Random tube matching nugget

[Randall]

Jess Oliver the designer of the Ampeg B-15 and others once told me in an email that he thought matching output tubes was over rated. They never matched them at the factory, and they were not trying to make hi-fi amps., so what is the point? Just something to toss out for ideas. He is a difficult person to disagree with.

[g1]

Nobody matched tubes back then, it's a fairly recent process. I think the specs were adhered to much closer back then, so the mis-matches were not all that substantial. Then again, Fender added the "output matching" bias pots in the 70's, so they must have been dealing with some hum issues due to mismatch.
Seems a bit ironic for him to say that about "hi-fi", I thought early ampeg were big on that.

[Justin Thomas]

Nah, they just used hi-fi TUBES so they wouldn't have to design a hi-fi amp. Which I'm sure has been the cause of MUCH swearing, angst, and modification, and plain thinking of amps with unobtainium tubes. Like 6BK11s! Maybe the hum balance was a way to dodge warranty service claims? You know, add a user-serviceable feature instead of taking it in. "Oh, it hums? Twiddle the knob on back & see if it fixes it. It did? Yay!" Amp runs fine til warranty runs out...

All of this is for fun, I propose no real facts here!

Justin

Edit: in a serious tone, Ampeg WAS trying to be different. They were proud enough to not copy others; hence the wacky tubes?

[Leo_Gnardo]

Ampeg's founders OTOH were hi fi nuts. Got a feeling they'd disagree with Jess on this one. And I'll disagree too for the most part.

IF it's your aim to get some grit in your sound, who cares if the output section is lopsided, that might even help. Besides, if you and your band play so loud nobody will ever hear the annoying hum that you get with mis matched output tubes.

For those who need a more refined sound, don't want to hear that hum from mis match, then you do have to match & bias & balance properly, treat your power amp as if it was hi fi. Got plenty of jazz and studio players who appreciate this approach.

Just make sure you got the right solution for each customer. I'll opt for the matched/balanced power amp unless the customer clearly doesn't want it and that's not happened around here. So far nobody's complained about minimum hum and maximum clean power. They find other things to complain about all right, but not that.

It can be a matter of what does matched mean anyway? On what parameters. Many vendors simply check emissivity. There's a growing number that also match by gain as well. How close is "matched?" Within 1%? 5? 10? On emissivity or also on gain? I'd have to say 5% is close enough for rock n roll, or just about any instrument amp. Maybe Jess was saying "don't get too precious about it, you don't need 1% matching." THAT, I could agree with. But if one output tube is drawing 20 milliamps, and the other 55 mA, that's gonna hum badly and likely sound distorted at a low volume. If Guitar Johnny's at the store and he hears that racket in one amp and not in the other brand, which one's he gonna buy?

[Leo_Gnardo]

Nah, they just used hi-fi TUBES so they wouldn't have to design a hi-fi amp. Which I'm sure has been the cause of MUCH swearing, angst, and modification, and plain thinking of amps with unobtainium tubes. Like 6BK11s! <snip> in a serious tone, Ampeg WAS trying to be different. They were proud enough to not copy others; hence the wacky tubes?

Early days, late 40's thru the 50's, Ampeg bass amps used EL37 output tubes, hi fi all right and rarely if ever seen now. (Would some current manufacturer copy THAT one! EL37 innards were assembled with tiny nuts and bolts. No quick n dirty spot welds here!) Sort of a super 6L6 before 6550 took its place. 6SL7, 6SN7, 5U4 in pre and rectifier, the standards of the time. By mid 60's I expect the founders were paying less attention to quality & maybe thinking about selling the company they had brought to a good level, maybe time to let the engineers have their way. Say there's this triple triode that saves time on TV assembly lines, let's use them! Of course they weren't much thinking whether compactrons would be available in the future. Nothing was supposed to be available in the future - except transistors - according to many. (And a very few who considered transistors to be a fad that would dissipate in short time, like rock n roll, that sure worked din't it?) In fact by the time Ampeg started using compactrons, TV's had pretty well gone 100% to transistors, and tube vendors were probably desperate to get rid of compactrons at any price, iow, cheap! Perfect, we'll use 'em, just imagine how flummoxed repair techs will be half a century from now when they're non-obtainium. There also - neither Ampeg nor anyone else considered their amps would still be in use 50 years later. How much 1915 gear was still in use in 1965? "Don't worry, in the future 50 years from now people won't need guitar amps, they'll all be listening to computer-composed satellite-distributed Gas Music from Jupiter." So you could say Ampeg was using odd tubes from day one, those EL37's could be replaced by 6L6's or KT66 or even EL34 if necessary and still sound all right. No such solution for replacing compactrons, whups too bad. Now back to our program already in progress "Return of the Son of Gas Music from Jupiter, part III."

[Justin Thomas]

Now wait just a minute! If they didn't plan on us using them 50 years later, why'd they make em so darn good? Why not use Chinese junk and flimsy PCBs and ribbon cables galore and 57 relays and MDF cabs?



Justin

[Enzo]

AMpeg wasn't making hifis, but they were concerned with not distorting. It is one thing to design a power amp and not worry about the tiny contribution of power tube mismatch, it is something else to concern yourself with a preamp that has enough headroom do it doesn't distort, especially on peaky bass signals.

[Leo_Gnardo]

Now wait just a minute! If they didn't plan on us using them 50 years later, why'd they make em so darn good? Why not use Chinese junk and flimsy PCBs and ribbon cables galore and 57 relays and MDF cabs?

Let's say the better MI companies didn't have their planned obsolescence program dialed in yet, the way Detroit did with cars. Seems betwen 1960-75 the meaning of "engineering" changed, from "build it so it won't break" to "build it so it will last to the final day of the warranty and not one minute longer."

Mesa did have a hi fi power amp line (maybe still does, I dunno) called Baron. I've never seen any in person, but the photos looked quality, along the lines of McIntosh. Wonder whether there was any special engineering involved or if they took their usual power amp circuits, OT's & PT's & packaged them to appeal to the hi fi crowd. I've worked on some of their MI power amps, Mesa Strategy 400 & 500, very well built and engineered except for one glaring error in some cases: volume controls mounted jam up next to the power transformer to guarantee a high hum level. DUH! Those with vc's on the back panel, no problem.

[Old Tele man]

Three historical nuggets to ponder:

1) "Back-in-the-good-old-days", most 'commercial' vacuum tubes from a single manufacturer had production-lot tolerances of ±10% at worst, with MIL-STD tubes having ±5%. Today's tubes are closer to ±30-50% tolerances at best.

2) "Back-in-the-good-old-days", the manufacturer vs. manufacturer tolerances were seldom worse than ±10-15%...remember they ALL wanted to sell HUGE numbers of their tubes to the military.

3) Thus, tubes from *same* manufacturer were typically (but, certainly NOT always) "close-enough" in matching, especially if they were from the same production-lot.

[pdf64]

...historical nuggets to ponder:

1) "Back-in-the-good-old-days", most 'commercial' vacuum tubes from a single manufacturer had production-lot tolerances of ±10% at worst, with MIL-STD tubes having ±5%. Today's tubes are closer to ±30-50% tolerances at best.

2) "Back-in-the-good-old-days", the manufacturer vs. manufacturer tolerances were seldom worse than ±10-15%...remember they ALL wanted to sell HUGE numbers of their tubes to the military...

Is there any evidence for the above?
I'm a bit sceptical; if tube 'variance from bogey' was that small, why would designers in the era of vintage production have bothered to fit amps with bias adjust / balance trimmers?
And why not put tolerances on tube performance characteristics?

My guess is that tubes have always been considered fit for sale / purpose if all performance characteristics were within eg +/-~50% of their type's bogey value.
The issue with current production is that manufacturers seem to have effectively outsourced their QA to customers / resellers, their business model / policy being to despatch anything that passes a desultory visual inspection, such that their raw output will include a significant percentage of tubes that are not fit for purpose, DOA, excessive 'infant death' rate etc.

[Enzo]

why would designers in the era of vintage production have bothered to fit amps with bias adjust / balance trimmers?

Because tube do not sit there as parts alone, they work in circuits, and those circuits have a WIDE variance. remember, most of the old Fender schematics have a note in the corner that readings will be within +/-20%.


In the good old days, tubes were what there was. RCA et al published design guides and tube manuals. They wanted to sell millions of tubes to huge commercial consumers of them. Nowdays tubes are a tiny niche market for people not looking for commercial bulk use. Compare the number of say Fender tube amps sold, to the number of MP3 players, flat screen TVs, computers, smart phones, home security/automation, etc. Much as we may love this industry, it is a TINY little corner of electronics. The people who make tubes today do not have the prospect of losing a $10 million tube contract from the consumer electronics industry. In the good old days, RCA and Sylvania and friends were doing research into materials and engineering in the processes of manufacturing tubes and improving the breed. Today, tube makers have bought pieces of the old tube making equipment and are trying to make the old products. Imagine someone's brother buys a fancy restaurant from a chef and then tries to cook the fancy meals himself.

I buy all my tubes from suppliers I trust, like CEDist or Magic Parts. All the tubes are tested, and I pay a premium for that. I could buy boxes of 100 12AX7s in bulk from Sovtek or whoever, and save on the purchase price, but I would then find 30% of them or more were not acceptable for resale. NO ONE bought a bulk box of 100 12AX7s from RCA and expected such a failure rate.

[Old Tele man]

Two answers to your question(s):

1) Read some "old" MIL-STD specification sheets for vacuum tubes; especially the "acceptance" test specifications.

2) I spent 7 years, 8 months, and 20 days ('63-'71) as an aircrewman in EC-121 "Warning Star" (2nd generation AWACS, if you care) in which ALL electronics used vacuum tubes. The AN/APS-20E (2MW / 2880 Mhz) search radar power supply used six 5881 (later 6L6GB) as regulator tubes. Tubes were "replaced" for two reasons: (a) scheduled maintenance (hours of operation) or (b) failure...and, typically NO adjustments to circuits were needed...just plug the new tube in and operate.

FWIW -- Bendix-manufactured tubes had "best" longevity reputation, followed by GE and RCA.

[GLPro]

Yeah Enzo, I used to purchase bulk tubes from a New York sensor outfit and I have a pile of EL34's and 6v6's in my unusable stack representing money I'll never recover (failure rates >30%); But in a small niche market of electronics, my small Enterprise purchase power has little leverage to move such folks to better QC programs....I suspect that quality products and their rejection criteria has always been driven by market share and price point.

Perhaps the warriors in shops like mine will take a few hits over time in an effort to continue to prop up a consumer market that ultimately rewards our customers with a fine working tool for their trade......it's probably always been that way. The rationale for the inclusion of tweaking apparatus in amps was probably driven by the same motives regarded by the valve manufacturing industry as overhead or just plain old pragmatism. After all, business plans evolve over time and our predecessors certainly knew as we know now that businesses would always fine ways to cut corners and maximize their margins.

Perhaps just forward thinking....they were building amps that would out last the current valve industry business models. I believe their forecast for such a trend was properly calculated.

[R.G.]

It consistently recurs to me that it is now possible to build much better and more consistent tubes than were ever built in the Golden Age of Vacuum, and most likely far cheaper than they are built in today's parasitic manufacturing market.

As an example, NASA and contractors are reviving the F1 rocket engine from the Saturn V. The F1 is the most powerful liquid fueled rocket engine ever built. Ever. The plans were not lost, as was thought, just stored so deeply that they could not be retrieved easily. But it's being re-engineered to modern manufacturing practices to eliminate the incredible amount of hand welding that was in the first ones.

Today, we have incredibly accurate and fast machines for making widgets quickly and reliably, to much finer tolerances that were ever possible with fixed tooling. We know more about materials. The functionality of a tube is almost purely a function of three things: the mechanical assembly accuracy of the internal parts placement, the vacuum chemistry of the materials, and the degree of vacuum. We can do all of these to a much higher degree than was ever possible before.

What we don't have is the economic will to go do it. The folks who could do this can make far more money making higher-profit-margin and higher-volume widgets for other purposes.

It's a pity.

[Old Tele man]

Agreed. Quality Control today is laughable...in the "good-old" days, non-conforming tubes were literally "crushed" rather than sold...today, everything is shipped (good or bad) with defect discovery left up to the end user. Back *then* matching was seldom (if ever) done, now it's the retail seller's ONLY real QC effort.