763 Blackvibe distorsion

Discussion in 'Shock Brother's DIY Amps' started by Baol1969, Oct 1, 2020.

  1. Baol1969

    Baol1969 TDPRI Member

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    Hi everyone,
    I'm having an issue with my new built RR763.
    The amplifier starts to distort at a very low level, between 1.5-2
    The signal (a 0.2V 5000Hz sine) is clean in V1A grid and plate, V1B grid and plate until after the .047 coupling capacitor, from then on the matter gets complicated for my skills because the signal is affected by both pre volume and master volume (trainwreck type-3) or NFB. I also disconnected the NFB (and the master volume too) but nothing changed.

    Anyway, this is my voltage chart:

    B+ : A 510V, B 508V, C 501V, D 486V

    V1a: plate 327V, grid 0V, cathode 2.35V
    V1b: plate 324V, grid 0V, cathode 2.40V

    V2a: plate 273V, grid 71V, cathode 115V
    V2b: plate 271V, grid 76V

    V3: plate 513V, grid -61V, screen 513V
    V4: plate 514V, grid -61V, screen 514V

    387VAC from the PT (Hammond 290D2EX) 218 after the 1N4007 diodes.

    Any advice, help or indication is very welcome.
    Thank you.
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Welcome to this forum Baol1969!

    Could you provide us with the schematic of your amp? And we love pictures; in fact, we need pictures :)
     
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  3. Baol1969

    Baol1969 TDPRI Member

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    Tom you're right.
    It's the Rob Robinette's Blackvibe RR763_BLACKVIBE_Schematic_small.png
     
  4. Baol1969

    Baol1969 TDPRI Member

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    IMG_20201002_000337.jpg IMG_20201002_000400.jpg IMG_20201002_000431.jpg
     
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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    How does a guitar sound through it?
     
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  6. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Welcome to the forum!

    Your voltage readings seem very high to me, even if your using diode rectification.

    Can you confirm all your readings, especially the very high plate voltage readings and -60V on your 6L6 grids?

    What brand of power tubes are you using?

    I see your first filter cap is rated for 350V. Maybe some of your parts are not fit for such high voltages.
    Edit: Just saw you are using two in series for the load cap, so that doubles the rating...what are your other electrolytic caps rated for?
     
    Last edited: Oct 2, 2020
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  7. Baol1969

    Baol1969 TDPRI Member

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    well, the sound was great when I tried at home (low volume), than I took it to rearshal, with another cabinet and I had like a motorboat noise that was possible to control lowering the master volume.

    yes, maybe I can take 'em one more time but this is what I read yesterday
    the grid current could be controlled with the bias pot

    tung sol rectifier and pre, jj for 6L6

    I followed the BOM directions

    22uF 500v
    70uF 350v
    50uF 150v
     
  8. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Ok, your caps could propably handle the voltage.

    I still think your voltages are way higher than they should be.
    I ran the PSUD2 program and came up with a B+ of 440V, which is pretty much the same as the Bandmaster AB763 schematic shows which also uses bridge recitfication and your transformer is listed as a Bandmaster type PT.

    Maybe you can check the wiring of the PT primary and seconday windings or give us a closer picture. Are you sure you connected the primary side right? What AC reading are you getting on the primary side?

    And where are you located? What's the wall voltage?

    Bildschirmfoto 2020-10-02 um 10.31.50.png

    Bildschirmfoto 2020-10-02 um 10.35.35.png

    Bildschirmfoto 2020-10-02 um 10.37.18.png
     
    Last edited: Oct 2, 2020
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  9. Baol1969

    Baol1969 TDPRI Member

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    I'm in Italy, wall voltage is 220VAC .
    Now I'm not at home and I can't actually check it but I should have joined WHT/BLK and BRN wires and took WHT and BLU to power switch.
    Should I try to connect it as for 230VAC or more?
     
  10. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Ciao!

    I live very close to you....wall voltages in Austria are more like 230-235VAC, probably the same in Italy.

    Connecting for 230V or even better 240V should get secondary voltages down a bit since you are using a smaller winding ratio. (More windings on primary side > less voltage on secondary side)


    1) BLK/BRN together (for 120+120V=240VAC)

    2) Hot wire of power chord (usually brown) > fuse > power switch
    > WHT

    3) BLU >neutral (usually blue) wire from powerchord.

    4) Green/yellow from powerchord to a seperate connection point on chassis

    5) ORG (shield wire) to ground
     
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  11. Baol1969

    Baol1969 TDPRI Member

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    The "official" voltage here in Italy is 220VAC, but changin the PT connections the voltages now are better:

    B+
    A 467
    B 466
    C 457
    D 444

    V1a plate 304, cathode 2.1
    V1b plate 303, cathode 2.1

    V2a plate 253, grid 64.5, cathode 103
    V2b plate 250, grid 68.9

    V3 plate 465, grid -56
    V4 plate 465, grid -56

    V5 351 from the PT and 198 at the 1N4007 diodes


    I haven't listened yet but the curve seems a little better.
    I notice that disconnetting the NFB the distortion decrease, I can rise a little more pre volume before the curve begin to show a distortion, shouldn't the NFB have the opposite effect of decreasing saturation?

    Edit
    I tried with the guitar but it's only playable at low volume, as you strumm a little bit harder the amp farts. Maybe I made some mess with the input jack (I used only one jack with a switch for high and low), later I'll try to bypass it, I got no other idea
     
    Last edited: Oct 2, 2020
  12. andrewRneumann

    andrewRneumann Tele-Holic

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    Hello,

    Check the second supply dropping resistor. The photo is a little fuzzy, but it looks like you have a 470R where you should have 4700R (4.7K). The difference between a red and brown stripe can sometimes be hard to distinguish.

    Good luck, hope this helps.
     
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  13. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Your voltages now look much better.
    The preamp plate voltages are still a bit high, but then you are only using 2 preamp tubes instead of 5 in a regular AB763 type amp...

    But there must be some other problem. Like Andrew suggested, check all your connections an values...
     
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  14. Baol1969

    Baol1969 TDPRI Member

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    Andrew, unfurtunately it's right, 4.7k (yellow, purlple and red). Do you say this because it seems to you that the values are still high? In fact D (B+4) is 34 V higher...
    but can this affect the low headroom?
     
  15. andrewRneumann

    andrewRneumann Tele-Holic

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    That’s why I looked there, but your voltage drops in V1A/B look consistent. Maybe Rob’s voltages are little low.

    Usually higher supply voltages increase headroom, but it’s one big system so who knows what the final result is?

    A lot of people have trouble with that Type 3 Master Volume post-phase inverter. You say you disconnected that and it made no difference?
     
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  16. Baol1969

    Baol1969 TDPRI Member

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    now that i fixed (thank you Tweedy!) the main voltage issue I tried to disconnect again the NFB and there are no "farts" when I hit hard the strings.
    I don't know if the problem comes from the NFB or from the Master Volume, asap I'll try to disconnect that too
     
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  17. trancedental

    trancedental Tele-Meister

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    Have you tried the 610v taps yet, the 290D2EX is usually more expensive as it has dual 710/610V options, the cheaper 290DEX has a single 650V tap but runs on 240VAC mains voltage. so would probably run lower secondary voltages in Italy.

    230VAC Power Transformers always run higher VDC in UK / Ireland. I've measured my mains voltage going past 255VAC late evening. :eek:

    Next day it's back down to 246VAC. o_O

    Best to take readings daytime, evening & weekends, check your mains VAC voltages as well.

    Glad it's working fine now.
     
  18. Baol1969

    Baol1969 TDPRI Member

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    Yeah, now I setted to 240Vac and all the values are right. Now I got to find out what's wrong in the NFB are (if is there).

    Thank you!
     
  19. D'tar

    D'tar Friend of Leo's

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    Positive feedback can present in different ways. Disconnect NFB if you have less distortion without it you may try swapping the OT primary wires at the power tube plates to correct the phase.
     
  20. Baol1969

    Baol1969 TDPRI Member

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    Great! It seems that was the matter.
    Inverted the OT primary and re-soldered the NFB, now it works.
    At home I can't push it hard but in the next days I'll try it.
    Thank you man.
    Thank you all!
     
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