64 Bassman question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by scottlr, Sep 6, 2006.

  1. scottlr

    scottlr Member

    Messages:
    22,773
    Joined:
    May 14, 2006
    Location:
    Born & raised in Texas; stranded in Iowa
    Seeing the 63 Bassman noise thread made me think about mine. But since MY noise isn't like the noise described, I thought a new thread was in order.

    Awhile back, I was using this amp for recording. It sounded great. But between tracks, I noticed it had developed a noise. Similar to someone gently blowing into a mic. Not super loud, but certainly noticeable, and even more certainly "not right". What could be causing this?

    Also, I am not an amp tech, so techie talk will probably go over my head unless very general and layman terms :)
     
  2. bob-i

    bob-i Member

    Messages:
    7,455
    Joined:
    Oct 16, 2005
    Location:
    Central NJ
    That sounds like resistor noise although it's possible that it's a tube too. I'd start by replacing the preamp tubes one by one. If the noise doesn't go away, take it to a tech for plate load resistor replacement.
     
  3. brad347

    brad347 Member

    Messages:
    4,811
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    definitely consistent with plate load resistors
     
  4. scottlr

    scottlr Member

    Messages:
    22,773
    Joined:
    May 14, 2006
    Location:
    Born & raised in Texas; stranded in Iowa
    Thanks, guys. If it is plate load resistors, is this an expensive repair?

    I sure wish I knew how to work on my own amps... ;)
     
  5. bob-i

    bob-i Member

    Messages:
    7,455
    Joined:
    Oct 16, 2005
    Location:
    Central NJ
    I wouldn't think so. The resistors are about $.50 each, you'll need 4, and they take about 15 minutes to replace all of them.

    :jo
     
  6. scottlr

    scottlr Member

    Messages:
    22,773
    Joined:
    May 14, 2006
    Location:
    Born & raised in Texas; stranded in Iowa
    OK, thanks again. I'll take it to my tech and see what he finds.
     
  7. brad347

    brad347 Member

    Messages:
    4,811
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    plate load resistors is such a simple fix that it might be a good time to get your feet wet. Just sayin'

    We could help you through it! :D
     
  8. scottlr

    scottlr Member

    Messages:
    22,773
    Joined:
    May 14, 2006
    Location:
    Born & raised in Texas; stranded in Iowa
    Hmmm.... without electrocuting myself? And where do I get them?
     
  9. devbro

    devbro Member

    Messages:
    1,128
    Joined:
    Apr 17, 2004
    Location:
    Los Angeles
    Hey Brad,
    Maybe you could help both of us and kill 2 birds with one stone.:AOK
     
  10. scottlr

    scottlr Member

    Messages:
    22,773
    Joined:
    May 14, 2006
    Location:
    Born & raised in Texas; stranded in Iowa

    I'm not sure I like the sound of that :jo

    maybe you should rephrase that?
     
  11. brad347

    brad347 Member

    Messages:
    4,811
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    It is easy to replace plate loads without electrocuting yourself IF you know how to discharge the filter caps properly. That is easy, but I don't want to advise you to do it lest it be on my conscience if you get hurt. That said, it's harmless if you know how to discharge your caps properly, as the amp need not be on when you change them. The parts are right on the surface of the tagboard, there are no polarities to observe, and it's just a few joints. Really good project to get your feet wet I think.

    To change plate load resistors, you would need the following tools and supplies:

    1. 1 test lead with alligator clips on each end
    2. appropriate number of appropriate value carbon composition resistors from Hoffman or any other supplier
    3. rosin-core solder
    4. de-soldering tool (pump or braid)
    5. soldering iron
    6. wire cutters, pliers, etc.
    7. (optional) DMM

    If you would like me to walk you through the process then I'd be happy to post it here (surely to be subject to some minor corrections by the resident experts), if you have or are willing to acquire the necessary tools above.
     
  12. scottlr

    scottlr Member

    Messages:
    22,773
    Joined:
    May 14, 2006
    Location:
    Born & raised in Texas; stranded in Iowa
    Thanks, feel free to post it. I have some of the stuff, and willing to get the rest. I needs my feet wet :)

    What's DMM?
     
  13. retro

    retro Member

    Messages:
    2,128
    Joined:
    Oct 5, 2005
    Location:
    Earth
    Hey Scott,

    Digital Multi Meter....

    Howzit?
     
  14. brad347

    brad347 Member

    Messages:
    4,811
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    STEP ONE: discharging the caps.

    Make sure the amp is unplugged and both switches are in the "off" position. Unplug the speaker from the chassis (if applicable) and unscrew all necessary screws to remove the chassis. Remove the chassis from the cabinet and place it on a stable working surface. Locate PIN ONE of the first preamp tube (v1), which is the one farthest from the power tubes. Here's how to locate pin 1 if you don't know:

    On a noval (9-pin) base tube like a 12AX7, there is a "gap" in the pins... a place where a 10th pin could be, but is absent. This acts as the 'key' that makes sure you insert the tube the right way. This is all obvious stuff. LOOKING DOWN AT THE BACKSIDE OF THE SOCKET from inside the amp, pin 1 is the pin that is IMMEDIATELY CLOCKWISE of this gap or space between the pins. To put it another way, we are not looking at the socket from the side you plug the tube in, but rather from the side the wires are soldered to.

    Once you have located pin 1, clip your test lead to it. Then clip the other end of the lead to the chassis of the amplifier. WITH THE AMP STILL UNPLUGGED FROM THE WALL, flip both the power and standby switches to the "ON" position. The amp of course will not turn on as it is unplugged. What WILL happen, however, is that this will drain the filter capacitors to ground slowly through the resistors in the circuitry of the amp. You will need to wait about 30 seconds for them to discharge fully. If you have a Digital Multimeter, then you can check voltage between ground and the point where the filter caps connect to the board and observe the charge quickly draining down. If no DMM, give it a full minute just to be safe.

    Sometimes caps can magically 're-charge' themselves a little bit (though not anywhere close to full voltage) even after discharged, so I suggest keeping that lead clipped between v1 pin 1 and ground for the rest of the procedure. Not necessary but recommended.

    Congratulations, your amp is now safe to work on! On to

    STEP 2: removing the old resistors
    Locate the plate load resistors using the pictures earlier in the thread. Heat up the solder joints with your iron, and use your de-soldering pump (recommended instead of a desoldering braid) to suck the old solder out. Do this on each end of each resistor, and with a little wiggling you should be able to pull them out with no problems. Needle nose pliers can work to your advantage here. I would recommend doing them one at a time if you're a first-timer, or at least making a drawing of everything you're removing and where it goes. Downloading a free board layout diagram from the ampwares fender field guide is a big help also.

    STEP 3: Putting the new resistors in

    Double checking the value of the resistors with your multi-meter rather than trusting the colored bands is recommended. Use your needle nosed pliers and wire cutters to pre-bend and cut the leads to the correct approximate length and shape to drop right into the holes on the tag board. Remember that while solder is to some degree conductive, you should NOT depend upon it to make your electrical connection. Make sure all leads in the eyelet on the tagboard are touching either one another or the eyelet itself. Also make sure they are situated in such a manner as to make a good mechanical connection that you don't have to hold in place... it shouldn't move when you let go, in other words. This ensures that you will have a good solder connection that won't go 'cold' as a result of something moving around before the solder has set.

    STEP 4: Put her all back together the way you found her and rock!

    self-explanatory.

    A final note of caution, it's easy for the uninitiated to fail to see the importance of certain steps in the process (pre-bending leads, etc) but I assure you cutting corners will often lead to problems. For a DMM, a cheap one from Radio Shack (about 20 dollars) will do nicely for most small-time projects such as this, biasing, etc. With a little care, this is a very easy and safe job. If you are at all inexperienced with soldering, I suggest getting some wire and making some practice joints first to really get the hang of it before you start working on your amp. Remember you are not melting the solder with the iron, but instead using the iron to heat the component leads to the point where THEY are hot enough to melt the solder. This is a common misconception among first-timers.

    Good luck!!
     
  15. scottlr

    scottlr Member

    Messages:
    22,773
    Joined:
    May 14, 2006
    Location:
    Born & raised in Texas; stranded in Iowa
    Hey buddy! Doing well!
     
  16. scottlr

    scottlr Member

    Messages:
    22,773
    Joined:
    May 14, 2006
    Location:
    Born & raised in Texas; stranded in Iowa
    Brad, OK, thanks. I got this printed out, and will go shopping tomorrow. I've been to the Fender Amp Field Guide many times, so I'll also go there for a diagram. Where do I get the resistors? Hoffman? Online?

    Maybe next I can learn to bias my amps, too!
     
  17. devbro

    devbro Member

    Messages:
    1,128
    Joined:
    Apr 17, 2004
    Location:
    Los Angeles
    Hey Brad,
    Do we need to use a heat sync when soldering the new resistors in place or are these resistors ok with heat?
     
  18. devbro

    devbro Member

    Messages:
    1,128
    Joined:
    Apr 17, 2004
    Location:
    Los Angeles
    Also,
    These are the plate load resistors right?

    [​IMG]
     
  19. scottlr

    scottlr Member

    Messages:
    22,773
    Joined:
    May 14, 2006
    Location:
    Born & raised in Texas; stranded in Iowa
    I'm not the one to ask :)
     
  20. devbro

    devbro Member

    Messages:
    1,128
    Joined:
    Apr 17, 2004
    Location:
    Los Angeles
    Sorry Scott, I meant Brad. I feel so helpless I bought a book today by Dave Funk so I can learn this stuff.:confused:
     

Share This Page