Low gain 12AX7 ECC83 ???

[SoulFetish]

Sorry I thought you asked if this meant the tube was getting weak.


Did they make a 12AX7 with different amplification factor? No, that would then be a different tube type.

I believe they did. Wasn't the 5751 designed to work as an electrical equivalent (for all other intents and purposes) to the 12AX7 with a mu of around 70?

[Justin Thomas]

The 12AY7 & 5751 are direct replacements, optimized to work at their best in an identical circuit as a circuit designed for a 12AX7, but neither should be considered a "low-gain 12AX7." They are all three very distinct tube types. It'd be like calling a 68k resistor a "low-resistance and 100k."

If I had a real 12AX7 with a gain of 70 or 45, I'd call it "broken."

Justin

[Enzo]

5751 was an industrial tube, designed for control circuits. The fact it is useful as an audio tube is a bonus.

[J M Fahey]

Good AX7 plate reads 168V

Good AU7 plate reads 68V

The International labelled 12AX7 plate reads 58V

So you have a re/mis-labelled 12AU7 , period.

Or any of the tubes listed in manuals as 12AU7 replacements.

[Audiotexan]

No need for a Tube tester: just plug suspect tube in a known preamp position, say the classic 1k5/100k cathode/plate pair.

A 12AX7 will self bias around 150/160V if rail is 250/260V , a 12AT7 will not be far off .... a 12AU7 will be way off.

I think plate will be much lower, probably less than 100V, wouldn´t be surprised at 60 to 80V , since U7 passes way more current than X7 in all cases.

While a well worn X7 will pass less current than expected (lowered emission as Enzo said), so plate voltage should be way higher than normal.

I bet that plate voltage difference should be enough to separate a weak X7 from a good but lower gain U7

Even if both show low gain.

Bloody brilliant Juan!
Thanks for the great tip!

Now to try and remember this the next time I decide to go wading into the tube stash. lol
(Regardless, I saved it for posterity! )