Tube qualities for PI...?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tedjac, Feb 10, 2006.


  1. tedjac

    tedjac Guest

    What tube qualities are most important for the Phase Inverter (invertor??) location in an amp? I've heard that matching internals is important. What about tube output? Would a 5751 work well there or is it's lower output undesirable? I have an assortment of preamp tubes including RCA 5751 blackplate (3-mica), Telefunken smooth plate, Amperex orange globe label, GE 6681, Mulllard "Webcor" (both a 7025 and a 12AX7), Tesla ECC803s gold pin grid plate, RCA 7025 greyplate. Is there any concnsus which of these tubes would be the best PI tube. My guess is the Tele smooth plate should be in V1... but who knows? The amp is a Fuchs ODS30SLX-HT. I think Andy uses JJs stock. Just trying to learn as much as I can about this stuff.

    Ted
     
  2. scottl

    scottl Member

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    Ted,

    Regular 12AX7 in a Fuchs. The PI section is tuned for a certain response around the 12AX7. Matched is meaningless as the circuit has an adjustment trim and it is intrinsically imbalanced as is.

    Note what tube Andy had. Then try replacing. Andy's tube will be scope set so you can always put it back.

    Scott
     
  3. tedjac

    tedjac Guest

    Thanks Scott... again! Thanks for being such a valuable resource, I really appreciate it!

    Ted
     
  4. Chiba

    Chiba Gold Supporting Member

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    High output, low noise - that's what you're looking for. I'm not as sure as Scott is that matched is 'meaningless' - I've got a 'matched' tube in my PI spot right now and it sounds a lot different (IMO better) than the 'normal' AX7 I had in there before (and yes, the previous tube was high quality).

    --chiba
     
  5. tedjac

    tedjac Guest

    I've got a "matched" RCA 7025 with nice high output test numbers that I think I'll try when the amp arrives (after I've played it a bit, of course!).
     
  6. scottl

    scottl Member

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    Chiba, PI balance is important. However, the Fuchs as an AC balance trimmer to set the balance. So, if the amp was adjusted for an imbalanced tube, the matched replacement will be out of balance. I agree on proper PI balance being noticeable. Just that it is totally not important on an amp that has an adjustment for it. Having said that, I prefer using a matched tube, then scope setting it. That way, all replacements in the future, if matched, will be in adjustment.

     
  7. tedjac

    tedjac Guest

    Hi again, Scott...

    So... does this mean that when my amp arrives that I'll never really know if the tube that was put in stock was balanced or just adjusted to balance? This will make putting new tubes in difficult because I'll never know how the adjustment was set in the first place. I don't have a scope or the expertise to set that adjustment with a new tube installed. Any suggestions... besides just forget it and leave the stock tube in?

    Ted
     
  8. fullerplast

    fullerplast Senior Member

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    I'm not Scott :p but if Scott were here he might say:

    "Ask Andy to use a balanced tube for the PI before he adjusts AC balance.", "Send him one if he doesn't have one."

    That's what he might say.....;)
     
  9. tedjac

    tedjac Guest

    Thanks... and I just might do that very thing... I have a nice RCA 7025 (early 70's) that just might be headed to New Jersey. As much as Andy seems a bit annoyed with this whole NOS tube thing... I want the best!
     
  10. Chiba

    Chiba Gold Supporting Member

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    Gotcha Scott - I had no idea the Fuchs had the capability to balance out the triodes of the PI. That's a neat feature!

    --chiba
     

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