7025 Tubes
Posted: 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.
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.
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.
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.