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JTM45 with EL34's, OT, choke questions

Discussion in 'Shock Brother's DIY Amps' started by Jason McG, Aug 1, 2020.

  1. Jason McG

    Jason McG TDPRI Member

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    my grid stoppers are 5.1k...
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    What is the pF rating on the snubber cap. (The cap between the PI plates.)
     
  3. Jason McG

    Jason McG TDPRI Member

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    47pf
     
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    That snubber cap allows very high frequencies to cancel across the PI. It discourages ultra high frequency oscillation.
    Using grid stoppers on the power tubes discourages high freq oscillation as well.

    What I have been envisioning... a possible problem with an oscillation above our hearing range. The tubes working overtime oscillating is the underlying factor and it presents in the audio range as that buzz we are hearing.

    I would hope 5.1k grid stoppers would be enough. That 47pF cap may not get the job done though. Can you clip in another say 100pf across the 47pF to raise that value to ~150pF? If you went higher than 150pF it may cut some audible highs but if it would stop the buzz at least we would know the tubes were likely oscillating.
     
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  5. Jason McG

    Jason McG TDPRI Member

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    added the extra cap, and put the 27 NFB resistor back in circuit, unfortunately the buzz is still alive and well.
     
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  6. Jason McG

    Jason McG TDPRI Member

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    I see some JTM50 layouts tapping the 8ohm OT secondary for NFB, wonder if that would make any difference?
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The different OT secondary taps would require a different resistor to be equivalent. Once equivalent there should be no difference.
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    As I said I am grasping at straws. Running out of ideas.

    I know we have been through this but I still wonder if a tube, or ground issue is behind the buzz.

    Looking back at the thread I notice the proximity of the OT to the PT. ???

    Possible winding trouble with OT?

    Perhaps a small cap from plate to cathode on the first tube?

    I think a positive feedback was ruled out. (Swapping wires on the OT).
     
  9. Jason McG

    Jason McG TDPRI Member

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    it's very quiet, otherwise, if the OT and PT were clashing, wouldn't it be all the time?

    I did get the mother of all foghorns when i first started it up, so then switched the OT leads, and then it was good, so think I can rule that out.

    I'm actually very happy with it with the larger resistor on the NFB lead (landed on 100k), cleaned it right up, so think I'll stop tinkering and enjoy the amp now.

    Just would be nice to understand why the standard 27k NFB resistor isnt working with my set up.

    Thanks for all the help and suggestions!
     
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  10. NTC

    NTC Tele-Meister

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    First of all, this won't solve your buzz problem. I am a bit unclear as to what schematic you are using, but I assume it is the 5F6A Bassman. The Bassman has a 2 ohm output, not 16. Everyone talks about the impedance reflected across the OT, but we are feeding back voltage. Output voltage on that 16 ohm tap is 2.8 times higher than for a 2 ohm tap on the same transformer. You want the feedback voltage - the voltage on the 5k resistor - to be the same for your design using the 16 ohm tap. Without showing all the math (too hard on the phone), the "correct" value is 85.5k ohms. Use the higher value that is closest. You will probably end up at 100k with 10% resistors.

    Are your screen grid resistors values that you find in an EL84 amp? Have you tried 6L6's?
     
  11. NTC

    NTC Tele-Meister

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    I missed the 5.1k grid stopper resistor above...
     
  12. Jason McG

    Jason McG TDPRI Member

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    Thanks NTC, I followed the Stewmac layout, but sourced all my own parts. They call it a 62 BritPlex, but it seems more like a late 60’s JTM50 from what I’ve read on Marshall history.

    My transformers and choke are close but not exact to the kit versions, so possibly that accounts for some of the oddities.

    I am keenly interested in the math that gets me to the proper resistor value, and appreciate your advice here! My ear got me to 100k, but I was just listening for where the buzz was gone throughout the entire sweep of the presence pot, but I’m still unclear why the buzz exists at all with the lower value resistor. If you can steer me how to check those voltages, that would be greatly appreciated!

    I’m just about to carry on with my cabinetry, boxing up the head, while I soak on the electronics puzzle a bit more.

    33D15EBB-31D1-42D0-8CD1-A6D5A2B8C999.jpeg
     
    Last edited: Oct 17, 2020
  13. Jason McG

    Jason McG TDPRI Member

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    .
     
  14. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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  15. Jason McG

    Jason McG TDPRI Member

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  16. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    :)

    Be careful what you ask for, you might just get it.
     
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  17. Jason McG

    Jason McG TDPRI Member

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    All finished, and rounds out the tweed collection!

    87457DA5-5F1C-4DF0-A49F-CBB09E0FBF8B.jpeg ED4A2FF3-4A3E-442F-8582-04B04CF38FFC.jpeg 3C808E41-B2F2-4A81-8B13-23DD52E628B1.jpeg
     
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  18. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    The Daltons
     
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