BF Vibrolux - biases too low; can't bias it any higher

Discussion in 'Amps and Cabs' started by Scott Whigham, Jan 26, 2012.

  1. Scott Whigham

    Scott Whigham Member

    Messages:
    3,536
    Joined:
    Mar 4, 2010
    Location:
    Dallas, TX
    So I bought a Bias Rite recently along with a new set of NOS RCA 6L6s for my 67 Vibrolux. When I hook the Bias Rite up, I get 427 for plate voltage but my tubes are running at 33/35. From talking with friends and referencing online docs, it appears that, at that plate voltage, I should be biasing it so that they are in the high 50s or low 60s.

    So I turned the bias pot all the way up - and it moved the tubes from 33/35 to 34/36. Ugh - it was barely a half turn and then I'd turned it all the way up.

    I then swapped tubes to a new set of Ruby 6L6s - they measured roughly the same.

    I then swapped one of the tubes to another old RCA that I have - it measured at 35.

    By this point, I know that it isn't my tubes.

    What would you suggest? Someone suggested I change out to a new bias pot. I can do that - I just don't want to do that unless I need to. I could take it to a tech but I'd hate to spend $75-$150 if it's something as simple as replacing a resistor or replacing a single pot.

    Stuff I think may be important:

    • The amp sounds kinda cold biased
    • As far as I can tell, all the caps/guts are original with the only exception being the 3-prong power cable.
    • All other tubes are original to the amp (or are NOS possibly)
    Thank you!
     
  2. smolder

    smolder Gold Supporting Member

    Messages:
    13,847
    Joined:
    Jun 16, 2008
    Location:
    Central Rocky Mountains
    The stock bias trim pot in most fenders is 10k plus a resistor. You can swap out the resistor to shift the range... Or you could replace the pot with a wider range pot (and swap out the resistor) for a wider range of adjustment. Some tube vendors offer power tubes in ranges that will help to stay within the range available.
     
  3. Guinness Lad

    Guinness Lad Silver Supporting Member

    Messages:
    15,343
    Joined:
    Nov 11, 2004
    Location:
    On top a mountain of Chocolate Chips
    You don't want it 50ma per tube. Mine runs at 420V and 36ma per tube and it sounds perfect. 50ma and above is totally out of control, you might as well get a Matchless. Considering your amp isn't designed for it, you will either kill tubes and/or components in no time. Your friends are mistaken.
     
  4. Sean French

    Sean French Supporting Member

    Messages:
    13,626
    Joined:
    Jan 7, 2007
    Location:
    Texas
    For 6L6 amps with 427 plate volts = 50% 35ma, 60% 42ma and 70% 49ma.
     
  5. chervokas

    chervokas Member

    Messages:
    6,864
    Joined:
    Feb 16, 2008
    FWIW, I think at that plate voltage 70% heat dissipation calculates out to something in the mid to upper 40s, not in the 50s or anywhere near the 60s mA. I don't know who is saying bias idle should be 60 mA or more in a BF Vibrolux but I think that's not a great idea. At best you'll burn through tubes quickly that way, at worst you'll red plate 'em.

    Personally I bias my '66 Bandmaster, which has a pretty similar output stage but even higher plate voltage, at around 33 - 35 mA idle current, which is probably in the range of 50% of max heat dissipation. When I look at how much current the tubes draw when I start playing hard I think 32-35 mA works ideally for idle current and all the big clean sound and headroom of BF Fender is still there.

    That said, if your bias adjustment circuit isn't giving you the full range of adjustment you should tweak it like suggested above.
     
  6. smolder

    smolder Gold Supporting Member

    Messages:
    13,847
    Joined:
    Jun 16, 2008
    Location:
    Central Rocky Mountains
    Like Sean said. You have to know how much voltage is on the plates, otherwise your just picking a point at random. Both of those measurements are essential to the equation.

    http://www.webervst.com/tubes1/calcbias.htm
     
  7. Blue Strat

    Blue Strat Member

    Messages:
    30,036
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)

    ...and anything over 60% is a bad idea.
     
  8. Scott Whigham

    Scott Whigham Member

    Messages:
    3,536
    Joined:
    Mar 4, 2010
    Location:
    Dallas, TX
    Man, I'm so glad I made this thread 'cuz I was obviously WAY off on my stinkin' thinkin'. Thank you to all.

    So now - it's just a matter of getting that bias up to around 40ma or so. It definitely sounds a bit lifeless/cold right now compared to how it sounded with the previous set of tubes (that I did not get to measure).

    Would you guys think I should replace the pot, the resistor, or both? Or should I just take it to a tech? I'm not averse to any option!
     
  9. flatpole

    flatpole Member

    Messages:
    51
    Joined:
    Nov 10, 2011
    What type of tubes were in there before? 6l6GC or a lower rated 6l6 like a 5881, 6l6gb, etc..
     
  10. Scott Whigham

    Scott Whigham Member

    Messages:
    3,536
    Joined:
    Mar 4, 2010
    Location:
    Dallas, TX
    I've always had 6l6GCs in this one.
     
  11. slider313

    slider313 Silver Supporting Member

    Messages:
    7,480
    Joined:
    Jul 28, 2005
    Location:
    NC
    If the caps are original, then all the tube changing won't help. Do the cap job and take it from there. 35ma is fine for a set of RCA 6L6's.

    The pre-amp tubes may be original, but that doesn't mean they're good. Weak pre-amp tubes can make an amp sound thin also.
     
  12. Sean French

    Sean French Supporting Member

    Messages:
    13,626
    Joined:
    Jan 7, 2007
    Location:
    Texas

    +1

    I've found old Fender' sound best at or under 60% dissipation but, over 50%.
     
  13. Scott Whigham

    Scott Whigham Member

    Messages:
    3,536
    Joined:
    Mar 4, 2010
    Location:
    Dallas, TX
    I had a previous set of tubes in it the week before and it was the greatest sounding BF Fender I've ever heard. I bought the Bias Rite, changed tubes (to the three sets I listed above), and now I can't get back to that sound. It was hotter, more vibrant that previous week (and I'd played it that way for six months). A tube blew (which is why I replaced/bought new) and I just haven't been able to get back to that sound. The sound I hear sounds like a cold biased amp - a bit lifeless and very little growl.

    Would changing the caps have an effect on the ability to bias or the bias pot?
     
  14. slider313

    slider313 Silver Supporting Member

    Messages:
    7,480
    Joined:
    Jul 28, 2005
    Location:
    NC
    What were the previous set of 6L6's and their bias point?
     
  15. Sean French

    Sean French Supporting Member

    Messages:
    13,626
    Joined:
    Jan 7, 2007
    Location:
    Texas

    I would suggest tring a pair of NOS 5881'.
    That's what I like in my Vibrolux Reverb.
    JAN Sylvainia and JAN Phillips are not too pricey.

    Give Mike(Blue Strat) a call at KCA NOS Tubes.
     
  16. Scott Whigham

    Scott Whigham Member

    Messages:
    3,536
    Joined:
    Mar 4, 2010
    Location:
    Dallas, TX
    KSDS (member here) came over and brought his Bias Rite over (that's when I knew I needed one!) and IIRC it was around 39-42.
     
  17. slider313

    slider313 Silver Supporting Member

    Messages:
    7,480
    Joined:
    Jul 28, 2005
    Location:
    NC
    The resistor on the bias pot should be a 27K 1/2 watt. You'll have plenty of range if you change it to 15K 1/2 watt.
     
  18. Scott Whigham

    Scott Whigham Member

    Messages:
    3,536
    Joined:
    Mar 4, 2010
    Location:
    Dallas, TX
    That's good info - thank you for that. I'll order one, strap it across the bias pot, and see what happens!
     

Share This Page