used 12at7

[pforeman]

I've been looking at used 12at7 tubes and see them advertised as having a reading of (for example ) 4725 / 4400 (on a Hickok tester).
Is there a value that relates to 100% ( NOS ) ?
Is there a value that relates to "don't buy it " - it's only got a few months left
Is there a value that relates to "used" but lots of life left?

In the tube data sheets I see several values for Ra from 10,500 to 16,500 depending on Va and Vg1.

Thanks
Paul

[SY]

Why not just get a new one? The JJs are the best 12AT7/ECC81 ever made and they're something like $10.

[pforeman]

Hi Sy,
Yes, I have a J & J and and old Amperex Bugle Boy.
No way to test the Bugle Boy to find out how "old" it is. Just thought that it had a little more believable sound stage.
I was originally trying to find the best output tubes (Tubelab S SE -triode only) but someone posted that it was the input tube that mattered most, so I finnegled a old Bugle Boy from a friends guitar amp...
I have a J&J, so just wanted to see for not too much money, I can buy a used 12at
or a new siemens (about the same $)
For $25 or $30 I thought I could try one of the "venerated" tubes, I just don't understand the results when posted about micro-ohms (?).
Thanks
Paul

[DF96]

It is more or less impossible to tell by measurements how much use a valve has had. The raw gm figure (presumably what is being quoted) varies quite a bit between guaranteed unused ones.

Measuring the change in current or gm when the heater voltage is reduced may give an indication that a valve is nearing the end of its emission life. A high ionic grid current shows a gassy valve.

Two options: buy new ones from a reliable dealer, or buy cheap NOS ones off the internet and expect to throw away some of them. Let other people buy the expensive NOS ones.

[SY]

Maybe micromhos? It's a measure of transconductance, but not meaningful without knowing current and voltages. I've tested most of the "venerated" ECC81 types and the JJ was by a large margin the most linear and microphonic-free.

[DF96]

One of the mysteries of the universe is why Americans measure gm in micro-mhos, but Europeans measure it in mA/V (=milli-mhos). 4400 micro-mhos = 4.4 mA/V.

Was the linear JJ 12AT7 the frame-grid version? I would expect a frame-grid to give greater linearity, if done properly, as it means the valve is basically planar as far as the electrons are concerned and this should lead to constant mu.

[SY]

We have a finer discernment than Brits.

[Salas]

Or milli Siemens. mS.

[DF96]

I think you are just afraid of decimal points, so prefer integers.

[Salas]

SI is Siemens. Backwards spelled Ohm in micro or milli is not for datasheets anymore. Japanese Jfet datasheets state in mS for instance. Which is mA/V. Even the Russian tube datasheets have mA/B.