Fender Bassman RI Tube Question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by BrianP, Nov 16, 2004.


  1. BrianP

    BrianP Guest

    I play harp and have recently acquired one of these amps and I am trying to set it up for less feedback with tube swaps. My question is about groove tubes power tubes. I would think I wanted Blue for a dirtier sound but I am not sure if they mean distorts earlier by dirty. I am looking to have it distort at lower volumes. I would use Tung Sol 5881's but I have no where around me (boonies) to have it rebiased. Any advice would be appreciated.

    Brian
     
  2. Jeff Flowerday

    Jeff Flowerday Member

    Messages:
    606
    Joined:
    Jan 21, 2003
    Location:
    Calgary, AB Canada
    You should rebias with every power tube unless unless you are replacing with the exact same color or number code from a vendor. Even then it's always smart to check the bias to make sure the tubes are OK.

    To lower the headroom and get it sounding extra sweet try the following recipe

    5R4 rectifier (Not the 5AR4)
    Jan Philips 6L6WGBs(5881s)

    Rebias to about 65% of 25Watts per tube and you'll be set.
     
  3. fullerplast

    fullerplast Senior Member

    Messages:
    6,793
    Joined:
    May 5, 2003
    For harp, and less feedback, you should try a 12AY7 in V1. I'd also suggest a 5U4 for the rectifier for harp use. You want some sag and compression. The Tungsol 5881's are a good choice for that amp.

    Unfortunately, the stock amp does not have adjustable bias so if you want to really set it properly you'll need to have a mod done by a tech. Most likely though, your tubes will be running colder rather than hotter, so you should be safe, if not optimum. If you do try new power tubes without checking the bias, at least look at them closely for a while to make sure the plates are not glowing a reddish color. If they are not glowing, you will at least be able to use the amp until you can get the bias checked.
     
  4. aeolian

    aeolian Member

    Messages:
    6,083
    Joined:
    Apr 5, 2004
    Location:
    Santa Cruz, CA.
    A 5U4 would lower the voltages a bit and a 5R4 a bit more, but is going that far down safe for the cathodes? Is there a chart somewhere that shows him how much?

    I would expect that changing V1 would make a bigger difference. I did find when putting a NOS 5751 in my DR that it was hotter than the cheap Groove Tube 12AX7 that was in there when I bought it. Sounded immensely better too. Tried a couple AT7's in there but the 5751 was magic for guitar. But I've heard that AT's or AY's are more common for keeping a harp amp from feeding back.
     
  5. Blue Strat

    Blue Strat Member

    Messages:
    29,924
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
  6. 908SSP

    908SSP Member

    Messages:
    5,794
    Joined:
    Jun 22, 2004
    Location:
    Michigan
    So are saying it is safe to move down that list but be careful and check if you move up?:confused:
     
  7. hasserl

    hasserl Member

    Messages:
    4,734
    Joined:
    Oct 24, 2004
    Location:
    So Cal
    That's a pretty good way to put it.

    If you use a rectifier that results in lower B+ you're not going to hurt anything. On the other hand, if you change to a rectifier that results in an increase in B+ than you could exceed the voltage rating of the caps, as mentioned (especially in move say from a 5Y3 to a 5AR4!), also with the higher B+ the bias may need to be adjusted to keep the tube operating within a safe range.

    There is also the matter of filament current to consider, as he mentions on that page. If you change from a rectifier that uses less current, no harm can be done. But if you change to a rectifier that draws additional current it is possible that the 5volt winding on the power tranny will be stressed, which could cause it to run hot.
     
  8. Jeff Flowerday

    Jeff Flowerday Member

    Messages:
    606
    Joined:
    Jan 21, 2003
    Location:
    Calgary, AB Canada
    Well it really does depend on how much the B+ is dropped by the rectifier. Get the tubes biased too cold and they'll start sounding like crap and you could start getting anomolies.

    Just bias it up properly!!!
     
  9. tonezoneonline

    tonezoneonline Member

    Messages:
    437
    Joined:
    Dec 15, 2003
    Location:
    central Illinois
    I like a 5U4 rectifier in these amps with a 12AY7 or 12AU7 in Vi.
    I also change the dropping reistor to lower the voltage on V1 to around 95-105VDC.Killer tone and less feedback for harp
     

Share This Page