12AX7 Transcondutance

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V_amp
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12AX7 Transcondutance

Post by V_amp »

Hello all.

I have found some used (but well conserved) 12AX7 tubes with good prices.
I would like kwon whta's the reasonable transcondunace to encourage-me buy these tubes.

The guy who is selling thes tubes said me that the transcondutance is over
1.25 mA/Ohm at grid voltage = -1V and anode voltage = 100V
and over
1.6mA/V at grid voltage = -2 and anode voltage = 250V.

But those are characteristics of new tubes!?

Could someone give some hint?


THANKS
FAB
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GibsonGM
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Post by GibsonGM »

Hi V-amp,

Look, unless the tubes have been really abused, the transconductance is likely to be somewhere in the neighborhood of those values. I would ask if they work, and if so, I'd buy them! But not for very much; new ones aren't a lot of money. Perhaps $3-4 U.S., each. I'd like to hear them in an amp, and gently tap each with a pencil eraser while the amp's on, looking for microphonics.

How does he know the gm for the tubes? Does he have an awesome tube tester??
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dotfret
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Post by dotfret »

Those look like the standard values I get from a supplier I use for HiFi stuff who uses an Avo tester. They are low compared to new, but acceptable as useful valves.

What you have to remember with used valves is that the S/N never gets better with age.

NOS 12AX7 are usually well over the standard 100 gain when new (114-120 is not uncommon for RCA or Mullard). The S/N ratio was higher when the valves were high gain, and the MIL spec valves selected out of batches had a much lower S/N - so the manufacturers picked out the valves with gain in the 90-105 region when new, because they were the ones with really good S/N.

So you can buy used valves with acceptable and useful gain characteristics, but you have to remember that a lot of the time those valves were pulled because they were getting noisy. It can be that they were pulled for preventative maintenance reasons, and they are useful.

You just have to be careful, a tester does not test everything. Check valves out before you buy them, in an amp, or pay prices that mean you don't take a big risk ...
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Post by V_amp »

Hello GibsonGM and Dotfret,

The guy who is selling the tubes said me that he has a tube tester.
I'm buying via e-bay, so I can't test the tubes before buy them.

The tubes are Miniwatt (four tubes) and maybe the seller has other brands.
I'm waiting some answers.

So I'll try, because the price is not expensive.
It will be a surprise but I know maybe the tubes needs to go direct to the waste.

What do you think (or could say-me) about the miniwatt 12AX7 tubes (and plugged on 18W head)?

I were using three 12AX7 Svetlana Reissue (made in russia), and I were not happy. So I put a Sovetek in the the first pre-ampli stage.
I think it's better now, but not as I would like yet


Thanks guys
FA
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Post by dotfret »

Miniwatt was a Philips brand - they could be any producer within the Philips group egs. Mullard, S&H, Philips, they would all sound good and smoother than Sovteks, which are a bit crunchy.
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