jtm45 clone bias WAY high..

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pelz, Sep 2, 2006.

  1. pelz

    pelz Member

    Messages:
    7
    Joined:
    Mar 19, 2006
    Location:
    South Jersey
    Just picked up a jtm45 clone and amp is running very hot. Also breaks up very early on the volume pot, at 11 o'clock. I checked the bias at the external bias terminals and they read 124mv and 119mv with the KT66's in it. I changed tubes to 6l6 and readings were 99 and 95mv with external bias pot all the way down. Any ideas what could cause this? Could a bad rectifier tube (5ar4) cause this?
     
  2. pelz

    pelz Member

    Messages:
    7
    Joined:
    Mar 19, 2006
    Location:
    South Jersey
    Thank you, off to the tech it goes.
     
  3. JacksonAmpworks

    JacksonAmpworks Member

    Messages:
    898
    Joined:
    Jan 29, 2003
    Location:
    Fort Worth, TX
    If the plate voltage is in the normal range for a JTM45 (around 450ish) then yes, those tubes are nearing meltdown. Assuming a plate voltage of around 450VDC, biasing the tubes at 70% of max plate disipation (30W) should have each tube idling at around 46mA each.
     
  4. hipfan

    hipfan Member

    Messages:
    2,302
    Joined:
    Nov 15, 2002
    Location:
    Evanston, IL
    Sounds to me like the amp's bias circuit probably was set up for EL34's. OTM's suggestion should be right on.
     
  5. pelz

    pelz Member

    Messages:
    7
    Joined:
    Mar 19, 2006
    Location:
    South Jersey
    Just got amp back from tech,it had a fried 68k resistor.
    OTM,you were right.
    Jacksonamps,you should be designing other amp makers websites.Your site is the most detailed I've seen.
     

Share This Page