Acceptable variances for "matched" power tubes?

[Regis]

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%?

Ok, my head just exploded.

If an amp overdrives in the woods, and there's no one around to hear it, does it make a sound?

I guess it depends on whether it has a master volume or a power soak.

[Enzo]

If a tree falls in the woods, and no one is around to hear it, how do they know the tree fell down?

[steve]

If a tree falls in the woods, and no one is around to hear it, how do they know the tree fell down?

More importantly: if a man says something in the woods and there is no woman around to hear him, is he still wrong?

steve

[Enzo]

Count on it.

[Steve B]

Well, I've been alone in the house & playing on my SF Princeton Reverb for the past couple of hours at good volume (LOUD to other family members), & it sounds really good!

So 6ma differences on the power tubes doesn't make things sound any worse. In fact, I've been really enjoying the sound, maybe even more than the matched set that was in there previously?!?! Psychoacoustics or the real deal, I like it.

Steve

[Roberto Lasco]

Matching is fine, but I need to say the sweetest DR I ever heard had the 6V6s 18mA apart !

[R.G.]

So 6ma differences on the power tubes doesn't make things sound any worse. In fact, I've been really enjoying the sound, maybe even more than the matched set that was in there previously?!?! Psychoacoustics or the real deal, I like it.
Steve

Oh, it's the real deal. Think about it - one tube with markedly different gain than the other? By definition, that's asymmetrical gain on the two half cycles, and a slight bit of even-order distortion.

Human ears LIKE small amounts of even order distortion.

This is another of the problems with a blind urge to have matched tubes - it may not be a good thing. This issue was pointed out in the Tube Amp FAQ years ago.

[stokes]

Copnsidering that all the components in a tube amp are +/- any where from 5 to 20% of their stated values or ratings or whatever I dont think tubes being a couple of ma apart means much.I have never measured the dc res from ct to either end of the winding of an OT and saw a match.Once everything in an amp gets cooking these small differences dont mean much.In a hi-fi situation you would want tighter tolerances and the hi-fi guys pay a lot more for the components just for this reason.Just look at the prices of Black Gate caps-$100+ for an electrolytic?Or some of the paper in oil signal caps.Wont ever see a guitar amp builder using high end parts like that cause we dont need them.

[Amp Kat]

I have never measured the dc res from ct to either end of the winding of an OT and saw a match.

Hmm I have and an exact to the very tenth of an ohm. The Jcm 900 OT's are very close and several other Fenders are too.

"Once everything in an amp gets cooking these small differences dont mean much.In a hi-fi situation you would want tighter tolerances and the hi-fi guys pay a lot more for the components just for this reason."

That's true but they put more emphasis on tube matching than expensive caps. The Marantz 8B has 4 individual bias pots for each tube with an analog meter readout and switch selector for each tube. The THD % + N is a much greater concern in a HI-Fi amp at least back in the day because the music they were playing was distortion free. Nowadays with all the insane gain and noise related amps it may not be an advantage to have such a low THD % allthough that's the way it's supposed to be. Hell a mismatched set and some hum may fit in quite well with the musical content !

[Carl / Zwengel Amps]

I'm probably going to get crucified for saying it but in my eyes the whole argument is pretty much academic. Think about it for half a second! We're overdriving the hell out of the phase inverter, pushing the power tubes to the point of grid blocking and saturating the cores of the opt. So we're going to get our panties in a bunch over less than 1% thd from a set of tubes that isn't perfectly matched?! UGH!!!!

Having gotten that rant off my chest, I dont know where some people are getting their tubes but mine come matched to within less than 5% on current AND transconductance. And that'll get you some pretty damn symetric output up to the point of driver or power tube clipping...at which point as I've said previously, you're really not going to hear much difference.

-Carl

[Rob Mercure]

many paths to thermionic bliss

The Marantz 8B has 4 individual bias pots for each tube with an analog meter readout and switch selector for each tube. The THD % + N is a much greater concern in a HI-Fi amp at least back in the day because the music they were playing was distortion free

Yeah, but in the early 1950s the MacIntosh managed get superior numbers w/o any matching with their 50-W2 and 50 watts out of a pair of 6L6Gs! Split the load between the plate and cathode circuit and coupled one output tube's cathode winding to the plate of the other tube and vice-versa. No matching and THD and IM numbers that are still state of the art for tubes. More than one way to skin a armadillo!

Rob

PS: Personally I like individual bias controls for each and every output tube on any amp - who knows what I might want to throw in there!

[Enzo]

Armadillo? Possum on the half-shell.

[Steve Conner]

A few things that sprang to mind about tube matching.

AC signal matching: Most tube amps have negative feedback round the output stage. Won't this force the AC signal currents in the two output tubes to match even if they have different gm's?

DC matching: I fitted independent bias pots on my first (Toaster) homebrew amp. I just stick in whatever power tubes I fancy and tweak them to about the same idle current. I put quite a big range of adjustment on the bias, and played with weird power tube combos like one 6L6 and one EL34, or one 6V6 and one EL34. I run it at 475V plate voltage and a choice of 475 or 360V screen voltage, and 6V6s seem to hold together at the low screen voltage setting.

My favourite odd power tube combo was one EL34 and one EL37. It gave an excellent dirty sound at high volume, but the EL37 was an old worn-out one that began to run away and red-plate, so I couldn't use it any more. I'd love a pair of EL37s but they seem to be awful expensive >_<

[R ski]

Tube matching for guitar amps isn't going to hurt.

When using tubes for Hi Fi it does matter, distortion isn't
desirable.

I never found a output transformer balance out symetrically.

Due to that, swap tubes to balance out.

[sloblues]

Ray, this is excellent stuff. When you say "I've got two GE 6550A pulls here, idling 17mA apart (50/33mA), and yet they pull 198/197mA with signal applied" do you mean that you apply say a .125vac 400Hz signal into the amp, then crank it to max clean, or just max, output and then measure the DC current draw and it is nearly the same, despite the big mis-match at idle? And that is more an indication of a good set of power tubes than mere idle biasing might indicate? Also, if you please don't mind answering, what is the effect of the negative dc biasing voltage on the amp output as a whole, or the power output or balance of the amp? In other words, if I have seperate bias adjustment for each tube, and one tube requires -53vdc to idle at say 36ma ..then the other tube takes -38vdc to achieve the same idle. Does this negative DC bias voltage affect the quality or shape of the AC input signal, and, especially, does using seperate bias adjustments, so 'matched tubes' are not required, actually pose additional 'balance' problems since small to large variations on the amount of negative dc bias voltage will be required for each tube?