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Thread: 12SN7 vs 6SN7 in a Croft pre question.

  1. #1
    Join Date: Jun 2015

    Location: London/Durham

    Posts: 1,584
    I'm Lawrence.

    Default 12SN7 vs 6SN7 in a Croft pre question.

    I just noticed the valves in my new Croft are 12SN7 not 6SN7 as I assumed. A I understand it's designed for a 6 I put the RCA 6s in in place of the GECs that came with it. Firstly I noticed more bass, but I thought there was a but less control over it. Also after an hour, the top was hot whereas it ran cool with the 12s. So I'm wondering if I'm taking a risk doing this, given I understand the Super Micro A was designed to use 6SN7s. Is it normal to run hot with pre tubes, it felt more like a power amp heat wise.

    I'm probably going to put the 12s back in, but are there risks in running 12s in a pre designed to take 6s? From the heat issue I'm assuming not, and given I prefer the sound it's a no brainer otherwise.

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  2. #2
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 43
    I'm Dan.

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    The only difference between a 12SN7 and a 6 is filament voltage. If it is working with the 12, does it take an inordinately long time to warm up? Do the 6's glow overly brightly. Next question, is this a brand new piece or bought used?
    What I'm getting at is, it shouldn't work properly with the 12's. Tube emissions will be too low,. On the other hand, if designed for the 12SN7, then you will burn the 6's very quickly.
    You need to check the filament voltage with a meter.
    Dan Santoni

  3. #3
    Join Date: Jun 2015

    Location: London/Durham

    Posts: 1,584
    I'm Lawrence.

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    Quote Originally Posted by Blackdog View Post
    The only difference between a 12SN7 and a 6 is filament voltage. If it is working with the 12, does it take an inordinately long time to warm up? Do the 6's glow overly brightly. Next question, is this a brand new piece or bought used?
    What I'm getting at is, it shouldn't work properly with the 12's. Tube emissions will be too low,. On the other hand, if designed for the 12SN7, then you will burn the 6's very quickly.
    You need to check the filament voltage with a meter.
    Bought used. The only reference I can find online says it's a 6 spec amp, although I know Glenn made all sorts of changes over the lives of his amps, especially in the old days. Which pins are the heater voltage measured over? I'll look for a spec sheet tonight.

    No it doesn't take long to "warm up" with the 12s in listening terms. It never really warms up in heat terms. And I like the sound with the 12s. So will be going back to them tonight/when the kids allow

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  4. #4
    Join Date: Apr 2012

    Location: London

    Posts: 332
    I'm james.

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    Heater is pins 7&8, measure volts with valves in place. Worth getting it right as it could stress the trafo if it's designed for 6.3V filaments and gets 12.6V filaments which will draw extra current due to running cool. It should be easy to see which are the correct valves as they should have a modest orange glow from the filaments, the same colour as the rectifier's filament.

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  5. #5
    Join Date: Jun 2015

    Location: London/Durham

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    I'm Lawrence.

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    Umm how about 86v and 90v?? Would having no input or output connected when I measure make a difference? Also when I changed the valves, I also changed a sovtek 12ax7wa in the power supply (I think) for a Mullard Ecc83.

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  6. #6
    Join Date: Jun 2015

    Location: London/Durham

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    I'm Lawrence.

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    May have found the problem, when I took the EL86 out to get to the 12ax7, one of the legs (the last one) looked bent and may not have been in. I replaced it and now I've got 1.9v and 2.2v across the 12sn7. Also the cv4048 is not glowing bright red anymore!

    Will now see if a 6sn7 gives 6v or not (assuming filament resistance is different to the 12).

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  7. #7
    Join Date: Jun 2015

    Location: London/Durham

    Posts: 1,584
    I'm Lawrence.

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    2.8/2.9v across the 6sn7 filaments not ideal but at least not 80v! Good job I measured the voltages or there could have been a nasty incident. Will try it out in a bit now I've identified and sorted the heat issue.

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  8. #8
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 43
    I'm Dan.

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    None of that makes any sense. Pins 7 and 8 are the filaments. Pin 8 is the first pin counter clockwise from the alignment key if looking from the top. Measure between pin 7 and 8 not each to ground. It will be either close to 6 volts or 12 volts. 2.9 volts won't even turn on a 12SN7. Also, unless the filaments are in series, it shouldn't matter if the tubes are in or not (at least not a lot). If you are getting a big difference, between tubes in and tubes out, then something is very wrong.
    FYI - If your measuring from the wiring side those pins are clockwise from the key.
    Dan Santoni

  9. #9
    Join Date: Jun 2015

    Location: London/Durham

    Posts: 1,584
    I'm Lawrence.

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    The issue was I think a pin of the el86 regulator wasn't in the hole. I measured between the two pins of the 6sn7 after the notch in the base going round clockwise looking from underneath (they are mounted horizontally). Maybe I need the two before ie counter clockwise from the notch looking from below..

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  10. #10
    Join Date: Jun 2015

    Location: London/Durham

    Posts: 1,584
    I'm Lawrence.

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    PS I think your 2 statements about the pin outs are contradictory.
    Quote Originally Posted by Blackdog View Post
    None of that makes any sense. Pins 7 and 8 are the filaments. Pin 8 is the first pin counter clockwise from the alignment key if looking from the top. Measure between pin 7 and 8 not each to ground. It will be either close to 6 volts or 12 volts. 2.9 volts won't even turn on a 12SN7. Also, unless the filaments are in series, it shouldn't matter if the tubes are in or not (at least not a lot). If you are getting a big difference, between tubes in and tubes out, then something is very wrong.
    FYI - If your measuring from the wiring side those pins are clockwise from the key.
    Sent from my BLN-L21 using Tapatalk

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