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pforeman 8th January 2011 06:11 PM

used 12at7
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.


SY 8th January 2011 06:12 PM

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

pforeman 8th January 2011 06:54 PM

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 (?).

DF96 8th January 2011 06:59 PM

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 8th January 2011 06:59 PM

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 8th January 2011 07:07 PM

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 8th January 2011 07:09 PM

We have a finer discernment than Brits.

Salas 8th January 2011 07:10 PM

Or milli Siemens. mS.

DF96 8th January 2011 07:16 PM

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

Salas 8th January 2011 07:19 PM

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.

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