New valves..must I re-bias?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by San, Nov 19, 2003.


  1. San

    San Member

    Messages:
    72
    Joined:
    Nov 17, 2003
    Location:
    Ipswich, UK
    HI folks,

    First off I have to say that I'm rather new to all this valve amp stuff so please forgive me if this question is just plain stupid!! ;)

    I've got a Fender HR DeVille which I love but I thought I'd try some different output valves (Sevtlana 6L6GC) to see if they had any effect on tone. So, I now have a matched pair of new tubes that I want to try out but I'm scared because I've read that if you change valves without re-biasing the amp (whatever that is!) I could cause perminent damamge to either a) the valves, b) the amp or c) myself!!

    Is this the case? Can I just replace the current valves (Groove Tube 6l6) and not worry too much about it or will I blow myself up? What happens if I loose a valve during a gig can I just do a quick swap??

    Oh yeah I just remembered that my amp has a bias control thingy somewhere inside the chasis. Does this mean I can do the job myself??


    Thanks in advance for the help guys and gals..

    San.

    (Tube amp virgin (sort of))
     
  2. VaughnC

    VaughnC Supporting Member

    Messages:
    14,561
    Joined:
    Jan 4, 2002
    Location:
    Pennsylvania
    There was a time when specs on tubes were fairly tight so one manufacturers 6L6 would bias very close to another manfacturers. However, these days this is not true....so it's best to check the bias when replacing output or rectifier tubes. You won't hurt the amp or yourself if you don't rebias but, if the tubes happen to be underbiased, they could be permanently damaged....which can be an expensive mistake with good NOS tubes. If the tubes happen to be overbiased, an amp will likely sound/feel stiff but will cause no harm at all. Bias meters are fairly inexpensive and can save you many headaches and trips to the tech once you learn how to use one. Many bias meters have adapters that plug between your tubes and their sockets so you don't have to take the amp apart to check the bias....but adjusting the bias, if needed, may be another issue. I use an Allesandro bias meter....it has meters built in so you don't need an additional voltmeter to use it.
     
  3. San

    San Member

    Messages:
    72
    Joined:
    Nov 17, 2003
    Location:
    Ipswich, UK
    Thanks Vaughn you've wiped some sweat from my brow, I'll check those meters out...

    San.
     
  4. myles111

    myles111 Member

    Messages:
    2,955
    Joined:
    Nov 19, 2003
    Location:
    Los Angeles CA

    You can do this yourself, but watch those high voltages. On the test point you want to see 60mV, which is 30mA per tube.

    If your stock tubes are White printing on the output tubes, then the GT range of 4-7 on the GT-6L6R will work fine, no need to rebias. If blue printing, then GT 1-3, and red is GT 8-10.

    This is covered on my GAB website.

    If you go to a different 6L6 (which I strongly recommend for nicer tone and touch dynamics), then a rebias is necessary.
     
  5. San

    San Member

    Messages:
    72
    Joined:
    Nov 17, 2003
    Location:
    Ipswich, UK
    Hi Myles, thanks for the reply.

    The stock tubes are white labelled Groove Tubes and I've bought some Svetlana 6L6's which I hope will sound sweet.

    Looks like I'll need to get the amp re-biased..

    Cheers,

    San.
     
  6. myles111

    myles111 Member

    Messages:
    2,955
    Joined:
    Nov 19, 2003
    Location:
    Los Angeles CA
    San,

    The White tubes are 4-7 range on the GT scale.

    If your Svets (the GT 6L6R2) are 4-5, your factory bias would be fine with these and no bias necessary from the White Sovteks. If they are not a #4-#5, but Svetlanas, then you should check the bias, as the Svets really vary quite a bit.
     

Share This Page