ongoing Bassman build issues

Discussion in 'Amp Central Station' started by 8urchevy, Mar 13, 2019.

  1. 8urchevy

    8urchevy Tele-Meister

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    Yes it's a 12ax7 . I will go through it again tomorrow. No time today unfortunately
     
  2. 8urchevy

    8urchevy Tele-Meister

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    So I was playing yesterday fumbling around with the controls and it got quiet..no I didn't touch the volume or gain knobs lol. Then cut out. I inadvertently then bumped the cable and it came back then dropped out. I'll be damned but I reflowed the input jack wiring and bam it's like a completely different amp. I'm guessing the signal was ****ty this whole time. I am still gonna rebias it to 100% when the resistors get here .
     
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  3. 8urchevy

    8urchevy Tele-Meister

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    Here is a quick recording of what sounds like with the repaired input jack wiring. Much better imo. Just audio from my cell phone which was fairly close to the cab. Sounds a bit muddy, but it isn't muddy in person. Should have the parts tomorrow to rebias it.

     
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  4. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Congratulations! Those pesky cold solder joints can be a pain to locate sometimes but glad to hear that you got it sorted out.
     
  5. 8urchevy

    8urchevy Tele-Meister

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    Thank you. Yeah I was completely baffled and there were several people here that dedicated their knowledge and time to help me sort it out. I can't thank everyone enough for that. Goes to show that there are awesome people left in this world after all.

    It was by mere happenstance that I found that issue, but nonetheless it is breathing fire now. I still need to sort out the odd voltage readings at the phase inverter, and reset the bias to a proper level for cathode biasing which will be Monday. I hope to soon do a more proper recording and sound sample/demo for you guys if anyone is interested anyway.
     
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  6. 8urchevy

    8urchevy Tele-Meister

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    Well, got it cathode biased at 83% huge difference. Slightly off on my resistor choice to hit 100% . Hopefully tomorrow I can make a real thorough voltage chart to figure out the odd phase inverter voltages .
     
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  7. D'tar

    D'tar Tele-Afflicted

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    Sweet.. Sounding better! Glad you found that!
     
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  8. 8urchevy

    8urchevy Tele-Meister

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    Ok so all my voltages are now in spec within 20% of schematic including the phase inverter at all points and intersections .

    Question. Since i only have one input and paralled v1 do I need the 270k mixing resistors?

    I am sharing one coupling cap to the gain/volume pot .Does the other half need it's own?

    Thanks fir all the help .
     
  9. D'tar

    D'tar Tele-Afflicted

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    Nice. Did you find anything to explain you previous readings?

    I cant answer this one for you. Do you have an as built schematic?
     
  10. peteb

    peteb Friend of Leo's

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    On two channel amps the mixing resistors cut the signal in half before the LTPI. Rob found he needed both resistors in his single channel amp in order to not over drive the LTPI, based on the sound. I would want more specifics. I would want to know the size of the signal. Measuring signal on the LTPI grid is a little dicey, I got a loud pop when I did, and we are not really sure what the signal should be or what the bias is. If you measure signal on the grid of the power tube, you can check on the power tubes and the LTPI at the same time. That’s the kind of information I like to know if it’s my amp.
     
  11. 8urchevy

    8urchevy Tele-Meister

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    That makes sense. I think that may be happening on my amp, overdriving the ltpi. I will try to wire that in tomorrow. This thing is a completely different beast already. So so close.
     
  12. 8urchevy

    8urchevy Tele-Meister

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    I wish I had a clue. I changed out a wire from the phase inverter that should have been a shielded cable and afterward I checked the voltages to provide you with a chart and I'll be damned if they aren't spot on. Clueless.

    Once I was satisfied I was going to draw one out. I will try to do that tonight as really the only difference is the input to V1.
     
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  13. 8urchevy

    8urchevy Tele-Meister

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    However, I do have a 270k on the wiper heading to the grid of V2.
     
  14. D'tar

    D'tar Tele-Afflicted

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    For the sake of conversation... Depending how the parallel triodes are wired mixers might not be needed. Say you bring them together before the volume pot(the stock mixers are after volume)... If your hitting the PI too hard just roll back the volue a touch. Are they sharing a plate load resistor? with a common cathode? A single coupling cap suggest a common plate load r?

    Your drawing will help.
     
  15. Sean Mac

    Sean Mac TDPRI Member Ad Free Member

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    Hi,

    The 270K value was used in early Marshalls but was raised to 470K in later versions.

    This reduced the interactivity of the two separate volumes.

    Your paralleled first stage might like a higher value than 270K.

    It's a long time since I studied this circuit but I seem to remember the second stage has a huge influence on the characteristic sound. A 470K might help you get it under control.

    Good luck with it. It's certainly been interesting to follow your progress...:)
     
  16. 8urchevy

    8urchevy Tele-Meister

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    They have their own load resistors and they are brought together at the volume pot sharing the same coupling cap.
     
  17. 8urchevy

    8urchevy Tele-Meister

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    I will try that .yeah i can be interesting to say the least lol . And I will try to upload a diagram of how I did the v1 into v2 soon
     
  18. Sean Mac

    Sean Mac TDPRI Member Ad Free Member

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    Hi,

    another thing that will influence the second stage is how the grid is referenced to ground.

    The Bassman and the four hole Marshalls use a path through two parallel pots.

    The voltage divider concept is another possibility.

    http://www.tdpri.com/threads/2104-jcm800-micro-amp.934878/

    Robrob has drawn a direct descendant here and you can see how the grid is referenced to ground in a later single channel version.

    The treble peaker is yet another possibility....:eek:

    Just to be clear, I have never touched a Bassman.

    My Superlead is wired to something very similar and sounds astonishing with both channels jumpered.

    It's a classic for good reasons :)

    Best wishes...
     
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  19. peteb

    peteb Friend of Leo's

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    I had the wrong part of the amp and the wrong signal drop, sorry.


    Like Sean said it is a voltage divider, not so much the value of the resistor but how it is used.

    Looking at the 5F6-A




    I see, either channel has this much signal left after passing it’s own mixing resistor


    270K + 500K / 270K + 270K + 500K = 75%



    Maybe it is important or maybe it’s not.


    Note, Rob added the mixing resistor in series to the grid, and then one from there to ground to take the place of the other channels mixing resistor and other resistance.
     
    Last edited: Mar 20, 2019 at 12:05 AM
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  20. 8urchevy

    8urchevy Tele-Meister

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