Bias on Sunn 300T/Fender Bassman 300 Pro

Discussion in 'Amps and Cabs [BG]' started by thombo, Jan 2, 2014.

  1. thombo


    Aug 25, 2006
    Denver, CO
    If anyone can offer some support on biasing my Sunn 300T, it would be appreciated.

    I blew the middle 100ma/250 volt fuse that protects the power tubes. I ordered new fuses, changed the blown fuse, and the cooresponding pair of 6550c's. The amp was loaded with the original Fender/Groove Tubes power tubes. I replaced the middle pair with two Ruby Tubes (from a matched quad set). I know Ruby Tubes aren't the best, but they are what were available. Here is where my problem starts.

    When I biased the amp, the manual read to set the voltage to 0.1 VDC, but the closest I can get is 0.082 (the bias screw is maxed clockwise).

    When balancing the amp, he manual says to set the voltage to 0.000 VDC, but the closest I can get is 0.002 (the bias screw is maxed clockwise).

    Any suggestions on what I do next? Swap the tubes again? Change another pair of tubes? Am I "close enough" or will I damage something?

    Here are links the the manuals, both say the same thing.
    Page 10

    Page 8
  2. While it is unlikely to cause any problems you possibly have no room left to re-adjust for drift on that pair. I would try a different pair of tubes. There are many online stores to get tubes from. GT's are not necessarily better than Ruby's they both buy tubes and rebrand them.
  3. Bill Hughes

    Bill Hughes

    Oct 17, 2013
    Your numbers sound OK. The absolute max is 0.136, but 0.08 is fine. The balance over to one side is strange. Try a new 12AT7, as it is in the bias loop, or you might have a dead 6550.

    The 6550’s should all be the same temperature. If you have a dead tube on one side its neighbors on that side will be pretty hot. Turn out the room lights and see if you’ve got any red plates. That’s a sign that you have a dead tube on that side.

    Be sure that your line voltage is correct at 120V.

    That power amp is an improved version of the SVT, cleaner and tighter. The preamp is identical to the SUNN / FENDER 1200s

  4. thombo


    Aug 25, 2006
    Denver, CO
    Thanks for the responses!

    I tried swapping out another pair of tubes for the remaining pair of Ruby Tubes, and my numbers were even further off, so my quad set won't work for me.

    Since I have a few extra fuses, I threw in all of the stock tubes. I am now able to bias the amp, but the balance still reads 0.002. Nothing has blown yet, all of the tubes are glowing evenly, nothing is hot-plating, and the amp sounds good. Could the bad balance have blown the fuse?

    Regardless, I'm going to go grab a new 12AT7 and a pair of Svetlana 6550's (maybe I'll be able to trade for my quad of Ruby Tubes). I'll start with the preamp tube. If that doesn't work, I'll throw in the Svetlanas. If the 12AT7 does the trick, I have a solid backup pair (which is good b/c I have two of these amps).

    Please, if anyone has any more input, or if I'm about to blow my transformer, I'd really appreciate it!
  5. The fuses will protect your transformer. Tube sets when matched will fall into a bias range, some require more negative bias voltage some less that is why the get "matched".
  6. BawanaRik


    Mar 6, 2012
    New Jersey
    The tube store get's good feed back. Those 6550s are mondo bucks going for NOS. They're a lot period.
  7. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    My 300 Pro's bias drifted as my 6550's failed collectively. The balance was always close (I replaced one dead tube already) but it worked until it fizzled out altogether at some small town redneck garage gig. It had SED Winged C 6550's that I ran into the ground when I bought it. The newer Svetlanas have been good ever since. :)

    Oh yeah, make sure that 12AX7 + 12AT7 pair is in good condition. Both were crap in my amp and they are critical to its proper operation
  8. thombo


    Aug 25, 2006
    Denver, CO
    In case anyone searches this topic in the future...

    I replaced the 12AT7, but it didn't help. Today, I took all power tubes and the 12AX7 that paired with the 12AT7 to my tech. They metered all the tubes and found one 6550 tube was mismatched (really low plate voltage), and the 12AX7 did not pass the bench test (so I replaced it). They tested about a dozen 6550's and eventually found one that would work with my other tubes. I came home, popped the tubes in, the amp biased and balanced perfectly, and my Sunn once again sounds epic!
  9. I Just picked up one of these (Fender badged) and it had a blown fuse -I replaced the fuse and all tubes worked and I was able to bias and ballance the lot - however the right hand side transformer when looking at it from the front ran so hot I could not touch it - the one on the left was luke warm. So if I am reading this correctly if a fuse blows it means valve failure? - could this cause my transformer to run that hot?