Questions on variations of the blackvibe circuit

Discussion in 'Shock Brother's DIY Amps' started by zorgzorg2, Sep 19, 2020.

  1. zorgzorg2

    zorgzorg2 Tele-Meister

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    Hi all,

    I'm starting ordering parts to build the excellent Blackvibe by @robrob, but I have some questions, and figured I would as you, dear amp gurus :)

    First about the output transformer.

    The one I ordered is a Hammond 1760J https://www.hammfg.com/part/1760J (being in Europe doesn't make it easy to order the recommended parts, sorry) and it has multiple possible impedances (4, 8 and 16 Ohms). Because of this I'm thinking of having multiple jack outputs for being able to switch cabinets with possibly different impedances.

    So I was wondering if connecting the three NFB from the 3 jacks would work, or if I need a switching system to make sure that only the NFB loop of the connected speaker connected?

    Another question to avoid the problem when there isn't a load on the OT (no speaker connected), why isn't a switched jack used with a resistor coupled when no speaker is connected? Or is shunting to ground enough and safe?

    Thanks in advance!
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Nope, just connect the NFB to the appropriate jack only, regardless of which jack is in use. The signal (voltage) is always present on the jacks.
     
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  3. andrewRneumann

    andrewRneumann Tele-Holic

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    I think you could use a resistor... that would be no different than a dummy load. It would have to be a high wattage wire wound type. But I think traditionally just grounding it is done because it’s simpler and it works, as long as the amp isn’t played for a long time in that condition.
     
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  4. EmWeAh

    EmWeAh Tele-Meister

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    On my last Blackvibe 6L6 built I used a 1760J OT as well. Since it was supposed to end up sitting in a head shell, I added the mentioned safety load resistor in parallel to the output speaker jack (470R 5W). I made all three impedance taps switchable using a APEM SP3T switch (short poles A1 and B5 for three selectable inputs and one fixed output with hooked up NFB).

    IMG_20200919_231827.jpg
     
  5. schmee

    schmee Doctor of Teleocity Silver Supporter

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    THIS. Whatever the amp was designed for (ie: 8 ohms, 4 ohms) connect the NFB to that one.
     
  6. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    The 1760J is a great transformer for 2x6L6!

    I‘ve used it twice so far and both amps sound really good.

    I‘ve also built an amp with a blackvibe type circuit before so let me know if you have other questions...
     
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  7. zorgzorg2

    zorgzorg2 Tele-Meister

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    Wow, thanks all for the answers!

    Ok, so I will go with a switch to change the impedance, and put a load instead of shunting to ground when the speaker isn't connected (going for a head too).

    So, from there more questions:
    - Using a switch, I have the possibility to have a different NSB resistor for each impedance. What should 4 and 16 ohm NSBs be? Or should I just use the 820 ohm for all?
    - Is it ok to leave the other impedance wires disconnected when one of the impedance has a load?
     
  8. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    If you connect the NFB resistor directly to one of the transformer taps (and not the side of the switch that goes to the tip of the speaker jack) your NFB signal will stay the same even if you choose a different output tap with the switch. Unused transformer taps do not have to be connected to a load.
     
  9. EmWeAh

    EmWeAh Tele-Meister

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    Yes, @tweedy_woodpecker is absolutely right and other have also already mentioned it. This is the correct way to do it. Basically in my picture above you just have to move the yellow NFB wire from the speaker jack tip to the 8 Ohm OT tap (in my case) on the switch. At the time of the build I accidentally wired it up like so and was too lazy to change it then, but will do it next time I have the chassis removed from the cabinet.

    The way it is wired in my picture it will change the amount of NFB induced into the circuit depending on the selected speaker impedance. Doesn't have to be a bad thing, but not the correct way to do if you want it to stay the same...
     
    Last edited: Sep 20, 2020
  10. zorgzorg2

    zorgzorg2 Tele-Meister

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    Ok understood. Here is the schematics implementing the changes: RR763_BLACKVIBE_Schematic_mod.png

    That's a load resistor for the speaker jack and a switch for the impedance selection, with the NFB connected to the 8ohm tap.
    Right?
     
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That looks good zorg. I typically use a 10 ohm resistor for the load.
     
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  12. FenderLover

    FenderLover Poster Extraordinaire

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    Hey @robrob. Not to derail this thread (sorry @zorgzorg2 ), but a cool variation on the theme might be a single channel '62 Vibrolux on a 6G2 chassis. Just punch a hole for one more 12AX7. An iconic rarity in a tidy package. The Sultan. Six String Swinger. Or a BrownVibe (that actually has a Vibe). ;)

    edit: this month's VG is featuring the '62 Vibrolux, and got me thinking...
     
    Last edited: Sep 20, 2020
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  13. zorgzorg2

    zorgzorg2 Tele-Meister

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    Thanks for validating!

    OK, next batch of questions, it's about adding a master volume: I'm wondering if I can use the Frondelli design here https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Frondelli_Master_Volume

    From what I understand, the bias and balance circuit added in the middle of the two 220k power tubes grid leaks means that the the schematic provided for the master volume can't apply here, is that correct? Otherwise I'll use the simple master volume.
     
  14. sds1

    sds1 Tele-Afflicted

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    I'm not sure I understand your question but I'll answer it anyhow and if I misunderstood, sorry. :)

    Each gang of the 250k pot becomes one the 220k grid leaks. So to be clear, you can use the Frondelli, just put the MV pot where the Blackvibe's 220k's are and remove the 220k's.

    Also make sure you don't miss that bit about the optional 10uF "AC Ground Cap", it applies here.
     
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  15. zorgzorg2

    zorgzorg2 Tele-Meister

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    @sds1 thanks, that did answer my question! I got confused by the schematic on the mod page, I thought the two resistors had to be connected with just one wire for the bias.
    Regardimg the AC ground cap, does it matter on which side it is connected?
     
  16. sds1

    sds1 Tele-Afflicted

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    Ooh great question! It matters... Both sides would get one in your case with the balance circuit. So you will need two of those caps.
     
  17. zorgzorg2

    zorgzorg2 Tele-Meister

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    Perfect! thanks! I'll come back tonight with a final schematic of the whole amp with the modifications.
     
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  18. Sparky2

    Sparky2 Friend of Leo's

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    Vacations of the blackvibe circuit?
    Oh, the artists who frequent the black vibe touring circuit vacation much like the rest of us, I suppose.

    Tone-Knob, a perennial headliner on the circuit, generally goes home to Ohio to visit with family when he's off the road.

    The Notorious B.I.Germanium usually spends a week at the beach with all the little ones.

    Rapper 'Lil Fuse likes to jet off to the alps for some snow skiing.

    So anyway, ........uh, wait;
    You said 'questions on the variations of the..', not vacations.

    Well, ****.
    Never mind.
    :(
     
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  19. zorgzorg2

    zorgzorg2 Tele-Meister

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    New version, now with master volume! RR763_BLACKVIBE_Schematic_mod.png
     
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  20. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That looks correct to me zorg.
     
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