SS rectifiers, biasing, headroom, BF Fender...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by cerebralpaul, Jan 6, 2008.


  1. cerebralpaul

    cerebralpaul Member

    Messages:
    257
    Joined:
    Apr 4, 2004
    OK, so I put a SS rectifier in my BFSR- to get the bias within proper parameters I had to turn the bias pot all the way down and it's still on the high side- so my question is: since my tubes are now running "hot" do I now have LESS headroom than if I just left the tube recto in there and ran the bias on the "cool" side?
     
  2. GearHeadFred

    GearHeadFred Member

    Messages:
    1,657
    Joined:
    May 4, 2007
    Location:
    San Diego, California
    I have experimented with SS and tube rectos in my BFSR.. my guess would be no - that the SS recto will give you more headroom.. BUT how hot are the tubes running?

    Why not add a resistor to the bias pot to give a larger range?
     
  3. Blue Strat

    Blue Strat Member

    Messages:
    29,829
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    What GearHeadFred said:) How hot is hot?

    Bottom line is, how does it sound? That's the only thing that counts.
     
  4. cerebralpaul

    cerebralpaul Member

    Messages:
    257
    Joined:
    Apr 4, 2004
    Well soundwise its fine- a tad tighter bass/more headroom but not a huge difference. Numberwise I forgot exactly, I printed out a reference from somewhere(maybe it was your site Mike?) and I think it might have been around 375v and 55ma which was just within "safe" parameters but definitely "hot". When I only heard a slight difference in headroom I got to thinking about my original question- if I put the tube rectifier back in and ran the tubes "cool" might that make a bigger difference?
     
  5. Blue Strat

    Blue Strat Member

    Messages:
    29,829
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Are you sure you have 375 or the plates? If you do, you've got a problem. You should have closer to 475 with a SS rectifier.

    If it is 475, you're running at 26 watts which is well out of the safe zone.
    60% of max dissipation is good, anything higher is a waste of energy (heat) and tube life.
     
  6. GearHeadFred

    GearHeadFred Member

    Messages:
    1,657
    Joined:
    May 4, 2007
    Location:
    San Diego, California
    Yes, the plate voltage should be between 440V and 470V on a healthy BFSR
     
  7. slider313

    slider313 Silver Supporting Member

    Messages:
    7,152
    Joined:
    Jul 28, 2005
    Location:
    NC
    Bring the tubes down to 40ma and then get a voltage reading.
     

Share This Page