Drifting bias

hywelghywelg Frets: 4302
edited December 2018 in Amps
As other Quad jammers will recall, my 1987 clone came over all poorly mid way through the afternoon, so we shut it down. Took it to our jam last Monday, got it out of the combo and powered it up. Sounded fine so I thought I'd check the bias, 84mA. SHIT.... turned the lights off but they el34 weren't redplating . So I adjusted it back down to 38mA and continued playing it. Was fine all night.

Now this has London Power powerscaling,, but other than this being faulty is there any other reason why the has would drift by such an amount?
0reaction image LOL 0reaction image Wow! 0reaction image Wisdom

Comments

  • DJH83004DJH83004 Frets: 196
    Having the London Power scaling kit on a fixed bias amp like your 1987 clone, does complicate things in terms of fault finding the bias drift, does it have any of the bias add on modules? (TBS or RBS), the tracking module is as its name suggests tracks the B+ volts and should adjust the bias volts accordingly, so that may be suspect.     
    0reaction image LOL 0reaction image Wow! 0reaction image Wisdom
  • Have you got grid stoppers on the EL34’s? Could be a thermal runaway thing. Or maybe one of them is breaking down, think I’ve seen this with valves getting gassy or heater-cathode issues. 
    0reaction image LOL 0reaction image Wow! 0reaction image Wisdom
  • ICBMICBM Frets: 71960
    hywelg said:

    Now this has London Power powerscaling,, but other than this being faulty is there any other reason why the has would drift by such an amount?
    From the title of the thread I thought it was going to be a DSL/TSL...

    :)

    If the bias has drifted on *both* valves, it probably points to the power scaling circuit as the culprit. If it's just on one, faulty valve or leaking coupling cap.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

    0reaction image LOL 0reaction image Wow! 0reaction image Wisdom
  • martinwmartinw Frets: 2149
    tFB Trader
    My first question would be: how has the raw bias supply been achieved? PS bias supplies need to be low impedance so can have issues if they are derived from an HT winding via a high value dropper resistor, typical on Marshalls.
    Other than that, the bias voltage is entirely controlled by the bias tracking system and pot.
    0reaction image LOL 0reaction image Wow! 0reaction image Wisdom
  • hywelghywelg Frets: 4302
    The bias definitely does follow the ht supply. Since it was both valves that were well out of whack I think that points to a fault in the LP PS unit. IIRC the supply is derived from a dedicated PT winding (Mercury PT) will check it all out when I next get to the studio. 
    0reaction image LOL 0reaction image Wow! 0reaction image Wisdom
Sign In or Register to comment.