Low gain 12AX7 ECC83 ???

Discussion in 'Glowing Bottle Tube Amp Forum' started by galaxiex, Feb 19, 2019.

  1. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    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.
    By my ear, about half the volume as any other 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 in V1, 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?

    100_2618.jpg

    100_2619.jpg
     
    Last edited: Feb 19, 2019
  2. Les Paul lover

    Les Paul lover Tele-Meister

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    I guess it's past its useful life. Probably why it ended up in your parts/spare lot?

    Some ECC83 are harder hitting (NOS RFT for example), some are smoother with slightly later break up, but a gain factor of 100 is a gain factor of 100.
     
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  3. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    Thanks for the input. :)

    I just figured this out.

    This International 12AX7 is mislabeled.

    It's really a 12AU7.

    Mystery solved! :D
     
  4. archetype

    archetype Fiend of Leo's Silver Supporter

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    Beat me to it. International Servicemaster tubes are just relabeled tubes by other makers. Do not trust the markings on these, as International didn't have a stellar track record for marking correctly.
     
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  5. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    Thanks for the confirmation. :)

    I'll be wary of these from now on. This is the only one in my stash so far.
     
  6. archetype

    archetype Fiend of Leo's Silver Supporter

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    Some of them are fine tubes. Just have a good look at the internal structure and any OEM markings to confirm what you actually have.
     
  7. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    Yep, this 12AU7 re-labelled as a 12AX7 is just fine... as a 12AU7...

    Looked pretty close, could not see the original markings.
     
  8. soulman969

    soulman969 Telefied Ad Free Member

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    Ha ha, a mislabeled 12AU7, that would do it wouldn't it.
     
  9. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    Yeah, here's a link to the vacuum tube hall of shame.

    Not all that uncommon to find wrongly relabeled tubes.

    https://vacuumtubesinc.com/index.php/hallofshame
     
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  10. soulman969

    soulman969 Telefied Ad Free Member

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  11. radiocaster

    radiocaster Friend of Leo's

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    Not necessarily, there is some tolerance. Could be 90 or 110, but not 50.
     
  12. Les Paul lover

    Les Paul lover Tele-Meister

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    Indeed, there will be a tolerance, but for valves produced within specs (that's another debate), it should be 100 ish.
     
  13. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    You probably know this already, but just in case - you said that the "quiet" tube actually was quite nice to your ears - a 5751 tube has a gain factor of about 70 iirc, so putting it between the 12ax7 and 12ay7 for gain -
     
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  14. radiocaster

    radiocaster Friend of Leo's

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    I'm sorry, but I've seen enough spec sheets to completely disagree with you.

    https://www.amplifiedparts.com/sites/default/files/associated_files/12ax7wa-wb-wc-sovtek.pdf

    https://www.amplifiedparts.com/sites/default/files/associated_files/12ax7-genalex.pdf

    [​IMG]

    Some test results in this thread:
    https://www.harmonycentral.com/forum/forum/guitar/acapella-28/1539590-/page4

    Even this 12AU7 has mu listed as 20 and 17.
    [​IMG]

    Another test:
    [​IMG]

    [​IMG]

    older ones a bit closer, but not exactly 100 either:
    [​IMG]
     
    Last edited: Feb 20, 2019
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  15. King Fan

    King Fan Friend of Leo's Ad Free Member

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    FWIW International Servicemaster (formerly IEC) sold a lot of rebranded Mullards. I'm not sure when they started, or when their name changed, so no idea if any of them are from the golden era (say first half of the '60s?). The plates / micas on that one don't look Mullard though.
     
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  16. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    I think it's a Sylvania, but that's just a guess.
     
  17. Les Paul lover

    Les Paul lover Tele-Meister

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    Like most common mortals, I have no equipment to measure valves, so I have to trust my good old getting deaf in a hurry ear.

    All of the the ECC83 I have tried, regardless of transconductance measure or not, had 0retty much the same gain. Some break up harder earlier (said RFT), some are smoother (said Tungsram), some have less high end (JJ) and so on.

    They vary from make to make.

    However, they all seem to deliver gain that is in the ball park of what we expect form ECC83 - I'll call that 100 ish, as that's what we expect from ECC83.

    I'd love to have a tester and measure all my 40 odds NOS and new production ECC83 and see what these tests translate as once the preamp valve is in an amp.

    My ears have tested ECC81, which behaved as expected..... much less gain than an ECC83.... :lol:

    I don't have tons of preamp valves, but few JJ, Harma (relabelled new sensor), Sovtek, groove tube Sovtek (one of the best new production I have), shuguang, RFT, and 60s Tungsram.

    It'd be lovely to see what's what in numbers.
     
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  18. galaxiex

    galaxiex Tele-Holic Silver Supporter

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    Handy with a multi meter and have a tube amp with standard 100K plate resistor and 1K5 cathode resistor?

    Know how to measure high voltage inside an amp safely?


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

    A good 12AX7 will self bias around 150/160V on the plate, if HT rail is 250/260V.

    Try plugging various 12A_7 tubes and see how plate voltage compares between them.

    A 12AT7 will be close to same as an X7, maybe a bit lower.

    A well worn X7 will pass less current than expected, so plate voltage should be way higher than normal.

    A 12AU7 will be waaaay lower, like 60-80V since it passes way more current than X7 in all cases.

    This doesn't give you tube gain number, but allows a quick easy way to compare tubes, weed out weak ones etc.

    Measured higher plate voltage = tube passing less current.
    Measured lower plate voltage = tube passing more current.
     
    Last edited: Feb 20, 2019
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  19. uriah1

    uriah1 Doctor of Teleocity Ad Free Member

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    Don't forget 5751..Love them.. AU is super low. Almost like AV..almost unusable
     
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