphonic tubes I've seen/replaced over the years - 9-pins, octals, power tubes, rectifiers (!) - they're out there, believe me.

Ray

[Amp Kat]

Regis,


IME high-gain combos are the worst for showing up loose 12AX7's (or beating good ones into pulp), followed by high-gain amps sitting on top of high-efficiency multi-speaker cabs. And of course all the other microphonic tubes I've seen/replaced over the years - 9-pins, octals, power tubes, rectifiers (!) - they're out there, believe me.

I 'm with Ray on this one and some 12AX7's that are microphonic in high gain amps may be perfectly quiet in another amp which brings me to another point. It's not always the tube that causes micrphonics. I've seen small ceramic disc capacitors used that are microphonic and even lead dress that can cause oscillation leading you to believe it's microphonic and other issues as well like pots. For the power tube mismatch, are you sure that your OT is evenly matched ? Peaveys are notorious for lobsided trannys so 5 to 10 ma's apart is the difference from side to side impedance wise. I really don't let it bother me if they are a few milli's apart unless they hum then it's a problem. Some amps with the bias balance can make up for the difference bringing them closer together. Then of course theres also individual bias pots for setting them like you want it but a rare thing in current production amps and they are getting more and more cheaply made by the minute. Theres also intentional tube mismatching for getting more asymetrical balance for producing a more even order harmonic content to the mix so for playing clean IMO you do want them somewhat close but for distortion depending on how you get it there could be some disadvantage to exactly matched tubes.

[Regis]

Dammit Ray and KB, I was up here on my soap box spouting truth to the masses, and you had to go and make sense and stuff. All of sudden my soap box is damn rickety and look like a fool, deservedly so.

Ray, I never heard a more clear explanation about the why's and wherefore's of matching. That's not hard, I always considered it a number but not what the numbers meant. Now I do.

KB, I also hadn't considered OT mismatching, how do I measure that? Come to think of it, one of my SF Bassmans has always been 5-8ma off when I measured it, maybe that's the problem.

I'm gonna save this thread and post it on my website, it's been very educational.

[Tom Phillips]

An easy diagnostic tool

All,
I will suggest that anyone who sees a mismatch may proceed by swapping the push & pull side tubes. If the mismatch does not follow the tubes then you will know there is something else going on and you can investigate further. Could be the PT, the bias circuit, leaky PI coupling caps etc.
Regards,
Tom

[Ray Ivers]

Regis,

FWIW, you came across as anything but foolish, and you certainly have every right to set your own tube-matching tolerances! I just wanted to take a look at the DC-matching thing from a different perspective... I wish it were more indicative of a truly well-matched set then it really is, but it's by no means a worthless waste of time, either. Lots of good info in the other posts here, too - for some reason I always forget about OT primary DCR imbalance; wishful thinking, perhaps.

I've got two GE 6550A pulls here, idling 17mA apart (50/33mA), and yet they pull 198/197mA with signal applied; go figure.

Ray

[Tom Phillips]

Re: Go Figure. How about...

Hi Ray,
Re: I've got two GE 6550A pulls here, idling 17mA apart (50/33mA), and yet they pull 198/197mA with signal applied; go figure.

Maybe the tube with the lower idle current has a higher Transconductance (gm). Seems like that could account for it "catching up" to the other tube when signal is applied. In addition the drive from the phase inverter could be a little higher on one side. That would also affect the total measured current when signal is applied. Seems to me that the combination of these two factors could easily account for the measurements that you observed.

What do ya think?
Tom

[Ray Ivers]

Tom,

Yes, I bet that the 33mA tube does have a higher Gm, which would give a lower idle current for the same -Vg. You'd think it would have lower current across the board, though, if the tubes were otherwise identical and were receiving identical drive... the tubes probably have differences in their Gm curves, and/or it could well be phase inverter imbalance as you mentioned - I'll check that out tomorrow (my last set of tests, then it gets buttoned up and returned to dustcatcher-status ;>).

Ray

[Roberto Lasco]

Ray, could I infer from your comment that matched sets that idle at lower currents (at same Vp/Vg) perform and would be considered "better" (due to higher transcondutance) than sets that idle at higher currents ? That would be interesting if true...

[R.G.]

I have a mildly heretical view on matching.

Matching is what you do when you can't design the circuit to be tolerant of mismatching. Since we can't redesign amps (very much, at least) we're stuck with matching.

What is a matched tube? In order:
1. matched tubes DC-bias at the same currents or close so we don't have magnetic offsets in the OT causing us one-sided saturation grief or out-of-current grief on one tube.
2. If tubes are matched at DC, that does not mean they're matched for AC gain. So second order matching involves gain matching as well as DC bias matching.
3. If tubes are both DC matched and AC matched, they might also be distortion matched, so that the little irregularities in AC response are also matched. I'm not sure if anyone has ever done this or not. 1 and 2 are easy.

You can do 1 and 2 by brute force measurement and selection. Or you can do it actively.

By actively I mean matching DC bias by matching the tube current with individual bias voltage settings. That largely eliminates the need for DC matched tubes.

You can also AC-gain match tubes by putting in a single sided post PI master volume of limited range. By turning down the available signal to the hotter gain tube, you can match the overal PI-to-output-tube-plate gain.

So for two added pots to tune in a new set of tubes, you can ditch buying matched sets.

I don't have a good solution to the matched-distortion squiggles.

Yet.

[Ray Ivers]

Roberto,

Ray, could I infer from your comment that matched sets that idle at lower currents (at same Vp/Vg) perform and would be considered "better" (due to higher transcondutance) than sets that idle at higher currents ? That would be interesting if true...

I think I'd have to say "yes". Given two otherwise-identical tubes, the one that idled at a lower current at some fixed negative bias voltage (i.e., the one with higher Gm) would actually be considered a "better" tube by the OEM, a tube tester, etc. In an audio-amp application, the higher-Gm tube would be easier to drive to full power, and would have more apparent gain.

Ray

[Amp Kat]

Regis. A simple resistance test across the primaries from center tap to each side will tell if each side is different. Find a 5150 OT and you'll most likely encounter it. It is an interesting point that Ray made because at quesient theres no signal and who knows what happens after it starts conducting and if frequency effects in the signal have any swaying one way or another. Tube testers really don't do justice because they will give you a gm reading and tell if there is a C.T.G short or heater short but they don't tell you how long you have left on the tubes or what happens in the real power transfer stage. I guess if you really wanted to scope it in depth you could hook up a Spectrum Analyzer and find some interesting things as that's most likely how they match them in the first place. Another thing is some tubes act much different when they've warmed up for a couple of hours and we know they aren't as stout as the NOS. This guy brought in a Siver Face Bassman the other day and said he hadn't ever changed the tubes (ten years!) so I yanked them out and of course an RCA GTA black plate and one gray plate and the bias on both sides is dead on ! Go figure,

[Enzo]

There is also the matter of expectations. When we arbitrarily decide that such and such amount of whatever parameter we are considering is the match standard, we should really consider also what the matcher considers matched. They may consider getting something within say 5% of another something is matched. We can then decide on our own that it has to be 1%, but that doesn't make 5% wrong.

If I want to match 100k resistors, how close do they have to be to be called matched? 2%? 1%? .05%?