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The Dynaco Tube Audio Forum

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    Saving hours on phono preamp tubes?

    Tom
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    Tom


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    • Post n°1

    Saving hours on phono preamp tubes? Empty Saving hours on phono preamp tubes?

    Post by Tom Tue Apr 19, 2011 9:38 am

    OK, another noob question.

    I have the SP-8 preamp and phono and line stage are on the same board. That means the phono tubes are "on" when not in use. Is there an advantage to powering down the filaments on the phono tubes (expensive 12AX7s) when not in use to extend their life? I don't see any reason this couldn't be accomplished with a separate power switch for the filaments.

    Any advice appreciated!

    Tom
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    Bob Latino
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    • Post n°2

    Saving hours on phono preamp tubes? Empty Re: Saving hours on phono preamp tubes?

    Post by Bob Latino Tue Apr 19, 2011 5:24 pm

    Hi Tom,

    If you pull the two 12AX7's on the phono part of the board then the filament voltage will rise slightly on the tubes on the line stage area of the board. Whether this will hurt the tubes on the line stage part or shorten their life, I can't say? If you are never going to use the phono part of the preamp, what I would do is put some cheap 12AX7 tubes in the phono stage area and save the good 12AX7's for another amp/preamp etc. This would keep the filament voltage about right for the line stage tubes.

    Bob
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    Tom
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    Post by Tom Tue Apr 19, 2011 9:23 pm

    Thanks Bob,

    The SP-8 passed the smoke test, phase I, (no tubes) tonight. So, I guess I'll have to get some cheap(?) 12AX7's for phase II.

    Sorry, I'm gonna say it. This is fun!

    Tom
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    Roy Mottram
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    Post by Roy Mottram Wed Apr 20, 2011 2:04 pm

    the SP8 preamp has regulated filaments and HV, so you can actually leave the phono tubes out,
    but you can NOT do this on the older SP6 version which has no regulation, or the voltages on the other tubes will go way too high
    Bob's idea was good, just put some cheap old tubes in unless you are actually using the phono section.
    Just rememember it will take them 3-4 minutes to cool off if you decide to switch tubes.
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    Tom
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    Post by Tom Wed Apr 20, 2011 6:37 pm

    Thanks! I went and got some Pentas today. Only $12 each so not too bad.
    Unfortunately, I didn't get to smoke test phase II but will put up another thread about that.
    Eventually, I'll get some good AX7s, but after I get a decent turntable.
    It's not that I'm cheap, I'm Poor, it's Different!

    Smile Tom
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    j4570
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    Post by j4570 Thu Apr 21, 2011 6:16 pm

    I see two solutions (the first making me think this is what I should do since I don't use the the phono much)-other than what Roy said since it's regulated, shouldn't matter:

    1) Build phono preamp in seperate enclosure with switch on front that would control power out to the turntable, only using it when necessary. Or use a larger enclosure with a seperate switch for the phono section.

    2) Wire up a second set of cheap tubes and a switch to swap between the tube sets. Of course this is overly complicated in my opinion, and number 1 is really better.

    JW
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    Tom
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    Post by Tom Fri Apr 22, 2011 7:55 am

    JW - I was thinking about a dpdt switch that would put a little load on the filament current in place of the actual tube filaments. Off position, load in line, on position phono filaments in line.

    Future project though.

    Tom
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