7025 Vs. 12AY7 In Princeton Reverb V1

Discussion in 'Amp Central Station' started by Radspin, Jan 2, 2018.

  1. Radspin

    Radspin Friend of Leo's

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    I like to experiment with tubes in my 1965 (original, not re-issue) Princeton Reverb. I've recently been trying a 7025 and 12AY7 in the V1 position, to experiment with getting a greater range on the volume control at lower volumes using the latter. Both tubes I've tried are prime examples of NOS RCA tubes.

    According to what little I've found online, the 12AY7 is a "warmer" sounding tube...but in my case I've found exactly the opposite...the 7025 has noticeably more bass, to the point where the amp even sounds bass-shy with the 12AY7. It's no surprise to me that the 7025 sounds "bigger" and bolder, but I am surprised at the bass attenuation with the RCA 12AY7. The 7025 also drives the reverb better, but is noisier when you turn the reverb up.

    I'm curious to hear what other peoples' experiences are. Also, what about trying a lower-gain tube in V1 other than a 12AY7? My whole reason for doing these experiments is to get more usable range out of the volume control at lower volumes.
     
  2. Mayas caster

    Mayas caster Tele-Holic

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    Well, I tried a 12ax7 in v1 of my '56 Deluxe and it didn't sound good and clear as with a 12ay7. It depends of the amp and the sound you are chasing I guess.
     
  3. Dacious

    Dacious Poster Extraordinaire

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    That's because the Princeton is set up in it's complement of capacitors and resistors, grid and plate bias to match the rest of the circuit to a 12AX7. It's probably dropping all frequencies slightly because it's amplifying less, which comes off most noticeable in bass.

    In the Tweed Deluxe, the amp is set up to work with the 12AY7, so it sounds smoother and doesn't apparently lose anything with slightly less gain. With the 12AX7 the one I built was a bit spiky and prone to early fartiness.

    While the 12AX7 family is somewhat pin compatible and you won't explode anything, some members are more linear, or not even regarding their amplification factor. A 6072 (12AY7) sounded like ass in a bandmate's Hotrrod Deluxe, whereas a 12AT7 smoothed it right out, reverb too! Usually 12AT7s are not what you'd choose.

    It's interesting that the Princeton is the ultimate version of the LTPI splitter from the Deluxe era, and the preamp is also derived. It loses a lot of gain through the reverb circuit.
     
    Last edited: Jan 2, 2018
  4. Radspin

    Radspin Friend of Leo's

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    Dacious, thanks for the info. My tech said it would be perfectly safe to use the 12AY7 and in fact I've been gigging with it like that for over a year, but I tried the amp with a different speaker recently and noticed the distinct lack of bass, which prompted me to put the 7025 back in and it made a huge difference.
     
  5. Dacious

    Dacious Poster Extraordinaire

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    Try a 5751 - the 12AY7 is a big drop in gain factor, like 40%, a 5751 is more like 90%. A 12AX7 is 100%>
     
    Last edited: Jan 2, 2018
  6. Radspin

    Radspin Friend of Leo's

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    Dacious, thanks again...I know I have a few 5751s around and they're good NOS tubes. Didn't think of trying a 5751.

    Update: I tried an RCA 5751 (don't know the particulars but the tube had that later 1970s red logo) and it didn't seem to make much difference in gain or tone, but the RCA 7025 still sounded better so I'm going with that.

    I can see--make that hear--why those old RCA 7025 tubes are so well thought of.

    Caveat--I haven't tried the amp at rehearsal or gig volume yet, where I suspect many of these tube-swapping subtleties will be overwhelmed once I'm playing in a band context.
     
    Last edited: Jan 3, 2018
  7. strat1

    strat1 TDPRI Member

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    It's easy to believe the 7025 tube sounds a bit better because it is a 12AX7 tube only built to more rugged specs which is what the amp was designed for. Fender used them quite a bit in blackface and silver face amps and they are regarded highly in guitar amps for their quality & tone. It also has a gain factor of 100 as does the 12AX7. Again, same type of tube.

    The 5751 has a gain factor of 70 which would tame the amp just a bit. I use these in aggressive sounding amps to tame it a bit. Me, personally I'd leave the 7025 in there. Here's a tube gain guide if you haven't seen it as of yet.

    https://www.kcanostubes.com/content/preamp-tube-gain-and-compatibility-guide
     
    Last edited: Jan 4, 2018
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