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12AU7s or 6N1Ps?

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  • 12AU7s or 6N1Ps?

    Hey All,

    I've got a guitar amp build in the planning stage. It's based on an amp I actually had once but ripped apart when I was young and foolish. The power section is a Williamson type topology that used 12AU7s for the gain/concertina and diff amp. Part of the preamp also used one 12AU7 tube.

    For my build I'm trying to use NOS tubes for reliability and long life. Hopefully sound quality will be in there somewhere.

    Since these tubes are pretty different from each other in many ways other than mu, I pretty much have to decide which one to start my massive amount of tweaking and experimenting with.

    12AU7s are pretty straight forward. Buy from the reputable guys and get the lower priced tubes. I might even be able to get really good low-use pulls down the road at radio swaps, etc. Maybe ebay, who knows? The advantage of 12AU7s is that the demand is not high like 12AX7s (relatively speaking).

    6N1Ps are cheap, but where to get them? I have been over many, many posts over at the asylum using the search function and can't quite get a handle on quality for these tubes and source. Many vendors on ebay selling these, but what are they? Junk? Any reputable dealers? Are there any *good* 6N1Ps in the area of ruggedness, reliability, sound quality, etc.?

    OK, we're talking about a guitar amp. Linearity doesn't count or matter. I'm aware of both tube specs regarding plate resistance, gm, cathode current, etc. Ruggedness, reliability, consistency, price, availability are the big factors, not necessarily in that order.

    So I'm looking for feedback and opinions on two questions.
    1. Which tube would you use and why?
    2. If you chose the 6N1P route, how would you go about getting them and from who?

    Thanks for any help.

  • #2
    Hey JWK-

    A 6N1P is very similar to a 6922. Sovtek makes them, Audio Research uses them a SRPP inverters for high linearity. For a Williamson scheme, I'd use an AX7 and a 12AU7 as follows: use the AX7 for a concertina, then feed each output to one half of the 12AU7 to drive the grids. Bias the AU's nice 'n hot!!
    You'll get the best of both worlds, good gain and plenty of drive current.
    I have drawn up a similar scheme using 12DW7's as a hybrid LTPI driving cathode followers with the 'AU' side. I thought I discovered a new PI, then found Fender tried it in a 80's Super Twin. Grrrr. But in the end I prefer high power-sensitivity power tubes like EL's etc. so don't need a PI to swing 100 plus volts! That's one reason I don't like 6L6's :-) don't get me started....

    Rock 'n roll,

    Austin Texas


    • #3
      The power section I have already tried uses a 12ua7 for both the gain/concertina and the diffy amp driver. It worked great and I really liked the tone and response, so no changes there. One thing nice about using a 12au7 for the concertina driver is that it's fairly easy to get the right voltage for the concertina's grid while still getting really good operating points for the gain stage, unlike a 12ax7. A B+ of 220 - 250 for the gain stage and 300 for the concertina is the shiznit.

      Yeah, 6l6s are a pain to drive for upper power/voltage amps, but not for the lower voltage stuff that gets you around 30 watts out a UL setup. A different sound from EL34s too, IMO. However, with the new offerings like JJ E34Ls, KT77s and EH 6CA7s it's getting harder to try and work around 6L6s.

      As far as the 12AU7 vs 6N1P thing, I'm mainly considering ruggedness and longevity. I can make both work just fine. Right now I'm unable to find a reliable source for 6N1Ps in the military "EV" or "EB" versions. The scuttlebutt is that counterfeiting (sp?) runs rampant in that part of the world. Those boxes of 50 for $62 on ebay sure gives me pause to think though...