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Removing 2 Tubes - Bassman 100

Discussion in 'Amps and Cabs [BG]' started by until.tomorrow, Mar 22, 2005.


  1. until.tomorrow

    until.tomorrow Guest

    May 5, 2003
    US - Milwaukee
    I found a Fender bassman 100 (early 70's model)for sale and I'm pretty interested in it but there are two problems.
    #1 the tubes are blown (WHAT?! How do all tubes blow without serious problem?)
    #2 the person selling it stated that he ran it using only two of the four 6L6 power tubes.

    Is there a way that I can test the output transformer of this thing without any tubes in it?(the only salvageable tubes that I have are in my V4 right now)

    Also is it safe to run the Fender bassman 100 with two 6L6 if the impedance was checked?
     
  2. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Yes, you can pull half of the power tubes...make sure you take out one from each "side" (imagine a line between the two center tubes). Mesa's 60/100 watt switch on some of their guitar amps essentially does the same thing.

    I'm not sure how you test an output tranny to see if it's good.
     
  3. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Also run it on one lower impedance setting, like put the 8 ohm speaker on the 4 ohm tap. That brings the load back to normal on the two tubes, more as if they were working with the other two in.

    It was covered a while back in another thread. and the explanation there was correct.
     
  4. Passinwind

    Passinwind I Know Nothing Supporting Member

    Is there a way that I can test the output transformer of this thing without any tubes in it ?

    What troubleshooting tools do you have access to?

    When you say "all" the tubes, do you mean the preamp ones as well as the power tubes?
     
  5. until.tomorrow

    until.tomorrow Guest

    May 5, 2003
    US - Milwaukee
    I have a portable multimeter but I could take the amp to my school if needed and use an oscilloscope, function generator, and DMM.

    The preamp tubes are fine. It needs new power tubes (4 6l6's). The seller stated that it blew a power tube while running with only two in which is why I'm concerned about if the output transformer was damaged.

    I think I'm going to order 4 Sovtek 6L6WXT+. meh ill be out $50...what's that to a poor college student with an addiction of buying amps
     
  6. Passinwind

    Passinwind I Know Nothing Supporting Member

    The output transformers on those amps are pretty bomber, I've never had to replace one. Here's a little tutorial on audio transformer test methods: http://www.geofex.com/ampdbug/outtrans.htm Be sure to read the warnings, and be careful.

    You can PM me for ideas on why the tubes blew and what to do about it, if you like. If the tubes were pulled without rebiasing, the increased plate voltage from only running with two could've eaten them. Lots of times it's no problem, sometimes it could be.
     
  7. Passinwind

    Passinwind I Know Nothing Supporting Member

    The rub is that a Bassman 100 doesn't have multiple taps, therefore you have to adjust the speaker load upward when you pull a pair of output tubes, unless you want an intentional mismatch. The Bassman 100 wants a 4 ohm load with 4 tubes, and an 8 ohm load with 2 tubes, as you said. A classic iteration is a Twin Reverb running only one of the speakers when two tubes are yanked.