Acceptable variances for "matched" power tubes?

[Ray Ivers]

sb,

two GE 6550A pulls here, idling 17mA apart (50/33mA), and yet they pull 198/197mA with signal applied"

The 198/197mA measurement was obtained with a clean 400Hz sine wave applied to the power amp input, just below clipping.

And that is more an indication of a good set of power tubes than mere idle biasing might indicate?

Well, I don't know about it indicating a "good" set of power tubes, but these two were certainly more closely matched at full power than at idle - and if I had to choose - which hopefully I'd never have to - I'd probably pick matched-at-full-power.

if I have seperate bias adjustment for each tube... does this negative DC bias voltage affect the quality or shape of the AC input signal...?

No, it shouldn't.

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?

If multiple output tubes each use a different DC bias voltage, they will each start to draw grid current at a different point on the driving waveform, resulting in moderate to severe AC imbalance reaching a maximum at full power (which some may like, and other's won't). I personally would still used matched tubes even if bias pots were provided for each tube.

My own preference is for a two-tube output stage with controls for a) DC bias b) DC balance and c) AC balance, but YMMV - this is just what has worked for me.

Ray

[Arthur B.]

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.

Balanced negative feedback and cross coupling would do the trick to match AC and DC characteristics.

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

I can't find a schematic for the 50-W2, but it seems it must be different from the usual Mcintosh since they cross couple the plates and the screens, not the plates and the cathodes. The other is part is of course the unity coupling circuit, which feeds back all the gain.

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!!!!

-Carl

What happens if the power amp isn't overdriven, and the distortion comes from the preamp, like most modern guitar amps?

[Typhoon859]

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

Hey, sorry to revive an old thread, but I was really curious about your view about a matching set of 0.0% tolerance in their full power and idle states and if with such a close match, it's likely that "musically", as you mentioned", they still might not be a match? If their so perfectly matched in this way, what are the chances the curves will be so different if "AC-matching"? Thanks!