Are there "bias test point" kits?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by exodus, Oct 19, 2004.


  1. exodus

    exodus Member

    Messages:
    1,812
    Joined:
    Sep 7, 2003
    Location:
    Sweet Home Chicago
    Are there kits out there that add on a bias test point ala Aiken amps, Tone King, etc. Its a feature that would really help a guy who has no idea what not to touch inside an amp, and don't like being dependent on amp techs.
     
  2. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    Not any "kits" that I know of, but all you need is a couple of one ohm, one percent resistors and a pair of test terminals. Normally, pin 8 (cathode) of the power tubes is connected to ground. You simply open that connection up and insert the resistor. The test point is now connected to pin 8. You measure the voltage across the new resistor as millivolts and due to the one ohm resistor and ohms law (I=V/R) that will correspond to the current in milliamps. The hardest part of this mod is figuring out where to put the test points (a clever solution for old Fender amps is to put a stereo jack in the old ground switch, one terminal for each tube).

    After saying all that, I'd say the easier solution is to just get a Weber bias probe which does this same thing inside an adaptor. You don't need to open the amp or make any mods.
     
  3. BUDDADRIVE

    BUDDADRIVE Guest

    >is to just get a Weber bias probe which does

    ditto. I have my tech get the B+ of my amps, then use Ohm's law and a bias rite or allesandro meter. Works for me.
     
  4. Slick51

    Slick51 Colonel Curmudgeon Silver Supporting Member

    Messages:
    505
    Joined:
    Mar 23, 2002
    Location:
    Northwest Corner of NC, 5 mi from MerleFest
    or get the Weber Bias Rite with the Voltage option...you can get the B+ from the bias probe with a switch...i.e. you can get it all direct from the Bias Rite and adjust per Ohms law.

    Slick51
     
  5. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,063
    Joined:
    Sep 23, 2004
    Location:
    Tejas

    Oh come on you guys, where is your sense of advanture.

    Cross over notch or at least
    do pin 3 to CT can measure
    mAmps.

    : )
     
  6. aleclee

    aleclee TGP Tech Wrangler Staff Member

    Messages:
    10,981
    Joined:
    Jan 4, 2002
    Location:
    5868 ft above sea level
    That's what I do. The one caveat is that you still generally have to "get inside" to access the bias trim pot to adjust the idle current.
     
  7. Slick51

    Slick51 Colonel Curmudgeon Silver Supporting Member

    Messages:
    505
    Joined:
    Mar 23, 2002
    Location:
    Northwest Corner of NC, 5 mi from MerleFest
    I do those methods too. I thought the point of the post was to help him do it in the easiest manner....

    Slick51
     
  8. exodus

    exodus Member

    Messages:
    1,812
    Joined:
    Sep 7, 2003
    Location:
    Sweet Home Chicago
    Exactly. I don't know jack about amps, electronics, tubes, etc. I don't have any of the necessary equipment (ie scopes, if using the "shunt" method). I also don't like being dependant on an amp tech every time I want a simple tube swap or need to swap at a gig or practice. I'm sure its not that hard, and I'm smart enough to learn it, but the thought of not knowing what to touch gets me a bit jumpy.

    I'd like to just have my tube amp modded and have test jacks installed with a thubscrew bias pot next to it- ala Aiken, or individual bias jacks and pots ala THD. I'm sure any competent tech could do it.
     
  9. Lonely Raven

    Lonely Raven Member

    Messages:
    1,003
    Joined:
    Jun 29, 2004
    Location:
    Bolingbrook, IL
    Sounds like an external bias pot (screw type like you
    mentioned) and a Weber Bias Rite with the VI mod is probably
    your best bet. That way you can always check the bias
    on most amps out there, and easily adjust the one you have.
     
  10. Slick51

    Slick51 Colonel Curmudgeon Silver Supporting Member

    Messages:
    505
    Joined:
    Mar 23, 2002
    Location:
    Northwest Corner of NC, 5 mi from MerleFest
    I'll send you an Excel spreadsheet I wrote to calculate all the currents, from 50% to 75% in 5% increments and the max just for the heck of it. You plug in the measured plate voltage and the max plate dissipation for the tube type used (from a chart also in the spreadsheet) and the sheet does the rest. It's in Excel 2000. Let me know if you want it.

    Slick51

    PS if anyone else wants it email me.
     
  11. Stringrazor

    Stringrazor Member

    Messages:
    1,531
    Joined:
    Jul 13, 2004
    Location:
    Southern California
    Slick, the "email" button on this forum never works for me "you don't have permission blah blah....". Would you please email a copy to stringrazor at yahoo dot com? Thanks!
     
  12. Slick51

    Slick51 Colonel Curmudgeon Silver Supporting Member

    Messages:
    505
    Joined:
    Mar 23, 2002
    Location:
    Northwest Corner of NC, 5 mi from MerleFest
  13. Stringrazor

    Stringrazor Member

    Messages:
    1,531
    Joined:
    Jul 13, 2004
    Location:
    Southern California
    I got my Weber Bias Rite Head Kit today. At $16 with assembly instructions, I found it a good deal. Assembly was a bit of a bear due to the many solder joints (16+) that have to be made in such a tight space (i.e. inside an octal base). I'm not an expert but I managed to build it and it works! :dude I can recommended it to anyone with decent soldering skills and a fair amount of patience.
     

Share This Page