What Are Octal Tubes Compared To "normal" Tubes? Can They Be Interchangeable?

Discussion in 'Amps and Cabs' started by Cgkindler, May 19, 2015.

  1. Cgkindler

    Cgkindler Member

    Messages:
    6,002
    Joined:
    Apr 17, 2013
    Location:
    Now Wetumpka AL
    I've read several times now about Octal tubes being used, and in one video (Fusion Bears) he had a clip of an amp with Octal tubes and it sounded awesome!!


    So, thought I'd ask a dumb question of the smart people of TGP!! :)

    Thanks all!!
     
  2. FFTT

    FFTT Member

    Messages:
    28,403
    Joined:
    Apr 6, 2007
    No, the octal preamp tube sockets use larger pins, like you see on a 6L6 or EL34
    12AX7's & EL84's have the smaller more delicate pins.
     
  3. SirGilmour

    SirGilmour Supporting Member

    Messages:
    9,893
    Joined:
    Aug 8, 2012
    Location:
    Fort Wayne Indiana
    Octals have 8 pins and other "normal" preamp tubes have 9. Octals are larger and generally have a warmer, rounder sound. They also sound thicker when overdriven IMO.
     
  4. SirGilmour

    SirGilmour Supporting Member

    Messages:
    9,893
    Joined:
    Aug 8, 2012
    Location:
    Fort Wayne Indiana
    Here are some octal preamp tubes.

    [​IMG]
     
  5. FFTT

    FFTT Member

    Messages:
    28,403
    Joined:
    Apr 6, 2007
    The octal preamp tubes are generally smoother than your typical 12AX7 and overall less aggressive.
    Some may find them perfect, some may find them too dark.
     
  6. J M Fahey

    J M Fahey Member

    Messages:
    2,162
    Joined:
    May 22, 2009
    Tubes are not dark, bright, smooth or aggressive by themselves but according to the circuits they are in, biasing, signal level, loading, etc.
    Bandwidth is flat from DC to Radio Frequencies, go figure.

    What is true is that **OLD** amps were darker, much lower gain, and intended not to distort ... the fact that they used octals is because "that's what was available way back then" , while modern amps are quite the opposite, and use Novals because "that's what is available today".

    Attributing sound properties to a tube base is like attributing sound differences to Tweed vs. Tolex .
     
    NewDr.P likes this.
  7. emdub123

    emdub123 Supporting Member

    Messages:
    1,284
    Joined:
    May 20, 2011
    I have a 1963 Ampeg Reverberocket that has four Octal tubes in the preamp. In comparison to my 1966 Vibrolux Reverb, it is less warm and less smooth. People have described the octal-era Reverberockets as sounding similar to the Fender Tweed era amps with reverb and I would say that's pretty close. In 1964, Ampeg began moving away from octal preamps, so I can't say how the '64 and later 9-pin Reverberockets sound by comparison, but my amp has a bit of raw balsiness that you don't get from BF Fenders. The other thing is that the octal preamp drives the tremolo and reverb, both of which are legendary in the early Reverberockets. To sum it up, I would say that my RR sounds very 1960s garage-rock. As an aside, it is very loud and can stay clean at band volumes.
     
  8. The Funk

    The Funk Member

    Messages:
    4,331
    Joined:
    Feb 27, 2007
    I find these assumptions to carry over to EL84 power tubes. People seem to think EL84s sound "small" or "thin" because they look small and thin. In certain circuits, they sound huge. I'm sure you could build a octal preamp circuit that sounds thin and strident, but I would assume that most schematics available today that use these tubes are based on older amps that were inherently darker or smoother or lower gain.

    The main difference between a 6l6 tube and an EL34 tube is how the circuitry around them reacts when they are biased into their correct operating range, not an inherent difference in the tubes themselves.
     
  9. FFTT

    FFTT Member

    Messages:
    28,403
    Joined:
    Apr 6, 2007
    My '64 R12R-M has the 7868 & 12AX7 configuration.

    From sweet and chimey to quite aggressive, all depending on the attack.

    I agree, way more mojo than comparable BF Fenders.



     
  10. NewDr.P

    NewDr.P Member

    Messages:
    2,309
    Joined:
    Feb 14, 2013
    Location:
    Sleeping in garbage Raleigh NC
    THANK YOU! The misconception you pointed out here is so widespread most people accept it as reality.
     
  11. FFTT

    FFTT Member

    Messages:
    28,403
    Joined:
    Apr 6, 2007
    That's kind of a matter of semantics and perception.

    Since the 60's Ampegs are cathode biased, the choice of tubes affect the sound of the amp as well as the circuit
    design affecting the sound of the tubes.
     
  12. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

    Messages:
    8,781
    Joined:
    Oct 14, 2005
    Location:
    Berkeley, CA
    I understand what you're trying to say here. The circuit ultimately determines the range of signal responses that pass through it. But I respectfully disagree with the idea that tubes have no influence, and no inherent, transferable character.

    I've carefully listened, A-B'd and made side-by-side comparisons of literally thousands of preamp tubes. There are characteristics that follow a particular manufacturer/time period. For instance, a black-plate Raytheon 12AX7A will always transmit more treble signal than RFT ECC83, and both of those will create more harmonic content than a short-plate RCA 12AX7A - it is how they operate. I think of audio tubes as filters to the pure signal - each dynamically messes up the original signal in their own characteristic manner. I have the experience to say definitively - I have heard these characteristics.

    This has been my experience, too - with 6SL7 types. There is some variation: Ken-Rad nickel-plate 6SL7GT have brighter/thinner tonal signature than RCA 5691, for instance. But compare each with 12AX7, in the same circuit, and you'll find that there is truth to the vague descriptors warm and rounder; I hear it. I made comparisons using an octal to 9-pin miniature adapter. Of course the amplification factor differs, but comparisons 6SL7 to 5751 also bear out the general characters, with the 5751 being more akin to the 12AX7. There is something about the larger octal plates that is unique.

    I happen to use a 6SL7, in an 8-9 pin adapter, in V1 of my Mark Kane built Liverpool amp. Yum!

    - Thom
     
    H. Mac likes this.
  13. SirGilmour

    SirGilmour Supporting Member

    Messages:
    9,893
    Joined:
    Aug 8, 2012
    Location:
    Fort Wayne Indiana
    I guess I shoudlve said: In my experience the octal based amps I have owned (6SL7 and 6SN7) were warmer, smoother and thicker sounding than *MOST* of my 12AX7 counterparts. I didnt mean it as an absolute but if you look at most of the octal based amps that are being built today, I think my description is accurate.
     
    Last edited: May 19, 2015
  14. FFTT

    FFTT Member

    Messages:
    28,403
    Joined:
    Apr 6, 2007
    I hear smoother, less aggressive used a lot in descriptions.

    It also makes sense, since Ampeg's founder Everett Hull detested rock and roll and considered distortion something to be avoided.
     
  15. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

    Messages:
    8,781
    Joined:
    Oct 14, 2005
    Location:
    Berkeley, CA
    Less aggressive does directly depend on the circuit, and the fact that a 6SL7, with mu of 70 (like 5751) will amplify less than a 12AX7 (mu = 100).

    - Thom
     
  16. FFTT

    FFTT Member

    Messages:
    28,403
    Joined:
    Apr 6, 2007
    I'm thinking in terms of comparing like models of vintage Ampegs with different tube configurations.
     
  17. sharpshooter

    sharpshooter Member

    Messages:
    3,300
    Joined:
    Jun 29, 2008
    The diff between the tubes is primarly due to a diff in design. To get an increase in power, you can either increase the voltage, or increase the currant. Older design amps ran at lower voltages, and required larger currant flow, (and bigger wire), along with a larger plate area to enable the required number of electrons to flow, (one watt requires one coloumb of electrons, at one volt)., so a smaller plate area tube at higher voltage can do the job of a higher area tube at lessor volts.
    Another concern is heat,, heat robs efficiency, and in the formula for heat, the value of currant is squared.
    Their is also a more exotic diff, in a tube, the electrons must migrate from within the metallic structure of the plate, to the surface of the plate, where they are "drawn off", in a tube with greater area, their is less, (for want of a better phrase), "internal pressure per unit area" to force the electrons to the surface.
    There is a school of thought, with good merit, that tubes with large areas just "coasting along", have a better fidelity than small area tubes being "pushed hard".
     
  18. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

    Messages:
    8,781
    Joined:
    Oct 14, 2005
    Location:
    Berkeley, CA
    You can obtain (or make) an adapter that converts an octal socket to 9-pin miniature, then compare. If subbing for a 6SL7, then a 5751 is more directly comparable than a 12AX7.

    - T
     
  19. FFTT

    FFTT Member

    Messages:
    28,403
    Joined:
    Apr 6, 2007
    I'm in the if it works, don't fix it camp with my Ampegs.
     
  20. emdub123

    emdub123 Supporting Member

    Messages:
    1,284
    Joined:
    May 20, 2011
    I don't know what other people's experience has been with 6SL7 and 6SN7 tubes, but I've found that MOST of the NOS tubes that I've purchased have been microphonic. When I first bought my Ampeg, it came with 5 or 6 extras which were all microphonic. I bought 4 or 5 NOS tubes, all of which were microphonic. Beware that almost all of the sellers on eBay conveniently say that they don't test for microphonics in this tube type....

    Having said that, in the '63 circuit what's horribly microphonic in the v1 slot may be just a little noisy in the v4 slot. Between the 10 or so tubes that I have, I was able to mix and match my way to a low-noise, great sounding amp. It's my understanding that this is well known, and part of the reason Fender and Ampeg went away from this tube type.

    If you do some research online, there are people who say that the new production russian 6SL7 tubes are generally not microphonic and that there are vendors who test for microphonics, so this may be an instance that you want to buy new production vs. NOS. Ironically, if you're patient, you can buy NOS tubes for about half the cost of the new ones, so it's not a clear cut choice.
     

Share This Page