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Thread: Low gain 12AX7 ECC83 ???

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    Low gain 12AX7 ECC83 ???

    I've been rolling tubes thru my 74 Silverface Princeton Reverb.
    It came to me with what looks like all the original RCA tubes.
    The amp is in nice shape and doesn't look abused.

    Edit; It has a Weber replacement speaker, original long gone.

    I have recapped it (it had all the original caps) and the HT voltages check almost spot on with the schematic.
    All within 2 or 3 volts. Fluke meter. The first HT node measures almost exactly 420 volts as per the schematic.
    This kinda surprised me, as I thought with the higher wall voltages these days it would be a little high.
    Wall voltage measures 121.8 at this moment.

    In my stash of tubes there is this International branded 12AX7/ECC83.

    I put it in the V1 socket and was surprised to find the amp much quieter.
    Not just a little quiet, a lot quieter with that tube.

    The amp actually sounds quite nice with this tube, just very quiet.
    At full volume there is just a hint of break-up.

    With any other 12AX7 that I have, the amp is quite loud at full volume and has lots of break-up.

    Is this a sign of the tube getting weak, or is there such a thing as a "low gain" 12AX7?


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    Last edited by galaxiex; 02-20-2019 at 01:01 AM.
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    Lifetime Member Enzo's Avatar
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    Is there such a thing as a weak tube? Of course there is. CAthode coating wears out, whatever...

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    Quote Originally Posted by Enzo View Post
    Is there such a thing as a weak tube? Of course there is. CAthode coating wears out, whatever...
    Yes, I know that, but that's not what I asked.

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    Lifetime Member Enzo's Avatar
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    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.

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    Ok, I suppose I can assume this tube is getting weak.

    When I first installed it and cranked full volume,
    I was surprised to not hear any hiss/hum/background noise.

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    Senior Member Pedro Vecino's Avatar
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    The model is Sylvania. With an internal structure practically identical to the 12AU7 of the same brand.
    I could have an error in labeling. With a tube tester you could check it.

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    Quote Originally Posted by Pedro Vecino View Post
    The model is Sylvania. With an internal structure practically identical to the 12AU7 of the same brand.
    I could have an error in labeling. With a tube tester you could check it.
    Thanks!

    I don't have a tube tester, but I could compare it visually with some other Sylvania 12AU7's that I have.

    Actually I have a big box of old and new tubes that I acquired years ago, and have been looking at tube testers on ebay...

    I'll try putting a 12AU7 in V1 to compare as well.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by galaxiex View Post
    Thanks!

    I don't have a tube tester, but I could compare it visually with some other Sylvania 12AU7's that I have.

    Actually I have a big box of old and new tubes that I acquired years ago, and have been looking at tube testers on ebay...

    I'll try putting a 12AU7 in V1 to compare as well.
    You might try the 12AY7. It's a medium gain low noise tube, more gain than the 12AU7. Fender used to use those as an input tube decades ago, though I forget what models.

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    Supporting Member mozz's Avatar
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    International service master was one of those late tube rebranders. They would erase original numbers and lettering and put what they wanted on it. Would not surprise me in the least that it was a 12at7, au, ay, av, az.

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    Quote Originally Posted by nevetslab View Post
    You might try the 12AY7. It's a medium gain low noise tube, more gain than the 12AU7. Fender used to use those as an input tube decades ago, though I forget what models.
    Thanks,
    I was just wanting to compare a 12AU7 to this International tube that is labeled 12AX7 to see if there is any difference.

    So I put a new JJ 12AU7 in V1 and the amp sounds exactly the same as this International 12AX7.

    Plus this International 12AX7 came out of a Sylvania box that someone had penciled "old" on the end flap.

    So it's either a 12AX7 that is exactly as weak as a good 12AU7, or it's mis-labeled as a 12AX7, when in fact it's a 12AU7.

    I'm thinking the latter...

    What are the odds...

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    Old Timer J M Fahey's Avatar
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    I bought 1000 PCC189 at 50 cents each.

    I designed a preamp specifically around them, bisaing them properly, again proper load resistors and supply voltage, etc. , and they work quite well. Mind you, they are "TV tubes", designed for VHF tuner service.

    But I can very well imagine a tube rebrander stuck with 5000 to 20000 tubes "nobody wants" .... the temptation to erase and relabel can be irresistible.
    Or simply profitably sell bottom of the barrel, returns or rejects.

    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.

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    Last edited by J M Fahey; 02-19-2019 at 11:38 PM.
    Juan Manuel Fahey

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    Lifetime Member Enzo's Avatar
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    Visuals only go so far. You can see the plate structure. What is very hard to see from the outside is the relative size and spacing of the cathodes and grids. And they are what determine the gain of the tube.

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    Quote Originally Posted by J M Fahey View Post
    I bought 1000 PCC189 at 50 cents each.

    I designed a preamp specifically around them, bisaing them properly, again proper load resistors and supply voltage, etc. , and they work quite well. Mind you, they are "TV tubes", designed for VHF tuner service.

    But I can very well imagine a tube rebrander stuck with 5000 to 20000 tubes "nobody wants" .... the temptation to erase and relabel can be irresistible.
    Or simply profitably sell bottom of the barrel, returns or rejects.

    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.
    Thanks JM!

    I just did this test.

    Rail is 248V V1 has Normal 100K and 1K5 resistors.

    Good AX7 plate reads 168V

    Good AU7 plate reads 68V

    The International labelled 12AX7 plate reads 58V

    So I'm really thinking it's labelled wrong.

    The tube operates just fine, no noises or microphonics, just low gain.

    I very much appreciate you sharing this info,
    I would not have thought to check this myself, but I won't soon forget!

    Thanks again!

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    Supporting Member mozz's Avatar
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    Found the link of the relabeled tubes. Everybody should read this. There were also "standard brand" and "National brand" who were tube relabelers.
    I actually have a bunch of "standard" tubes in my fathers old tube caddies. I guess they were cheaper and made the tv repairman a few extra dollars. I don't think the "standard" put their label on the tube, just the box, but chances are they were lower grade or gain, factory rejects. Another i have seen was "El Menco" tubes in a Heathkit power supply, clearly 6l6gb made by GE.

    https://vacuumtubesinc.com/index.php/hallofshame

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    Quote Originally Posted by mozz View Post
    Found the link of the relabeled tubes. Everybody should read this. There were also "standard brand" and "National brand" who were tube relabelers.
    I actually have a bunch of "standard" tubes in my fathers old tube caddies. I guess they were cheaper and made the tv repairman a few extra dollars. I don't think the "standard" put their label on the tube, just the box, but chances are they were lower grade or gain, factory rejects. Another i have seen was "El Menco" tubes in a Heathkit power supply, clearly 6l6gb made by GE.

    https://vacuumtubesinc.com/index.php/hallofshame
    Interesting read, thanks!

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    Quote Originally Posted by Enzo View Post
    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?

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    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

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    Lifetime Member Enzo's Avatar
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    5751 was an industrial tube, designed for control circuits. The fact it is useful as an audio tube is a bonus.

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    Old Timer J M Fahey's Avatar
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    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.

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    Quote Originally Posted by J M Fahey View Post
    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! )

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