1. Welcome to TalkBass 2014! If you're new here, we just went through a major site upgrade. Please post all concerns and bugs to the Forum Usage Issues forum. We will be monitoring that forum. Thank you for all of your feedback.

    The TB Android app is working, you may need to uninstall/reinstall. The iPhone app is now updated and should work after you upgrade. TalkBass is responsive to any screen size, so we recommend using your mobile browser for full functionality.

    Please read the TalkBass 2014 FAQ for lots of great info on the new software.

'67 Fender Showman bias oddity

Discussion in 'Amps [BG]' started by edwinhurwitz, Oct 16, 2013.

  1. edwinhurwitz

    edwinhurwitz Supporting Member

    Joined:
    May 13, 2003
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: DR Strings, SMS, D-TAR
    The amp has some mods, mostly increased filter capacitance, Mercury Magnetic output transformer, and coupling caps adjusted to let more low end through. I also have a Bandmaster which has gotten a similar treatment, along with a negative feedback adjustment pot.

    The Showman functions great, it has Philips 7581As in it and sounds wonderful. The only thing that's odd is that the bias current fluctuates as I measure it, maybe by 10-15%. The Bandmaster is dead stable. Any ideas what could cause this? I'm not noticing any real issues with the amp, in fact, it can get pretty loud and stay really clean through my Sunn 200S.

    Oh yeah, oddity #2 is that the vibrato channel is quite a bit lower in gain than the normal channel. I've tried switching out the preamp tubes, but no change. I expect some difference, due to the vibrato circuit, but this seems to be quite a bit more than I expected.

    Thanks for any ideas! I do love this amp.
  2. B-string

    B-string Supporting Member

    Joined:
    Nov 21, 2008
    Likes Received:
    25
    Have you eliminated a noisy driver/PI tube (7025 or 12AT7).
  3. Psychobassguy

    Psychobassguy

    Joined:
    Oct 14, 2013
    Likes Received:
    0
    The new OT/filter bank could be oscillating. Have you scoped the output? How are you measuring bias current?

    This makes me think "oscillation" even more. Since the vibrato is 'controlled' oscillation, it would be interacting with the (most likely infrasonic) AC on the rails and phase cancelling. What does the amp's output look like with no signal?
  4. edwinhurwitz

    edwinhurwitz Supporting Member

    Joined:
    May 13, 2003
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: DR Strings, SMS, D-TAR
    Hm, no scope. I should get one together. I certainly can't hear anything that sounds like oscillation and it doesn't seem like it's power is being sucked up by oscillation.
  5. Psychobassguy

    Psychobassguy

    Joined:
    Oct 14, 2013
    Likes Received:
    0
    I'm guessing that this oscillation is at an extremely low frequency. If the negative feedback loop to the OT still wired as stock? If so, you might want to consider lowering the feedback resistor slightly.
  6. beans-on-toast

    beans-on-toast

    Joined:
    Aug 7, 2008
    Likes Received:
    29
    As I recall, in this amp the bias circuit is connected to the tremolo. Does the warble in the bias increase or decrease as you adjust the bias? With these amps (it depends on the year after 68 they used a different bias fixed/cathode circuit), the bias setting, too hot or too cold affects the quality of the tremolo. There's a sweet spot that optimizes power tube bias and tremolo.

    You mentioned that it has a new mercury magnetics output transformer. In that amp, the OPT, choke and PT are in close proximity to each other and the tubes. Is the orientation of the MM transformer the same as the original? Is this a bigger transformer than the original? If the problem is caused by an oscillation, it could be something induced by the new OPT. It's awkward but you could try holding a copper or aluminum sheet that will act as a shield between the OPT and the tubes and see if it changes anything.
  7. Pimmsley

    Pimmsley

    Joined:
    Nov 29, 2012
    Likes Received:
    0
    Motorboat (low freq oscillation) no ?
  8. LowEZ

    LowEZ Supporting Member

    Joined:
    Mar 29, 2011
    Likes Received:
    0
    The tremolo is defeated with a foot-switch or dummy plug, right? If you just have the speed and intensity knobs turned fully counter-clockwise it might sound defeated but still be affecting bias.
  9. BurningSkies

    BurningSkies

    Joined:
    Feb 20, 2005
    Likes Received:
    11

    PBG, It's nice to see you back. :)
  10. jastacey

    jastacey Supporting Member

    Joined:
    Feb 8, 2009
    Likes Received:
    0
    +1 to that !!!
  11. edwinhurwitz

    edwinhurwitz Supporting Member

    Joined:
    May 13, 2003
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: DR Strings, SMS, D-TAR
    What's considered extremely LF? If I were to try to judge it off what the meter reads, it is probably around 2cps.

    The feedback loop is stock. I can give this a try pretty easily.
  12. edwinhurwitz

    edwinhurwitz Supporting Member

    Joined:
    May 13, 2003
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: DR Strings, SMS, D-TAR
    The amount of fluctuation seems to be constant, regardless of the bias setting. I wasn't aware of the interaction between bias and tremolo. I'll have to look into it.

    The transformer orientation is the same. It is a bigger transformer, but not as big as the previous transformer, which was actually a Hammond 1650T, which also sounded really good. Its downfall was that it was so big the amp wouldn't fit in the original case. I don't recall if it had a similar issue.

    I'll try the shielding trick, I have some copper sheeting that would be easy to screw into place, but something tells me that it's probably not the issue. The current transformer isn't physically a whole lot bigger than the original.
  13. edwinhurwitz

    edwinhurwitz Supporting Member

    Joined:
    May 13, 2003
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: DR Strings, SMS, D-TAR
    I don't hear anything that sounds like motor boating, at least what I assume sounds like motorboating, the amp sounds great.
  14. beans-on-toast

    beans-on-toast

    Joined:
    Aug 7, 2008
    Likes Received:
    29
    I think so too but it doesn't hurt to eliminate it. You can hold it in place or tape it for the purpose of a test. This allows you to move it around on the cap and choke side as well. Screwing it sounds temporary permanent.
  15. B-string

    B-string Supporting Member

    Joined:
    Nov 21, 2008
    Likes Received:
    25
    I hope you have tried a different set of output tubes?
  16. edwinhurwitz

    edwinhurwitz Supporting Member

    Joined:
    May 13, 2003
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: DR Strings, SMS, D-TAR
    Oh yeah, definitely. The tubes in there now are low hours Philips 7581As, just wonderful tubes. They had been in my McIntosh MC30s prior to this. They are very robust.
  17. AL30

    AL30

    Joined:
    Oct 15, 2009
    Likes Received:
    0
    What type of bias circuit is in that amp and how are you measuring the bias? Is the bias fluctuating on one tube or both?

    Check you power tubes, check the components leading up to (and in) the bias supply, make sure all your voltages are within spec.

    AL
  18. edwinhurwitz

    edwinhurwitz Supporting Member

    Joined:
    May 13, 2003
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: DR Strings, SMS, D-TAR
    It is a standard AB763 Fender Showman circuit. Classic.

    The bias is fluctuating on both sides of the transformer.


    Voltages all seem good. I should double check the bias cap, but it's fairly new.
  19. BassmanPaul

    BassmanPaul Gold Supporting Member

    Joined:
    Aug 25, 2007
    Likes Received:
    8
    Edwin, who did the modifications and what is the schematic version? For the 6G14 layout, the vibro circuit is not connected to the bias in any way. The schematic differs from what is used in say the Twin Reverb where an "opto-isolator" is used. In that scheme the bias voltage is used to switch the oscillator tube on or off. The current flow should by no means affect the bias voltage.

    Check the AC voltage from the PT tap and see if it varies (about 70V AC as I recall). If no, check the DC voltage at the minus of the bias rectifier. If that is stable follow the path through the components until you find where it starts to fluctuate. I would look again at the caps in this circuit. Make sure the cap positives are grounded properly. Look for stray wiring that maybe shouldn't be there. If you find any trace them to see whare they are going. All techs, I'm certain, have found miss wires from the factory in some units they have worked on.

    Good luck, keep us informed and don't touch anything you shouldn't!! :)
  20. edwinhurwitz

    edwinhurwitz Supporting Member

    Joined:
    May 13, 2003
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: DR Strings, SMS, D-TAR
    I did all the work and it's an AB763. Nothing is too radical (unlike my Bandmaster, which I did some really crazy things to before I knew what I was doing). I'll be the first to admit I'm not the world's greatest tech, but when it comes to AB763 circuits, I can at least accomplish some basic stuff and, at this point, not do too much damage and certainly not electrocute myself (I did enough of that in high school fooling around with amps).

    I'll check the voltages starting at the power supply and follow them through the bias circuit.


    Thanks to everyone for chiming in! :)

Share This Page