Results 1 to 7 of 7

Thread: 1973 Fender Princeton non reverb bias question

  1. #1
    Forum Member
    Join Date
    Dec 2021
    Posts
    16

    1973 Fender Princeton non reverb bias question

    Good day. I have a 1973 Fender Princeton no verb. Until recently it had a set of tung sol 6v6’s but was sounding ratty. When I checked the bias one tube was at 20ma the other 10. Aha - bad tube - this explains it. I put in a set of NOS tubes I had and the amp sounds great, with good strong tremolo. Yet… when I checked the bias the tubes at 15 mA. Should I bring them to at least 20?

    thoughts? By the way the amp was modded to add a bias pot by myself so changing the bias current is not a huge deal.

    and for all those that helped with my 1973 Pro Reverb questions I retuned it and it sounds great.

    thanks

    warren

  2. #2
    Forum Member phantomman's Avatar
    Join Date
    Aug 2007
    Location
    Ten miles from the Mexican Frontier, in Arizona
    Posts
    7,297

    Re: 1973 Fender Princeton non reverb bias question

    What's your plate voltage look like at the 15 mA idle-bias figure?
    "When injustice becomes law then rebellion becomes duty."

  3. #3
    Forum Member
    Join Date
    Dec 2021
    Posts
    16

    Re: 1973 Fender Princeton non reverb bias question

    Quote Originally Posted by phantomman View Post
    What's your plate voltage look like at the 15 mA idle-bias figure?
    i actually only use an octal bias probe to check current.

  4. #4
    Forum Member
    Join Date
    Aug 2008
    Location
    SW New Mexico, USA
    Posts
    1,658

    Re: 1973 Fender Princeton non reverb bias question

    You need to know the voltage to figure the bias starting point.
    Check this out:

    https://robrobinette.com/Tube_Bias_Calculator.htm

  5. #5
    Forum Member
    Join Date
    Dec 2021
    Posts
    16

    Re: 1973 Fender Princeton non reverb bias question

    Checked the plate voltage. Around 500 v. Biased my new JJ’s at 17ma per tube.

    thoughts?

  6. #6
    Forum Member phantomman's Avatar
    Join Date
    Aug 2007
    Location
    Ten miles from the Mexican Frontier, in Arizona
    Posts
    7,297

    Re: 1973 Fender Princeton non reverb bias question

    Too high. You should be seeing circa 420 VDC.
    "When injustice becomes law then rebellion becomes duty."

  7. #7
    Forum Member
    Join Date
    Dec 2021
    Posts
    16

    Re: 1973 Fender Princeton non reverb bias question

    Just to point out this is the non reverb version with a 5U4GB rectifier. What would I do to lower plate voltage?

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •