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AA764 Tremolo Depth

Discussion in 'Shock Brother's DIY Amps' started by Linkjr, Sep 14, 2020.

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Does the higher voltage on the V1 help the trem at all? It may or may not.
     
  2. Wally

    Wally Telefied Ad Free Member

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    Linkjr, imho and in agreement with lowerleftcoast’s post #16, you need to do a fresh voltage chart fro: one end of the amp t9 the other so as to keep simple minds such as mine on track....easily confused I am. This needs to be done before anything is changed. I for one have to do these things in some type of systematic manner or I get lost.
    for instance, your first voltage check gave us plate voltages on V1 but the cathodes read zero???. Now, you give us two cathode voltages but no plate voltages for V1. IF the canoe voltages actually changed from zero to 1.4 and 3.1, then the plate voltages Had to have changed. If the plate voltages did not change, then it is very likely that your first measurements for the cathode voltages were I; error. As Lower said,you need to recheck the voltages. That means all voltages, ime and imho.
     
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  3. Linkjr

    Linkjr Tele-Meister

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    @Wally working on it, yeah lets get our evidence together and divide and conquer.

    @Lowerleftcoast Unfortunately I did hear it with the extra 10k and it sounded weaker on first try but now i can barely hear it at all with or without the spare 10k across the existing one.
     
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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    We have more things to try. Let's get the voltages first.
     
  5. Linkjr

    Linkjr Tele-Meister

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    @Wally @Lowerleftcoast thanks guys for getting us this far here below voltages I remeasured this evening making sure I reset to read the lower ranges V2 pin 2 seemed to be fluctuating.

    All figures in vdc unless stated

    V1 12ax7
    1 151
    2 0.3
    3 1.42
    4&5 heater
    6 204
    7 1.0
    8 2.2
    9 heater

    V2 12ax7 middle
    1 123
    2 25.5 fluctuating
    3 1.62
    4&5 Heater
    6 328
    7 122
    8 58
    9 heater

    V3 6v6
    1 -
    2 Heater
    3 325
    4 328
    5 .045
    6 -
    7 Heater
    8 20.6

    V4 5y3
    1 -
    2 340
    3 -
    4 290vac
    5-
    6 289vac
    7 -
    8 334
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    How about replacing the 470k on V2 with one of the spare 100k resistors.

    If you have a red LED, use it to replace the V2 4k7 cathode resistor and the 25uF cap. The short leg of the LED goes to ground.

    If you do not have a LED try to get the resistance of the 4k7 cathode resistor down to about 1k to 1k5. If nothing else put a 10k parallel to the 4k7. Keep the 25uF cap in place when the resistor is there.
     
  7. Linkjr

    Linkjr Tele-Meister

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    Ok have ran parallel a 10k to the 4k7 unfortunatly no change to the trem , there seems no trem at all.
    And have replaced the 470k on v2 with a 100k no change
     
  8. Linkjr

    Linkjr Tele-Meister

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    Ok guys were getting somewhere!

    In this current config i have a 750ohm bias resistor i swapped it whilst contemplating and have the 100k on v2, i tried another tube in v2 as @Guy Guyatone recommended back in #5 i must admit i didnt try this as my tubes were straight out the box new seems he was right!

    I have got trem back maybe too much as beyond 5 intensity it stops and also volume is very weak and lo fi.

    I'm hoping putting back to stock will resolve the current situation.
     
  9. Linkjr

    Linkjr Tele-Meister

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    Ok it's back with a 500ohm bias resistor and 470k on v2 and back to where this started.

    Thanks for this advice, I have done this and the tremolo intensity sounds a bit better, I can see the LED faintly glowing and flickering.

    Would intensity improve further if I was to put back to stock and put a 10uf cap across the 4k7 instead of the 25uf?
     
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I don't think so. The 10uF would cut off some of the lower frequencies which may not be helpful here. IIRC the valvewizard site recommends the 100k plate resistor and the LED cathode to give the thickest modulation. Perhaps the forward voltage of this particular LED is making a difference.
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I read through valvewizard.co.uk/trem1.html. When using the cathode cap and resistor, he suggests a larger ~100uF bypass cap to get the oscillations started. He says the oscillation starts in the very low Hz range (~6Hz). He says the oscillation takes some time to be at full strength.
     
  12. Linkjr

    Linkjr Tele-Meister

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    @Lowerleftcoast I also tried another fix for this particular problem by replacing the 68k in this circuit for a lower value of 47k. trem intensity greatly improved but trem intensity on full and not engaged resulted in v low volume.
    your suggestion did not adversly affect the volume.
     
  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Do any of the attempts get V2 pin voltages anywhere near the schematic voltages?
     
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  14. Wally

    Wally Telefied Ad Free Member

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    okay, so....what is the function like now? “Back to this started” is vague for me. Does the amp work Correctly with the trem switched Out? Is the interaction still a problem as it was in the very beginning? Are your voltages still the same as they were on Monday? If so, why is the V2P8 voltage so low?

    yes, this is the way it is....systematic. With every change, one wants to knows voltages and one wants to observe the changes in operation.

    and...I started this post before a customer came in and before Lower’s observation.
     
  15. Linkjr

    Linkjr Tele-Meister

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    @Wally

    Yes the amp sounds great with the tremolo switched out, If I understand correctly I'm looking for 175vdc on p8 of the v2 12ax7 whereas I had only 58vdc
    Yes I was judging only on the sound and not being more scientific in my approach shame on me!

    Voltages of amp in current state to follow
     
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  16. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    Once you’ve gotten the voltage situation figured out, if you feel like the trem is still too weak, you can lower the value of the 68k resistor.
     
  17. Linkjr

    Linkjr Tele-Meister

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    All figures in vdc unless stated


    V1 12ax7

    1 155
    2 0
    3 1.4
    4&5 heater
    6 264
    7 0
    8 3.46
    9 heater


    V2 12ax7 middle

    1 144
    2 0.006 fluctuating
    3 1.50
    4&5 Heater
    6 328
    7 141
    8 154
    9 heater
     
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  18. Linkjr

    Linkjr Tele-Meister

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    @Dan_Pomykalski
    Thanks for that , i did try using a 47k before which i read on another forum before i put the led in but it gave v low amp volume with trem switched off and intensity on full
     
  19. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    Also now that I think about it, I don’t know if this actually increases the strength of the trem anyway... I think it might just cause the intensity to take effect sooner...
     
  20. Linkjr

    Linkjr Tele-Meister

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    No it definetly increased the depth of trem in mine at the expense of useable vol with it disengaged and on full intensity
     
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