6100 problem

Discussion in 'Marshall Amps' started by Xoph, May 18, 2019.

  1. Xoph

    Xoph New Member

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    I brought a 6100 head out of storage after several years and I'm not getting any sound.
    I'm looking for help diagnosing it:

    - When I plug an external preamp into the line input, I get sound, so the power amp seems OK.
    - When I connect the effect send to an external power amp, I get sound and all the preamp channels/controls seem to work.
    - However, when I connect the line out to an external power amp, I get no sound.
    - So I connected the 6100 effects send to the 6100 line input, and that gives sound, but bypasses the master controls.
    - I fiddled with the effects loop knobs and buttons, thinking I just overlooked something simple, but no sound.

    Thanks for any suggestions/help
     
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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Connect effect send to effects return using a guitar cable and re-test the amp.
    And welcome to the forum Xoph.:cheers:
     
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  3. johan.b

    johan.b Well-Known Member

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    Loop in parallell mode?
     
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  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Try all modes, it sounds like the effect jack internal switch is bad.
    Plugging a cable from send to return bypasses the bad part.
     
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  5. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    :welcome: To the forum. Cheers
     
  6. Xoph

    Xoph New Member

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    Thanks everyone.
    The problems got worse (rather, a new one appeared):
    One of the power tubes blew (one of the inner ones). I pulled that tube, moved an outer tube into its position, and put the amp in 50-watt mode. Now I get no sound out except for slight hum at the speakers. The fuses that can be accessed from the back test fine with an Ohmmeter. I will check for internal fuses next. Appreciate any other suggestions.
     
  7. anitoli

    anitoli Well-Known Member

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    Was the output fail protect LED lighting up? If so you will need to replace that fuse as well.
     
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  8. Xoph

    Xoph New Member

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    No rear fuse LEDs light up. I tested those fuses by the LEDs with an Ohmmeter ; 0 Ohms.
    This is an EL34 version. Power tube plate voltages are reading around 530 VDC and heater around 3.5 VAC (relative to chassis).
     
  9. anitoli

    anitoli Well-Known Member

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    What is the rating on those fuses? they should be 500ma.

    530 VDC is high. Was this measured with the tubes out?
     
  10. Xoph

    Xoph New Member

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    Those two fuses are marked T500L.
    530 VDC plate voltage was with the tubes in.
    I get a very slight sound from the amp: Turning the volume up pretty high gives a very slight and broken up sound like the sound you get several seconds after you turn an amp off but there's still a little bit of charge. (still using a preamp into the power amp insert like before)
     
  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Did you try reseating or checking cables/connections through CON104-CON206?
     
  12. Xoph

    Xoph New Member

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    Yes checked all power and large connector seating.

    Made some progress: Something was fishy around R239, so I switched to triode mode (which does not use R239). The power amp works in triode mode. R239 seems open/fried ; don't have a replacement handy. Is there anything else near R239 that is critical to test?
     
  13. Xoph

    Xoph New Member

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    Actually I'm not sure of R239's role when in triode mode, so if someone has a more complete explanation about the triode mode working, let me know :)
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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  15. anitoli

    anitoli Well-Known Member

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    R239 was a resistor on the EL34 heads. When the amps were converted to 5881 they removed the screen grid filter cap and replaced that resistor with a link.

    530 is a tad high. take out all tubes and measure the voltage on pin 3 all sockets, what are you getting?
     
  16. Xoph

    Xoph New Member

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    Still 530V with tubes pulled.
    This is the EL34 version, with a resistor. Biased for 36mA. Stable in triode mode.
     
  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I was wondering if R239 was the extra grid resistor.

    The earlier stuff has nothing to do with power tubes. So what is the deal?
     
  18. Xoph

    Xoph New Member

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    > The earlier stuff has nothing to do with power tubes. So what is the deal?

    A tube+resistor blew and distracted me from the original problem, where I think it acts stuck in serial effects mode. Thought I better first make sure nothing else is ready to blow. Might go check out the switches and clean the scratchy controls now that it looks like triode mode is OK.
     
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  19. Xoph

    Xoph New Member

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    The whole amp works now that I replaced R239 and poked around at the series/parallel switch several times.

    I still have comments/questions.
    - Bias: I'm getting -43V at the pot but the schematic suggests -50V. I've got a plate voltage of 490V now and cathode current of 36mA (only measured one of the tubes). This seems OK to me for a hot biased EL34, but I don't know about the 7 V difference at the pot. Anything to watch out for there?
    - Why is there an extra resistor coming off V11 control grid pin (5) on these amps?
    - Has anyone confirmed that 4 Ohm cabs are supported properly by swapping black and green OT wires? Another thread speculates about affecting negative feedback, but it is beyond my skills: http://www.marshallforum.com/threads/6100-speaker-out-swap-to-4-ohm.77143/

    FYI, plate voltage dropped a bit (by about 12 V) when I switched back to high power mode, but I don't know how it went back down to 490 (maybe related to R239? Replacing that brought the screen voltage (g2) back up -- it was far too low before, in pentode mode). The parts are still original, so possibly something is unstable or drifting off spec.

    Thanks to everyone who gave input.
     
  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    You are good to about 41mA at that plate voltage. 36mA aint hot. You should measure all of them to know what is going on.

    Can you give a picture of R239? What are the stripe colors on the component?
     

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