Need to work on a Fender Super Twin but I'm a little rusty

Discussion in 'Amps and Cabs [BG]' started by jbigfoot, Apr 8, 2016.

  1. jbigfoot


    Oct 10, 2015
    Columbus, Ohio
    I just bought a Super Twin at a redonkulously low price because it wasn't working, and I fixed it with such a simple repair that I'm turning cartwheels. Now the real work begins: I need to bring it back to life and then convert it to a Studio Bass (my dream amp since the mid '70s).

    The latter shouldn't be much of a problem. But the biasing...that's a different story. I haven't done some serious work on these things since I had a full head of hair.

    I'll be putting in a variable bias circuit; and I have an old bias device that you plug into the socket of a 6L6 and it interrupts pin 8. At the risk of sounding like a stupid old man, how do I use this thing again...? And what should I set the bias to...?

    The current tubes are GE (except for one) and the amp is highly microphonic (which I don't think is a fault of the power tubes). IMG_0035m.jpg
  2. beans-on-toast


    Aug 7, 2008
    There are two types of these bias probes.

    One has a 1 ohm resistor inside, you set your meter on milli volts and the voltage reading is equal to milliamps. Take the bias probe, don't connect it to the amp or plug in a tube. Use your ohm meter to read across the two wires, it it reads 1 ohm, this is the type that you have. If it read infinite, you have the type describes below.

    The other brings two wires out, from and to pin-8. It is intended to be connected to an ammeter. It directly reads mA. The meter has to be connected for the tube to work while the amp is operating. The meter provides a path to ground. Disconnect the meter and the cathode is not connected to ground.

    To set the final bias, the amp should warm up for at least 20 minutes. The tube can be in the bias probe while it is warming up.

    If you can recall where you bought the probe, there may be instructions online that explain what current to set the tube at.

    Do you have a schematic for the variable bias circuit that you intend on using?
  3. MIMike


    Jan 1, 2013
    West MI
    Beans is obviously the one to address you actual question, so I'm going to go slightly off topic.
    You say you want to convert it to a Studio Bass, but unless you will not be transporting it or gigging it, might I suggest an option? I had a Super Twin that I just got rid of that had already gone through the head case conversion. That 2x12 monster is very unwieldy, and converting it to a 1x15 only improves portability slightly. If you go for the head-only mod, then you can use it in various configurations. I have a 72 Bassman 100 that I gig with a converted original cab in a 2x12 format. The Super Twin worked with this, as well as my 1x15 EV cab. Different flavors and levels of portability, but more options than a combo amp.
    Side note of interest; the Super Twin head balanced much better than the Bassman when lugging it by the strap handle...this made it seem lighter than the Bassman, which probably isn't true, but it was definitely easier to carry around. This is probably because it has two gigantic transformers on each side of the head creating more balance from side-to-side.
  4. jbigfoot


    Oct 10, 2015
    Columbus, Ohio
    My apologies for such a delayed reply, I've been deeply involved in some other things right now (and sick as a dog, too!).

    I'm going to have to check the bias probe tester, it's an old one with no name or other markings on it.

    I don't have the schematic (yet) for the bias mod.
  5. jbigfoot


    Oct 10, 2015
    Columbus, Ohio
    Thanks for the advice. I won't be leaving it in the original cabinet configuration, an open back just doesn't cut it for us. I'll be doing a head-only modification and using it with two 2 X 12" cabinets -- or something along those lines. With that much power, I want to spread the wattage among several speakers, and to have those speakers in cabinets large enough to breathe.