12AX7 What is Mu?

[dumbassbob]

I see RCA 12ax7 tubes on ebay ,and a seller will have one Tested on a TV-7 @48/50,with 32 being fail...Then have one on a different Tester @98/102,with 70 being fail...What Is a Good RCA 12AX7...I need a good RCA for V-1 in my 63 Bandmaster ,all my other tubes just do not sound right ,Most of my tubes are for my old 100watt Marshall,Mini Watts,Mullards ect,there All to bassy or "dark"What is a Strong tube....

[Leo_Gnardo]

And I have a box full of "famous legendary brands" 12AX7/7025/ECC83 that all read "good" on my tube tester - the needle's in the green so they're good, right? Which all shows to go ya, what good is a tube tester. As has been said at least a hundred times on this fine website, the real tester is your amp. You can take "mu" as "theoretical maximum gain" for most purposes. And your amp will deliver about half that gain, maybe a little more than 60% at best. And the figure doesn't tell you a thing about bright, dark, noise, hum, clack, clatter, buzz, hiss, rumble, crumble, bloom or any other adjectives used for describing tube attributes whether good or bad. You have to plug 'em in and listen, bottom line. Whatever the seller says, it's blarney designed to get you to part with your money. Plug in & listen.

[dumbassbob]

But at $100.00 a pop..., So What should a NIB RCA read? 500/500?

[J M Fahey]

And the figure doesn't tell you a thing about bright, dark, noise, hum, clack, clatter, buzz, hiss, rumble, crumble, bloom or any other adjectives used for describing tube attributes whether good or bad. You have to plug 'em in and listen,

+ 1

Tubes are ruler flat from DC to at least tens of kHz or even a few Megaherz, depending on type, not brand.

Any statement on the contrary is anecdotical at best or con men babble at its worst, or honest people who read way too many forums and publicity and repeat that blindly.

What should a NIB RCA read? 500/500?

??????????????????????????????????????????????????????

Do you know what those numbers mean?

What does the phrase: "Contents: 500 % pure water" mean to you?

a) very pure, high quality water

b) nonsense

Pick one.

[Steve A.]

Most modern OD channels are way too hot for me so I am usually looking for lower mu tubes to swap in and smooth them out. I have no special mu tester and just use my ears...

Steve Ahola

P.S. I usually avoid NOS tubes, looking instead for used tubes which have been tested and sell for a fraction of the price.

[tedmich]

I got your lower Moo right here!



or your lower mew

[Enzo]

Well, tedmich, you just beat me to the two gags I was going to write.

Bob, the mu means the amplification factor of the tube, look it up on the sheet.

The tube tester readings are meaningless.

ANy tester will have a meter, usually just a good/bad meter. If it has a scale it is an arbitrary one. When the guy says tubes tested on one tester at 99, with 70 being fail all it means is that on his tester the good/bad line was at 70 ON HIS METER, and his tubes tested a lot higher than the bad level. The other tester said 50 with 32 fail. They measured the same tubes. One scale had numbers roughly twice as large for the same results. Look at it like this:

my ceilings measure 10 feet tall, with 7 feet being fail (too low). With a different ruler I found my ceilings 3.33 yards tall with 2 yards being fail. See? Different number systems describing the same thing. And if you were from Canada, you'd probably want it in centimeters anyway.

All your tube seller was doing was reporting that on two different testers, the tubes tested way up into the GOOD region, instead of just barely.

[Alf]

Correct me if I'm wrong but what I've always thought is important, is how much ma/ Volt a tube can deliver and what my meter says about his.



If the number is close to the number of the datasheet (1600 for a good tube) the tube sounds fresher and if the number is significantly lower the tube sounds more tired ( more worn out).

What is the view of the more experienced people on this ?

Alf

[stokes]

"What is the view of the more experienced people on this"? You have the views of some of the most experienced people on this board already.A tube tester is basically just a good/bad instrument.I have had a number of good testers over the years and they will help weed out "bad" tubes but it still comes down to my ears in the actual amp.I currently use a modern tester with a computer interface which gives all the parameters of a tube,gm,plate resistance,mu,plate and screen currents and even a curve tracer.While it is much more informative,it still comes down to performance in the intended circuit.As for sellers citing tester results,its still a crap shoot

[Justin Thomas]

So basically,

Tubes were never designed to "sound good in my amp." They were designed to "function as designed." Read as, "diode, triode, tetrode, pentode, etc." As stated, they are linear from DC to well above the audio range; EVERY manufacturer would claim the same thing for any given tube type. And, there was an awful lot of licensing going on - if you didn't make that 6L6GC to the original patent specs or better, the inventor-company would sue your ass for misrepresenting their product, because you are only licensed to make the original design, not brand your own as something new... (huh, I wonder if anybody does THAT nowadays...)

Maybe the mfr. knew about variances in sound between brands, maybe not. I highly doubt they gave a crap - they had MUCH more important things to worry about, like whether that radar system reliably identified the Russian bombers invading the realm. Or the power output in that TV station. Or whether or not the police dispatch transmitter worked over the entire district.

If it met the specs, didn't short, wasn't gassy, and didn't melt down, it was good. Tubes were used for EVERYTHING - guitar amps were but a tiny fraction of intended use. And tube testers wouldn't know what the heck job the tube was doing...

I doubt anyone is licensing designs and following specs anymore, as there are no companies making new designs. Aren't they all just basically bootlegging? I mean for the tubes WE use...

Justin

[Alf]

"What is the view of the more experienced people on this"? You have the views of some of the most experienced people on this board already.

Sorry Stokes , I don't mean to offend anyone , English is not my first language.

I would like to know if what I'm saying in my message is true or pure nonsense.

[stokes]

Sorry Alf,didnt mean to sound like you offended anybody.Just letting you know the replies thus far were from some of the most experienced here.

[J M Fahey]

Can I double like this?

As of tubes, NO original Factory is still making them, period (most aren't even alive) and 98% of Worls tube production is Russian and Chinese ... who are justly famous for NOT respecting intellectual property rights.

Or maybe Russians bought and paid US technology back in the 40's, probably to help them against the Nazis , there must be a reason the Soviets made US type tubes and not European ones, and suspect shared that technology with communist Chinese, they were bed partners until 1960
Very smiling Mao and Nikita in 1958:

so chinese also "inherited" US designs.

As of Czechoslovakian tube factories, they were established, again because of the War, as Telefunken factories, so now you know what pattern do those EL34 follow.

And EI was originally established by Yugoslav *Military* , who wanted "independence" from foreign suppliers and, of course, military equipment fully run on tubes (like everything else), and who built their factory? ... again Telefunken, who supplied machinery and processes but most important, raw materials, specially "mystery" cathode coating.
So early EI tubes were VERY good, essentially MilSpec Telefunken.
Suppose later, when relations soured, they started using some home brewed mixtures, not as good.

So my point is that there are not that many *original* designs, most are OLD, and those surviving today are somewhat legal or mostly illegal copycats.

And they must follow old blueprints and formulas as close as they can, doubt there is much creativity left for really new designs.

Are some making KT90 and even KT120? ... suspect it's the same old KT88 design with slightly enlarged plates and maybe a higher emission cathode , not really different, just a little extra helping of the same sauce.

[km6xz]

Tubes really are low tech compared to the materials and precision in most engineering now so any country could make tubes as well as any other. Tubes however did not benefit from the advances in chemistry and materials because they were low volume and the whole potential market not being large enough to invest in the production equipment and engineering or even replace aging and worn out production equipment so we are stuck with old methods and materials until tubes fade away.

Because they could be produced by any level of sophistication, many countries were making their own tubes, often with unique designs. I come across interesting tubes every day here that have no western equivalence and even those which are said to be copies of US designs are not. When tubes started getting harder to get and more expensive a few people went to China and the USSR to find sources.

A common tube was found cheap and good quality with a rough operational range as the 6L6 was renamed 6L6 instead of its original name "6П3с-е" so lots of tubes bought for $0.25-.50 where relabeled 6L6 and got such names as Groove Tube and Sovtek. That was the beginning of the tubes being sold as something they were not, but they worked fine. The same production line or surplus caches got different labels and prices that spanned a range of 3:1.
Materials never got better and QC got worse, and the customers were taught to accept a wide variety of measured parameters as being good, a feature, so the re-branders could sell the increasing percentage of out of spec tubes that in prior generations would have just gotten crushed as bad tubes. Now they take the bad tubes and label it "clean" (low gain), middle gain(close to spec) or high gain(usually grid out of alignment) but with more subjective names.

So what is the correct spec? Who knows or cares, they are all variations on a theme and depending on the circuit conditions and preference for distortion onset, a tube can't be pegged with a particular sound. It is one component in a system of components and the best one can predict is "in my amp, with my playing technique, with this acoustic space and pickups, and these settings I like my sound". That makes sense. Internet claims that "XYZ is bright and airy with better HF" or "ABC brand has a strong mid bump and shimmering upper mids" clearly identifies someone who DOES not know what the hell the are talking about. Don't waste money on tubes that have a reputation for a type of sound. It doesn't.
A system can sound different when all the variables that are involved in circuit conditions, in naturally non-linear systems, are taken into account. Just normalizing the operating point of two different "sounding" tube erases almost any audible difference between them. You plug a tube into a different set of operating conditions and you should expect different transfer function but setting the operating conditions of any sample tube and suddenly the tube "sounds" like another. Being non-linear, the curves of the transfer function really changes the results if you select a different operating point on those curves.
People will argue that they test by keeping everything set one way and just swapping tubes proves the difference heard is because the tube "sounds" different. Think of it in the terms of the analogy of people evaluating a car by setting the exact same degree of pedal deflection when comparing a car with different size tires or engine tuning. Too many variables that give a false conclusion to those who ignore those variables.
Besides, tone is more in the players fingers than amp anyway so tube rolling is a fool's errand that helps identifying people NOT to pay attention to. The less experience and less the talent and less something to say, increases the odds that the player believes the the tubes or super special capacitor or lack of it accounts for the lack of interest listeners show.

[Alf]

This is a picture I found on the internet- Sylvania
This was quality control in the twenties and thirties.