# Tell me what I'm missing here



## J-75 (Jul 29, 2010)

Anybody else notice sets of 'matched' 12AX7's in pairs, and on up to a set of 6 being offered for sale on CL? I realize the value in finding an internally matched dual tube used as a PI, but what's the point in matching up individual tubes used sequentially, or in totally different roles in a circuit?


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## Soultone (Jan 3, 2011)

I think that matched sets of 12AX7s are needed in some HiFi applications.


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## mhammer (Nov 30, 2007)

That makes sense. For guitar, we don't really need channel-to-channel balance, since we only run one at a time, or if we run two we usually want them to be different from each other. For a high-end tube listening system, it helps to know that each preamp side provides identical gain and doesn't need to be monkeyed around with to achieve that.


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## gtone (Nov 1, 2009)

It's my understanding that early grid-leak biased (as opposed to cathode biased) pre-amp circuits work best with balanced pre-amp tubes. Mind, many of these amp circuits are so ancient that they ran octal pre-amp tubes and not 9-pins.


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## Wild Bill (May 3, 2006)

As a tech, I can't think of ANYWHERE to use a "matched" 12AX7! I think it is just more audiophile mojo, coming from people who have never read a true electronics book in their life!

In an amplifier circuit, gain is set almost entirely by the circuit parts values, given using the same tube, of course. Any differences from the tube itself tend to be swamped out. The sections of a 12AX7 are used in sequence but when you multiply the gain factors by several stages the total amplification is so large as to render any small differences between tubes meaningless. That cascaded amplification of the signal will eventually be fed to the output tubes. Variations between good tubes of the same type will be "mice nuts" by the time they come out the speaker. The human ear is incapable of hearing them.

So each preamp side DOESN'T have to be balanced! Any balancing is too trivial to be heard after being run through the entire circuit.

As for the phase inverter, again the differences are nowhere near enough to be audible but more than that, the usual long-tailed phase inverter is of a design that would render a difference in gain between the sections but THE CIRCUIT COMPENSATES FOR THAT!

Those old engineers weren't dumb, you know!:bow:

Besides, how are they matching such tubes? Actual dynamic gain? Or plate current from a bias level, like an output tube? Completely different when placed in a circuit. Still trivial, though.

However, it's an easy way to get more money for your tubes!

I would LOVE to get a few of these so-called "gurus" together and do some blind testing. Most of them refuse to attend anyway. A bit of a google will turn up some "Penn and Teller" BS history. One of my favourites is the time some techs got some audiophile magazine writers to do a blind test on the sound of different speaker wires. The joke was that one of the choices was an old piece of barbed wire!

And yes, they fell for it, hook, line and sinker!

Wild Bill --- Busen Amps


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## Adicted to Tubes (Mar 5, 2006)

Quote:As for the phase inverter, again the differences are nowhere near enough to be audible but more than that, the usual long-tailed phase inverter is of a design that would render a difference in gain between the sections but THE CIRCUIT COMPENSATES FOR THAT!

Hmmmm....dumble amps use a PI plate balance trim pot that really does make a difference in tone.I have one in my latest build and I would have to say it really works. You can balance or unbalance the PI plates AC signal that rides on the DC and it alters the tone quite remarkably.So yes,a different balance of tube in the PI position can make a difference in tone.The offset of plate resistors of 82k and 100k is supposed to 'auto balance' the plates when an amp uses negative feedback but it can't compensate for every tube,so a balance trim pot does the rest.

NO IT'S NOT PHSYCOACOUSTICS. IT REALLY WORKS!


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## Printer2 (Apr 8, 2012)

Adicted to Tubes said:


> Quote:As for the phase inverter, again the differences are nowhere near enough to be audible but more than that, the usual long-tailed phase inverter is of a design that would render a difference in gain between the sections but THE CIRCUIT COMPENSATES FOR THAT!
> 
> Hmmmm....dumble amps use a PI plate balance trim pot that really does make a difference in tone.I have one in my latest build and I would have to say it really works. You can balance or unbalance the PI plates AC signal that rides on the DC and it alters the tone quite remarkably.So yes,a different balance of tube in the PI position can make a difference in tone.The offset of plate resistors of 82k and 100k is supposed to 'auto balance' the plates when an amp uses negative feedback but it can't compensate for every tube,so a balance trim pot does the rest.
> 
> NO IT'S NOT PHSYCOACOUSTICS. IT REALLY WORKS!


I think he was saying the circuit is the dominant factor in terms of a 'matched' tube or not, not that you can not hear the difference in the tone between a balanced and unbalanced PI.


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## Wild Bill (May 3, 2006)

Adicted to Tubes said:


> Quote:As for the phase inverter, again the differences are nowhere near enough to be audible but more than that, the usual long-tailed phase inverter is of a design that would render a difference in gain between the sections but THE CIRCUIT COMPENSATES FOR THAT!
> 
> Hmmmm....Dumble amps use a PI plate balance trim pot that really does make a difference in tone.I have one in my latest build and I would have to say it really works. You can balance or unbalance the PI plates AC signal that rides on the DC and it alters the tone quite remarkably.So yes,a different balance of tube in the PI position can make a difference in tone.The offset of plate resistors of 82k and 100k is supposed to 'auto balance' the plates when an amp uses negative feedback but it can't compensate for every tube,so a balance trim pot does the rest.
> 
> NO IT'S NOT PHSYCOACOUSTICS. IT REALLY WORKS!


Printer2 nailed it, Adict! Howard's PI balance alters the tone by causing distortion! The use of 82k and 100k is to make the gain of the two PI sections equal and therefore as clean as possible. It was never designed as a tone control or a distortion adjust.

Also, you have to be REALLY careful when looking at anything built by Howard Dumble! He deliberately tried to confuse things, in order to discourage anybody trying to copy one of his circuits. So he would do things that didn't really matter. For instance, he first developed his overdrive back in the late 70's or so. Metal film resistors had come on the market and anyone working in audio knew they were MUCH quieter than old-fashioned resistors like carbon comps. Plus, their stock tolerance was 1%, instead of the 10% of carbon comps or 5% of carbon films. That meant that you would not buy a 100k resistor. The standard 1% jumps meant you would likely buy a 998k or whatever.

Howard loved these strange looking values because most copiers know very little about electronics. If they did know, why would they have to copy? They would see all these strange values and assume that the differences were very important and they would waste their time going out to buy the 1% values. If they were really "new" they might not even know where to buy them or know to go to an industrial supplier, since smaller shops at the time would not have even stocked metal fims.

In reality, for a tube circuit, especially one designed for guitar, the more precise values didn't matter a whit! It was like measuring with a micrometer then cutting with an axe! But, it was one of Howard's ways of slowing down the rubes and keeping his "secrets".

For a while he would actually pour auto body potting compound all over his wiring, to make it even more difficult to trace his circuits. Made it a bitch to fix one of his amps, of course!

Finally, Howard wasn't some all-knowing guitar amp god. Even folks who have electronic degrees and years of of experience don't know everything. Howard knew enough to experiment and try some ideas. He came up with a FABULOUS type of overdrive! That doesn't mean he knew that a PI balancing pot was absolutely necessary. He knew that there was a difference in gain between the two sections, NOT from the tube but due to more losses in the circuit on one side than the other. He knew that the circuit was designed to balance both sides so that both sides of the signal would be equal when they were combined in the output transformer. He knew where to put a trim pot.

(The AikenAmps website has a fabulous technical FAQ about the long-tailed Phase Inverter. Really explains it very well!)

That doesn't mean that a trim pot is necessary! It just means he tried it out! Guaranteed nobody before him had even tried it! After a while he began to realize that the pot just wasn't worth the money or the bother and he stopped using it.

So the trim pot doesn't "balance the tube". It just can ADD distortion if adjusted far off from the stock values. You might of course hear the difference but when you tweak it to be "clean" and look at it on your scope you will find that the signal is balanced over a range and the centre of that range will be the stock value. Changing the tube won't matter, unless one side of the side is actually screwed up or failing.

Part of the reason these issues crop up is that most folks today don't realize that the physics behind how tubes are made makes them rather close in things like gain. Tubes that fall out of range are scrapped when they come off the production line. Having two sides of the same tube having a difference in gain THAT IS SIGNIFICANT is VERY hard to make happen! It's just the way tubes work and how they are made. This idea of matching gain with preamp tubes would have made engineers from the Golden Years of Tubes laugh their asses off! Those guys understood tubes FAR better than probably ANYONE alive today! How many PI balancing pots have you seen in equipment build by those old guys, even in the high end audiophile stuff like Fisher and MacIntosh or McCurdy? 

The answer is none!

I was probably one of the last people hired to work in what was left of the Canadian Westinghouse Tube Division, back in the late 80's. The big factory had been closed at least a decade before. From about 3000 workers they were down to maybe 8 old folks hoping to last it out till retirement. Westinghouse no longer built tubes so they would buy from surplus dealers and rebrand them as Westinghouse and honour the warranty. I learned so much from those old experienced people and I am forever grateful. I can tell you for certain that they did not believe in any of the mojo that is being spread today. Once a customer on the phone was asking if I could source other brands than Westinghouse as he believed that a certain brand sounded better in his hifi amp. Being new, I didn't know so I asked one of the old engineers.

He laughed at me for at least 3 days!

Wild Bill - Busen Amps


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## Printer2 (Apr 8, 2012)

This tube has an airy crispness to the top end but the midrange sounds a little wooden on some material. Best listening is by candlelight at a temperature of 68F with a hint of fog outside.


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