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Biasing issues, JVM Satriani

Discussion in 'The Workbench' started by Karl Brake, Jul 16, 2021.

  1. Karl Brake

    Karl Brake Active Member

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    Strange conundrum. I put new power tubes in my beloved Satch a year ago...sensed that I seemed to need higher volume settings with the band. Figured I needed a rebias. Opened it up, and it was way low. I put in a new matched set to be sure, but the pots top out at about 40 ma per pair. It sounds fine, everything functions, no burned or discolored parts, amp is quiet and sweet toned as always. It's a 2016 model I bought used but mint a couple of years ago. I'm baffled...techs are hard to find around where I live. Am I crazy or missing something simple?
     
  2. yladrd61

    yladrd61 Well-Known Member

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    Try a different set of tubes.
     
  3. RLW59

    RLW59 Well-Known Member

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    There's a resistor in the bias circuit that can be changed to alter the range of the bias pot.
    -----------------------
    This section of the Marshall Forum is for general discussion about Marshalls. Generally soft topics -- "which Marshall is right for me", "this Marshall vs that Marshall", buzz about new models, things like that.

    The amp techs don't always check this section. They hang out in the "Workbench" section. Questions about malfunctions/problems with amps will get more answers with better info in that section.

    (I have no idea exactly which resistor needs to be changed, nor whether to go for a higher or lower value.)
     
  4. Karl Brake

    Karl Brake Active Member

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    Thanks. I did. Same thing.
     
  5. Karl Brake

    Karl Brake Active Member

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    My beloved Satch 410 suddenly doesn't allow me to bias higher than 40 ma per side. I've had this 2016 model for some time, changed and rebiased with no problem. I noticed the amp seemed to be lacking some punch, so I checked bias. Way low...tried two sets of tubes, same thing. Has me baffled. With the raised but too low bias, it sounds actually great, but it's weird. No strange noises or weirdness. Any thoughts out there? Decent techs have become scarce in my locale. I know a resistor can be changed to affect bias range, but I don't know why it was normal, then not. Do these kind of resistors, like, fade in function over time? No mods on the amp, excellent condition, been in quarantine with me, so hasn't been pushed or abused. A puzzlement.
     
  6. Pete Farrington

    Pete Farrington Well-Known Member

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    Maybe your meter needs a fresh battery?
     
  7. Karl Brake

    Karl Brake Active Member

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    That would be just like me. The meter lights up and seems to function normally, but could that actually affect the reading?
     
  8. Pete Farrington

    Pete Farrington Well-Known Member

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    Sure.
    Often a battery symbol will appear on the display, but it’s not universal.
     
  9. Chris-in-LA

    Chris-in-LA Well-Known Member

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    You may need to test the resistors in the bias circuit to find out why you’re having this problem.
     
  10. Karl Brake

    Karl Brake Active Member

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  11. Chris-in-LA

    Chris-in-LA Well-Known Member

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    But, you already knew that. Do you have a schematic that shows where the bias circuit is?
     
  12. Karl Brake

    Karl Brake Active Member

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    No. And the Satriani is hard to find...not the same as the standard. I know very little, not a tech. I'll have to drive it over to the nearest guy. Honestly I've never tested a circuit...just had the now retired Marshall certified guy in my area teach me how to bias. I'm very conservative about touching the guts lemme tell ya.
     
  13. mickeydg5

    mickeydg5 Well-Known Member

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    It has a separate bias adjust for each side of the push-pull. If all he gets is 40mA on both sides with several tube sets then the issue is most likely not the bias monitoring system.

    @Karl Brake Are you using the bias measurement points on the amplifier?

    Are you adjusting both trimmers because you did not mention two? One can affect the other if way off balance.

    Make sure the multimeter is setup correctly as well.
     
  14. spacerocker

    spacerocker Well-Known Member

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    Try your amp with just 2 power tubes connected at a time.

    Try 1 & 4 Fitted (2 and 3 removed)
    2 & 3 Fitted (1 and 4 removed)
    2 & 4 Fitted (1 and 3 removed)
    1 & 3 Fitted (2 and 4 removed)

    What bias readings to you get with these configurations? The amp should work as a 50W amp with any of these configurations. However DO NOT run the amp with just 1 and 2 fitted, or with just 3 and 4 fitted!

    Let us know what bias figures you get?
     
  15. Karl Brake

    Karl Brake Active Member

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    Yes to all of the above. It has two pairs. I've biased different amps, including this one, normally a number of times. I biased this one at 70 ma per side/pair, as measured from the 200mA setting on the meter a year ago. Using both pots and the two bias points and the ground point. I've had amps have problems before, but this one is quirky.
     
    Last edited: Jul 17, 2021
  16. marshallmellowed

    marshallmellowed Well-Known Member

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    I never had any issues with the bias range on my HJS. Are you sure you're measuring it correctly? Where are you placing your probes when checking the bias?
     
  17. RLW59

    RLW59 Well-Known Member

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    What brand/era of new tubes did you put in?

    Supposedly, the final batches of SED Winged "C" had a lot of duds. Many tube vendors are selling them at high prices based on SED's previous reputation and the fact that there will never be any more new SED's.
     
  18. Karl Brake

    Karl Brake Active Member

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    I tried new JJs and Tung Sols. Depressing. It's been stone cold reliable thus far, and it's my go to amp. Gotta visit a tech in the next town. I'm too nervous to try the 2-tube at a time test outlined earlier. I appreciate your input y'all.
     
  19. Karl Brake

    Karl Brake Active Member

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    Black probe on middle post, red on either side one at a time. Clipped the black lead to the chassis, same reading.
     
  20. marshallmellowed

    marshallmellowed Well-Known Member

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    OK, had to ask, you never know. Are both sides measuring 40mv max, or just one side? Also, did you have a recent power tube failure? If it was fine before, and you've had no tube failures, have you verified your meter is reading correctly?
     

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