62 Bandmaster help?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jcs, Mar 26, 2020 at 10:36 PM.

  1. jcs

    jcs Member

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    Amp is full serviced by a very good tech; caps, Sylvania 6L6GC biased 60%, known good preamp tubes thru played several known good cabinets at 4 and 8 ohms...

    This amp has the presence knob btw, what is going on is the amp has a 'slight bit' of 'static-crackling noise' at lower to medium volumes BUT after the amp has been on and played for say 20 or more minutes it seems to go away.

    A variety of guitars etc and the amp sounds very good at clean volumes once warm, so, could there be some bad resistors at work here until it gets warm?

    Using a compressor helps a bit as well BUT at medium slight gain tones it still gets a bit of a crackling sound.

    When you drive the amp very hard with various OD pedals it seems to go away somewhat.

    What else could it be? Its the 'edge of distortion' sounds that sound this way compared to say my 63 Tremolux which excels at 'edge of distortion'.....Change out the big bottle Sylvania 6L6GC for say RCA Blackplate 6L6GC for comparison?

    Once the amp is warm the clean tones are very good; presence knob allows lots of dialing in along with the treble knob for those unfamiliar with this 6G7A circuit.

    Any ideas? try some different preamp tubes (all are 12ax7)?

    Thanks in advance!
     
  2. xtian

    xtian Member

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    Some crackling, but only during a warmup period? Noisy carbon comp resistors is one possibility. It would be useful to narrow down the area of the circuit causing the issue. You can do this by playing Battleship with the tubes. That is, start by pulling out all the tubes except the rectifier and power tubes, and power up. Do you hear the crackling? Then we know the issue is in the power section, not the preamp. If no cracking, put one preamp tube in (phase inverter) and try again, until you hear the crackling again.
     
  3. jcs

    jcs Member

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    I should add, im not really hearing crackling with out playing.....its mainly when the amp is pushed just a bit....thanks for the info and i will try that (though the amp is one of Fenders 1st SS rectified designs and we checked the diodes)!
     
  4. zenas

    zenas Member

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    Have you talked to the tech about it?
    Did he change ALL the electrolytics? Some will leave the cathode bypass caps, I've found those can make noise.
    I've had about zero problems with carbon comps in old Fenders but I think living in a place where the humidity is low during our 8 months of winter has a lot to do with that.
    The reality is this could be a lot of things.
     
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  5. jcs

    jcs Member

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    Yes ALL electrolytics changed, bias circuit rebuilt entirely (adjustable bias added).

    Man, this thing rips with OD pedals stacked in such a way that any cracking/static is barely if at all apparent.

    My humidity really never gets below 50% in the house which is good actually but can get to 65% rainy seasons.

    The really clean tones and heavy OD tones are great...btw, the transformers are barely luke warm after an hour or so on.
     
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  6. gldtp99

    gldtp99 Member

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    A minor static/crackling sound that only appears when the amp is operated under certain conditions (pushed to edge of distortion) could be almost any component, solder joint, pot, ground connection, or tube.

    One would have to narrow down the possibilities somewhat to begin the troubleshooting process.

    I went through a somewhat similar process when I rebuilt my 1973 Marshall Superlead----- taking the amp from an abandoned derelict and turning it into a great sounding reliable amp.

    The very last issue was a maddening slight crackle that would appear and disappear at random (there were many, many other issues that were solved before this particular issue).

    The problem turned out to be the Bass pot that the Track/Wiper connection would not clean up no matter what kind of cleaner I used on it (and how many times it was cleaned).

    I also could not remove the pot, take it apart, and physically clean the internal parts because the original knob would not come off the pot shaft (the set screw would not budge and stripped out)--- I eventually had to break the original knob into pieces to remove it (I tried to carefully drill out the set screw to no avail).

    I installed a new pot and a similar year used knob and the amp was finally working 100% (consistently) for the first time since I'd owned it.

    But I'd gone through quite a process of troubleshooting that particular intermittent crackle before I found that the problem was the pot itself and was finally able to correct the problem (install a new pot in this case).

    So in your '62 Bandmaster someone has to go through the process of finding out what the exact problem is and proceed to fix it.

    I don't know how this could be done online. It takes a logical troubleshooting process best done by a person experienced with the typical problems found with old amps.

    But ruling out a bad preamp tube (100%) is a good place to start.

    [​IMG]

    [​IMG]

    [​IMG]

    My '73 Superlead as I first got the amp---- the "Before" pics-- :cool::cool::cool:

    This amp needed a lot of work.
     
  7. xtian

    xtian Member

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    I would approach this by using a sine wave generator. If you can get the amp to misbehave with a fixed input signal, then both hands are free to chopstick everything for issues. A scope might also help "see" where the noise gets introduced.
     
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  8. gldtp99

    gldtp99 Member

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    Here's a new vid by Uncle Doug where he goes through the troubleshooting process to find and fix a noise problem in a Blonde Blackface AA763 Bandmaster head (I have another Blonde Blackface AA763 Bandmaster sitting across the room from me right now---- and two earlier 6G7-A Brownface Bandmasters in the next room).

    The main problem with the amp in Uncle Doug's vid was a noisy old RCA 7025 preamp tube---- but he went farther with his work and also replaced some 100k Plate Resistors, and a Volume Pot (and a few other things) to do his best to insure that the old Bandmaster would continue to operate properly.

    He shows a logical troubleshooting approach using a scope and explains what he is doing step-by-step.

    Another tech may not use the exact same approach or the exact same steps but a logical approach to troubleshooting is definitely the way to go when chasing down noise problems in old amps.

    Uncle Doug ended up with a nice sounding old BF Bandmaster to return to his customer--- :cool::cool::cool:

    Vid:

     
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