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Preamp tube type gain question

Discussion in 'Amps and Cabs [BG]' started by tombowlus, Jun 17, 2005.


  1. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    Can anyone help me to clarify the relative gain values for the 12**7 type tubes? I know that various iterations have X% less gain than the 12AX7, but it'd be nice to have a reference, or a scale, showing the relative gain values for 12AX7, 7025 (same 12AX7, right?), 5751, 12AU7, 12AT7, etc.

    I've seen varying figures from varying sources, and I'd like to straighten this out.

    Thanks, Tom.
     
  2. Here's a rough for a couple of types:

    12AX7 (7025) - 100
    12AT7 (5751) - 70
    12AU7 - somewhere from 20-40

    Hope it helps,
    Jay
     
  3. Chef

    Chef Moderator Staff Member Supporting Member

    May 23, 2004
    Columbia MO
    Staff Reviewer; Bass Gear Magazine
    What I have writtne down on a 12ax7 box from my amp tech parallels what Jay was saying.
    But, replacing 12ax7's with 12au7's in my 1965 fender bass did not result in a 60-80% loss in volume. Instead of the amp grinding and becoming useless (to me, I'm a relative clean-ophile) at 2.5 on the volume knob, I can now run it a 6 or 8, and it souds pretty dang good. Before it sounded like the guitar intro to "Iron Man" by Black Sabbath;)

    Now it's just nice...whatever I lost in gain, I got back in use-ability.
     
  4. Passinwind

    Passinwind I Know Nothing Supporting Member

    The other posts mirror the standard data sheets, but those don't tell the whole story. If you have a circuit that's at or close to unity gain, it may not matter that much which variant you use. If you have one that requires high current supply, like say a power amp stage, the 12AU7 may have more effective power gain, even though the voltage gain is spec'd at ~1/5 that of a 12AX7. Then there are the issues that Chef brought up, which are quite valid IMO.

    Incidentally, there are some circuits where the various types are not good plug 'n play replacements for each other. The Fender reverb drive stage is one of those in some cases, at least in my experience; it can eat 12AX7s. I've read about swapped-in 12AU7s taking out low wattage plate resistors occasionally in some preamps, although I've never seen this happen in anything I've worked on myself, and I usually swap with impunity. There are associated resistor changes to optimise for the tube swaps though, and I usually do those for long term modifications in my own equipment. I'm a huge fan of 12AU7s, FWIW.
     
  5. My first post was a bit sterile, but I was just trying to answer the gain question. I, too, prefer a clean signal, so both the 5751's (AT7's) and the AU7's are great tubes for me.

    Jay
     
  6. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    Great help, guys! And Jay, your first post is just what I was looking for. I knew the AT7's and 5751's were about 70%, but I did not know the rough value of the 12AU7's.

    Thanks again!

    Tom.
     
  7. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    I have a general idea of the gain values, but most of this is coming from internet message postings. Is there any "definative" source for the gain values of the various preamp tube types? Or would that be Psycho Bass Guy? ;) Man, he has been a huge help in the past on tube issues.

    Also, is there any source for sorting out the different "sub-types", like 12AX7A, 12AX7LPS, etc?

    Thanks, Tom.
     
  8. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    Here's the best source that I have found so far, but it doesn't seem to have any kind of compiled chart or table.

    Dr. Tube data sheets
     
  9. Go here and search for whatever values you need. Sometimes they don't have ALL the info for all tube types, but it's the most comprehensive tube reference site I've found that you don't have to wade through pages and pages just to get to an audio tube.
     
  10. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine