6SL7s compared to 12AX7s

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Leonc, Sep 6, 2006.

  1. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    I'm curious as to how 6SL7s vary from 12AX7s, not so much w/rt their specs but with respect to how they affect an amp's tone and gain structure. I'm interested in hearing about your experience with them both in pre-amp and phase-inverter applications. They're basically dual-triodes with a shared heater right...similar to 12AX7s? Do they not put out as much gain?
     
  2. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    So, OTM, as someone who doesn't understand the tonal implication of 6SL7s only being able to take 6.3V on the filaments; is it that they won't produce as much again as the 12AX7?

    Am I also picking up that if 12AX7s can take 6.3 or 12.6, there are some circuits in which they're designed for only 6.3 and others for 12.6? As more-or-less a layman, how do I distinguish which circuits puts 12.6V on the 12AX7s' filaments and which put only 6.3?

    Also, in those that only use 6.3...do you think the 6SL7 offers anything tonally unique or interesting?
     
  3. bobgoblin

    bobgoblin Supporting Member

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    I can't offer any technical response, but since my main gigging amp for nearly 2 years was equipped w/6sl7's, & the amp I frequently use now is a 12a*7-type, I will say this:

    The 6sl7's did not put out nearly the same amount of gain anywhere on the dial. I could wind the Ampeg up nearly full tilt & still not have any real grind (archtop, no pedals, phat cat pups). There was also an insane amount of bass that was difficult to shape (lack of t/m/b controls on the Ampeg). For a jazz-type sound, I recommend a better amp, or tube-type.

    The 12a*7/7025's in the Fender (using the clean channel only, to compare) are a lot clearer, but thinner-sounding, too. I can overdrive this channel w/any of the guitars I've plugged into it: stock Strat, LP, Guild...That would seem to me like the 12a*7-type is a bit gainier/livelier.

    Now, I from what I understand the architecture between the '60 Ampeg & '83 Fender are completely different, but, both are PCB-free & I thought
    the comparison might help.
     
  4. hasserl

    hasserl Member

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    With 12AX7's the 12v heater rating is for series stringing, not parallel. Most all the familiar amps we talk about around here, save the Peavey Classic series, are parallel strung, not series. Many series strung amps are transformerless designs. For the purposes of comparing the tonal qualities of the tubes, ignore the differences in heater ratings.

    Also, the two tubes are not indentical electrically; the 12AX7 has an amplification factor of 100, the 6SL7 has an amplification factor of 70. There are differences in plate resistance also, with 250v on the plates the 12AX7 has plate resistance of aprx 62500 ohms, the 6SL7 at the same voltage has plate resistance of 44000 ohms.

    I'm not going to try to argue anything with Old Man Tele when it comes to math and formulas and so forth. Hell, I'm lucky if I can count my change at McDonalds. That guy has a formula for anything and everything having to do with amp circuits. I'm lost on 90% of his posts. But many people that have experience with similiar preamps using the two different types of tubes claim there is a difference between them, though the differences are hard to articulate. Terms like Bigger and smoother, warmer, etc. I have one amp with a 6SL7 in the PI slot, how it would differ if that were a 12AX7? I don't know. It's an old RCA amp, and that's what they used so that's what I kept in there. I just got done working on an old Ampeg with a couple fo 6SL7's in the preamp, that is a very righteous sounding amp; big, smooth and warm seems to describe it well. Is that due to the octal preamp tubes? I don't know. It also uses a 12AX7 in the preamp, so who knows.

    One way to tell would be to use the 8 to 9 pin adapters that are available out there to stick a 12AX7 in place of the 6SL7's in an amp and record the amp with both to hear what the difference is.
     
  5. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    OMT and bobgoblin - thanks for your replies. Which Ampeg are you referring to bobgoblin? Was it an old reverberocket or mercury? Do you know if the tubes in it were good?

    Interesting hasserl. I had a feeling that you only got up to 12.6V if you were combining the triodes. So...your contention is that in most circuits, in which the pre-amp tubes triodes aren't combined, there are tonal differences that appear to contribute to warmer/smoother sound? Are you able to get any gain from your RCA amp?
     
  6. conundrum

    conundrum Member

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    As OTM said; Electrically, they are almost identical to a 12AT7. However, tonally, they are fatter, their resonant peak is lower in the midrange, and the distortion characteristic is very different. If distortion was like woodgrain, then 6SL7's would have a very straight grain, whereas 12AX7's are much messier. Everything is very ordered and clean (I know it's an oxymoron, but if you hear one you'll understand):D
     
  7. hasserl

    hasserl Member

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    Oh God yes! The thing is a gain monster. Although I stuck with the tubes RCA used in the riginal amp, I didn't stick with the original circuits. The amp was gutted and I started all over, using a Clubman as inspiration. That circuit, with a slight change to the plate load resistor value of the parallel 1st stage, is capable of a lot of gain. The stock plate load resistor sets it very soft, and is why Clubman amps are known for high headroom. Change that resistor to say 47k and it is a very different situation. But I digress.

    The Ampeg I recently worked on is a R15R Super Reverb, and it too has plenty of gain. I did have to do a slight reqwiring of the circuit to rid the amp of an excessively loud and obnoxius hum. After which I had to add a voltage divider between stages to cut some of the gain as it was too much IMO.
     
  8. bobgoblin

    bobgoblin Supporting Member

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    Hey, LeonC, its an old Ampeg Mercury. Fantastic amp, still own it, love playing it, probably taking it out tomorrow for the Thurs. night gig.

    The original tubes were still strong (tested fine), but I've since replaced them w/NOS RCA's (6sl7) & JJ's (6v6). I'm on my 2nd set of power tubes since I bought the amp (the last set was faulty, or sustained some kind of damage, as one of the tubes literally disintegrated into dust during a gig last year).
     

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