7025 Tubes

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JMPGuitars
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7025 Tubes

Postby JMPGuitars » Thu 07/11/19 10:07 pm

Here's a weird/funny story. I decided I was going to test out 7025s instead of 12AX7s for a couple applications. 7025 tubes are supposed to measure exactly the same as 12AX7 tubes, but with less noise or hum. If you look at all the old datasheets from tungsol, GE, Sylvania, etc... they show the same characteristics for typical use:

Ra 26.5k, Va 250V, Ia 1.2mA, Vg -2, gm 1.6mA/V, mu 100. That's all common between 7025/ECC83/12AX7. JJ doesn't even bother using a separate datasheet.

So I made the mistake of ordering 10 TAD 7025 tubes. Supposedly premium, hand selected, all that BS.

I tested 7 out of 10 and then decided to return them. TAD decided to make their own datasheet with completely non-standard values to sell substandard tubes.

The values I got with the TADs averaged:
Ra 300%, Va 250V, Ia 40%, Vg -2, gm 45%, mu 90%.

That to me looks like dead tubes. Any 12AX7 coming back with these readings would be thrown out...or used to loosen new sockets. ;)

Not only that, they were inconsistent in their crappiness. The best one of the lot actually hit 68% gm, and 58% Ia, with only 144% Ra. The worst one had 46% gm, and 34% Ia, with 462% Ra. That's a serious swing.

I emailed TAD about it, and somebody responded with some nonsense, and assumed I was testing on an AT1000 (I'm not). He made excuses saying that a tube in a tube tester won't perform as well as in real life. Also (mostly) nonsense. If two tubes are supposed to perform the same, then both tubes will perform equally well or equally poorly in the same testing circumstances. However, I use a tube tracer that actually can set the real life voltages for the tube...and trace them to get real life curves like real manufacturers do. And guess what? Even my old used and abused ECC83/12AX7 tubes I tested performed significantly better than any of these 7025s. The most abused old ECC83 tube I had still tested better. That one got 83% gm, 95% mu, and 70% Ia.

Anyway, I'm obviously over the TAD hype. :roll:

Thanks,
Josh

PS. TAD's datasheet shows similar numbers to some of what I found if you follow the curve. But that doesn't make them good. That makes them defective tubes they traced to make them look ok. Those tubes are either defective, or not 7025s.
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crgfrench
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Re: 7025 Tubes

Postby crgfrench » Thu 07/11/19 10:20 pm

I think TAD is like GrooveTubes -- they don't manufacture anything, they just package and market the stuff OEMs passed over as not good enough from the factory.
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Re: 7025 Tubes

Postby zuceno » Fri 07/12/19 3:57 am

IIRC I have seen it said, by an ex-employee, that TAD buy in large batches of tubes and test them individually, rejecting anything they don't like, often up to almost half a batch. What they do with those rejects I don't know.

What I do know is how the Philips group used to select tubes from their ECC83 production batches.
First of all, you need to know that the specification sheet gain value, 100, is actually a nominal figure. The typical gain of production ECC83 was well above that, around 112-118 was usual.

The next thing you must appreciate is that there is a relationship between gain and signal to noise ratio(S/N). The low gain, but in specification, tubes from a production run have a far better S/N than the high gain ones.

So Philips would select out the low gain tubes and test the S/N. After that was checked, they would be reserved for special applications.

IIRC (I can't find this written down anywhere) the ECC83WA had a gain between 100-105, ECC83WB had gain between 95-100, and ECC83WC had gain between 90-95, with the S/N being much better than standard production and improving as you move from A to C.

Note that they never managed to produce low noise high gain types consistently. The closest they got was the 5751, and while the spec gain for that is 70 they are often a lot higher.

What the TAD man said about what happens in "real life" has some truth, despite the fact that he was trying to deflect criticism. In many cases the "stage gain" is not absolutely proportional to the amplification factor. So a stage designed to give a gain of 50 with a u=100 tube may give a gain of 48 with a u=90 tube - there are other factors in play here.

From what has(n't) been said, I would guess that TAD's selection process involves picking out low noise tubes, and low gain tubes with better S/N are what they have supplied.
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Re: 7025 Tubes

Postby JMPGuitars » Fri 07/12/19 10:58 am

It's fairly typical for any 12AX7 to have some variation in mu, but gm, Ra, and Ia all need to be within a specific range to be considered both 1. a good tube, and 2. an ECC83 variant.

An old tube may maintain the mu, but once gm and Ia degrade, the tube is out of spec. Their tubes START out of spec (by far).

The TAD guy responded saying that I'm right, but wanted me to test in a cathode biased setup. I told him apples to apples is apples to apples, and even their datasheet curve shows bad values compared to a real 7025.
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