SF Fender Twin help

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Rich M, Dec 28, 2005.


  1. Rich M

    Rich M Member

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    O.K., I've been going through this amp for about 2 weeks now and it is making me nuts. I replaced all the electrolytics and did a blackface conversion and I think that helped the tone . I also removed the MV and put a PPIMV in there. The remaining issue with the tone is this kind of buzz overtone over loud signals,clean or dirty. I changed the tubes, cleaned up some ground wires (what a rats nest!) Nothing. I can't even seem to make it worse. I can tell the amp would sound great if I could get rid of this and I really want to figure this out, but at this point, I'm starting to look longingly at my Superbass on a chair waiting patiently for me. I read G. Weber's book to try and understand Fenders. The only posslble thing that I can find to try is increasing the swamp resistors on the OTs (they are 1.5K). Will this help. Man, I just want this to end ! Please help!
     
  2. Shea

    Shea Member

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    Maybe it's biased too cold and you're getting crossover distortion?

    Also, does this happen when the is mv turned up fairly high? If the pots aren't matched very well, then that could cause some buzziness at very low settings.

    Shea
     
  3. Rich M

    Rich M Member

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    Tubes are at ~37 mA. Buzziness even with MV all the way up.
     
  4. Rich M

    Rich M Member

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    I would thing crossover distortion would be worse on quiet passages when the signal itself is smaller. If you bang a chord it goes away once the signal decays a bit.
     
  5. mr coffee

    mr coffee Gold Supporting Member

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    you didn't go all weber/torres and change values of caps around did you?

    what kind of preamp tubes are in there..
     
  6. slider313

    slider313 Silver Supporting Member

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    Check all your grounds on the input and pots. If you removed the shielded cable from the preamp tubes to the input and volume pots, put them back. The grounding is not going to be complete and parasitic osolation is common. Also if you converted the bias supply from fixed/cathode to fixed,go over your connections and make sure the resistors and the cathode bias cap have been removed from the circut. 37 ma. should be about 440 plate volts
     
  7. Rich M

    Rich M Member

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    Input cap was changed to 0.001 from 0.01, did this buzz before/after change. I have standard complement of tubes, tried an 12AX7 in PI, didn't change buzz. Haven't changed other caps, checked bias points - all seem o.k..

    Slider,
    No monkeying with shielded inputs. No fixed/cathode removal necessary (I think it's a '72).
     
  8. Swarty

    Swarty Member

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    Are you sure it is not a speaker or something buzzing on the cab? Try an extentsion speaker and put it far away from the amp.
     
  9. Rich M

    Rich M Member

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    Tried it both in combo box and into a Marshall 4x12 w/ Weber MASS.
     
  10. AL30

    AL30 Member

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    Did you swap out all of the 100K plate resistors? These are common sources of noise. Also, check for drifted resistors. I have a '72 and a few of my resistors had drifted out of spec.

    And you are correct - once it's up and running these are fantastic sounding amps - don't forget the Tube Screamer.

    AL
     
  11. Rich M

    Rich M Member

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    I swapped a couple (V1) but gave up. I'll try that as I have some nice carbon comp 1/2W. I will check for drifted resistors as well. I thought it sounded great with my TS, but the cleans in particular had this annoying overtone. Thanks for the tips, guys. I'll report back.
     
  12. slider313

    slider313 Silver Supporting Member

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    Does this happen in the normal channel as well? It sound like it could be a cold solder joint on a cathode bypass cap. I check by hitting the eyelet with the iron and if the connection lifts, it should be redone.I use a solder vacuum and clear the eyelet and make sure there's enough lead for a lift free joint.
     
  13. Rich M

    Rich M Member

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    Yes. Will check the joints.
     
  14. Wakarusa

    Wakarusa Member

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    Don't forget to check the screen resistors while you're in there. Another common problem with these are weak/dry solder joints on heater wires, particularly if someone has been inside moving things around.
     
  15. mr coffee

    mr coffee Gold Supporting Member

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    without the MV? what year is the twin.. if it's a pull boost one there is some other stuff in the verb circuit that needs to be straightened around also. did you check the mix resitor at the reverb ?
     
  16. Rich M

    Rich M Member

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    Hey Todd, been trying to get a hold of you re: that back panel. I will get off my lazy ass and change the screen resistors. One was smoked REALLL good when I got it. Come to mention it, the insulation on one of the heater wires was damaged, probably from THE FLAMES JETTING OUT of that resistor. Maybe replacing that section of wire could be it. Year? I think someone said '72 (Todd?) It had the pull MV. What the hell does that thing do anyway. THe schematic says pull for "pure sound". What reverb stuff needs checking? I should mention, I have the reverb and tremolo turned all the way down and the tank disconnected with the chassis up on the bench. Well I go do a little soldering now and get back to youse. Thanks!
     
  17. Wakarusa

    Wakarusa Member

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    Awww crap. My humble assistant said the panel was taken care of, yet I see it tucked away in the corner over there. I'll get it out. Next year we don't suspend the periodic assistant beatings for the Holidays ;)
     
  18. Shea

    Shea Member

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    I really doubt the reverb circuit has anything to do with it. Yes, the reverb circuit is a little different from blackfaces, but that doesn't cause buzziness, unless you somehow rigged it so that the pull boost is on all the time. But with the pull boost off, there should be no buzzing and the reverb should sound fine. It does in my mv twin.

    Shea
     
  19. Rich M

    Rich M Member

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    Todd,
    Before you launch the panel, are you sure that it is "wing-shaped" in that era amp? The chassis looks similar to this :
    http://boston.craigslist.org/msg/121632022.html
    I'm concerned the raised end will interfere with the power chord.

    I changed the screen resistors - no change. One thing that has me concerned is that the grid of V4B (reverb mixer?) has about 100 mV on its grid. On the other side of the 3.3M (between the 3.3M and the 0.022 uF coupling cap) I get ~660mV. I initially thought this was the problem, but since these nodes are leaked to ground through 3.3M + 470K, this is a very small current and I thought more reflective of poor design. Is this incorrect? Is the 0.022 too leaky?
     
  20. Rich M

    Rich M Member

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    This works out to an equivalent series resistance of the 0.022 of 1.3G ohm also, it wouldn't explain that same buzziness on the Normal channel (I think).
     

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