'63 Bassman is noisy - help

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by devbro, Sep 4, 2006.

  1. devbro

    devbro Member

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    What would you do to quiet this thing down......?
    Just bought a '63 blonde 6G6-B Bassman and its a little noisy. I tried it for the first time last night and I love the tone but I experienced 2 noise issues. All the trannies are original but I cannot tell how old the caps are. Its got 2 Jan Phillips NOS 6L6's and EH 12AX7's in V1 & V2. V3 & V4 are no-name tubes. Below are pics of the caps.
    http://www.masterlynx.com/images/bassman/doghousecaps.jpg
    http://www.masterlynx.com/images/bassman/couplingcaps.jpg

    (1) I hear a periodic interference beeping like morse code but it's not a radio station thank god.
    (2) Its EXTREMELY sensitive to single coils. I have a Suhr with the silent single coil set-up and I've never heard hum like this with my Shiva or OD-100. I spent most of the night facing the drummer...eeek.

    What do you think? Caps, tubes, resistors...Maybe this is normal...HELP..????????
     
  2. brad347

    brad347 Member

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    pic 1: yr filter caps:

    All are original save one which is a cheap cheap cheap tiawanese cap. CHANGE THESE CAPS ASAP and you will noticed much improved performance and lower noise!

    The blue mallorys on the tag board are all original as are the astron 2-in-1 electrolytic cathode bypass caps. Stock mallorys = "great." Stock cathode bypass caps = "debatable." These caps (the yellow-orange cylinders) are probably in need of changing. However, if you like the way it sounds and there are no problems, leaving them in poses no danger to your amp. But if you want improved performance, they are definitely past their life and should 'technically' be changed. There are multiple schools of thought... some people like to leave them alone if they're working.

    Are you fully grounded with a 3-prong plug, properly installed? If not then that is the first order of business.

    Your interference means a tube or some other component is most likely microphonic. Start swapping tubes beginning with v1 and v2 on down the line. There is also a very small chance you have a microphonic cap or some other microphonic component.

    You might also want to consider re-routing the chassis grounds to a star-grounding scheme. There's also a chance that the sensitivity to single coil hum you experienced was exacerbated by something in teh venue. Good luck.
     
  3. devbro

    devbro Member

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    Thanks Brad,
    That's exaxctly the kind of feedback I was hoping to get.

    Are you saying I should replace all the doghouse caps or just the odd looking one and what exactly do you mean by "improved performance"?

    It has a three prong cord with the ground simply soldered to the inside of the chassis (not sure what start gorunding is). I think you're right about the pre-amp tubes that's probably a good place to start. Any recommendations?
     
  4. AL30

    AL30 Member

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    I agree. A cap job would be a good idea. That means replacing all the electrolytics. All the caps under the doghouse and all the electrolytics on the board(s). I would take out those orange double caps on the board and replace them. You can replace them with 2 single caps.

    New tubes might not be a bad idea either. Pre-amps can get noisy.

    Plate load resistors are also a source of noise in these amps. I usually just replace them on my amps but it may not be necessary on yours. Try the caps and tubes first. Here's a little info

    http://www.obsoleteelectronics.com/Fender_Tech/fender_tech.htm

    Also, check the 1.5K and the 470ohm resistors on the power tubes. These are exposed to a lot of heat and, over time, can start to look like a BBQ.

    AL
     
  5. brad347

    brad347 Member

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    change all of the filter caps in the doghouse. "Improved performance" equals lower hum, quieter operation, a little more apparent power, removal of 'ghost tones' or intermodulation distortion, and perhaps a tighter low-end, mainly.

    Electrolytic capacitors are generally rated for a 15 year lifespan. Less if the amp isn't used regularly. Regardless of patterns of use, your caps are way older than that already, and you're on "borrowed time" as far as that's concerned. The fact that at least one has (obviously) failed and been replaced already, it means the others aren't far behind, if they're not gone already. If a filter cap goes electrically leaky, then increased hum would DEFINITELY be apparent.

    RE: the tubes, as far as new production EH tubes are fine, JJ tubes are good... they're all inferior to a quality NOS tube though. NOS tubes however are getting more expensive so it helps to prioritize. Do you use one channel most often? If you mainly plug into "normal" and never use "vibrato," then put your best tube in v1. If you mainly use "vibrato" and never use "normal," then put your best tube in v2. The phase inverter spot is usually worth having a good tube in as well.

    If you're having symptoms consistent with a failing or excessively microphonic tube, then you can just try moving the tubes around in the amp (or removing them altogether) to see if the problem moves. For instance, if you're getting some squealing or interference, remove v1. Does the problem stop? no? Then put it back and remove v2. Problem stop now? You get the picture.

    Good luck, and personally I would get those caps replaced before I do too much more playing with it. I certainly wouldn't take it on a gig with the caps like that.
     
  6. devbro

    devbro Member

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  7. bob-i

    bob-i Member

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    They'll do fine. Make sure you watch the polarity, + and -.

    Actually you'll need 4. The 2 on the board are double caps, each contain 2 caps. Notice how they have 3 leads, 2 + leads and one - lead. You'll need to connect the - leads of the 2 replacement caps together and to the ground point on the board, then the 2 + leads.
     
  8. devbro

    devbro Member

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    Thanks Bob,
    Let me make sure I understand. I get (4) of the http://www.tubedepot.com/cp-at-25-25v.html caps and twist them together in pairs as you described.
    Can I substitute the (2) twisted together with (1) of a larger value so I don't have to double them up or is this paired method tonally superior?

    thanks.....
     
  9. bob-i

    bob-i Member

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    No, that's not it. There are 3 leads on the existing caps. 2 + and 1 -. On the replacements you'll twist together the 2 - leads, not the 2 positive.

    That will give you the 3 leads needed.
     
  10. danieldroukas

    danieldroukas Member

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    You don't want to twist the 25uF caps together at both ends. You can twist the negative end together because they both go to the same point: the preamp stage ground (which in fenders is just the chassis). However, the positive ends need to go their respective preamp tube cathodes.

    Look carefully in your second picture. You'll notice that on the - end there is one lead coming from the 25uF/25uF double cap. On the + end there are TWO leads. This is exactly as I described you could do in the previous paragraph: twist the negatives together and send the positives to where they're supposed to go.

    PS...
    I've got a 62. Consider yourself very fortunate, these things sound gorgeous.
     
  11. bob-i

    bob-i Member

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    Also, I don't think plate load resistors have been mentioned. These carbon comp resistors get noisy over time. These resistors are connected to pins 1 and/or 6 of the preamp tubes. Some are 100k and some are 220k, check this schem/ layout for more details.

    http://schematicheaven.com/fenderamps/bassman_6g6b_schem.pdf

    To keep the amp orignal I'd use carbon comp resistors.

    Many solder joints can go bad from constant heating / cooling. Remove the old solder and reflow the joints.

    So there are 4 things I'd do....

    1) Replace the electrolytics
    2) Replace the power tube resistors as Al said earlier
    3) Replace the plate load resistors
    4) Reflow any solder joint that look/suspected bad.
     
  12. bob-i

    bob-i Member

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    Exactly. the confusion was my fault, I wasnt' clear before.
     
  13. devbro

    devbro Member

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    Wow this is some great technical advice. I think I'm clear on twisting together the 2 negative leads and leaving the positive leads apart. Like this right?
    [​IMG] but Are both the new caps the same value (25uf/25v)? In other words, are (2) 25uf/25v caps twisted together as above is a direct replacement for the single cap currently there right?
     
  14. bob-i

    bob-i Member

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    You got it. :dude
     
  15. devbro

    devbro Member

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    Thanks Bob.
    Given the picture below of V3 depicting pin 1 &6 , which are the plate load resistors you mentioned?

    [​IMG]
     
  16. bob-i

    bob-i Member

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    B and C. They connect to the high voltage.

    I'd replace the caps first, if you still have noise, then go for the plate loads.
     
  17. AL30

    AL30 Member

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    Don't forget the electrolytic on the Bias Supply Board. It's easily missed. If you've got the amp opened up you may as well replace this one too.

    AL
     
  18. devbro

    devbro Member

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    Is that the single paper cap next to the power transformer?
     
  19. danieldroukas

    danieldroukas Member

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    yep, should be. Replace it with anything between 25uF to 100uF. But I would strongly suggest you increase the voltage rating on that bias cap. I think stock the cap is 50V, replace it with something around 100V. On startup I sometimes read up to 65V on the cap, so I double the observed voltage and find an appropriate replacement.
     

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