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Trem is not working on a vibrochamp build

Discussion in 'Amp Tech Center' started by Jerry Bee, Mar 24, 2020.

  1. Jerry Bee

    Jerry Bee TDPRI Member

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    Hello!
    I need your precious advice.
    The trem on my recent vibrochamp build is not working. With the intensity on 1 there is a 60 cycle hum, when I increase intensity the it becomes less, but also the overall volume of the amp decreases until it is dead quiet.
    I checked the speaker, tubes, wiring etc., but I don't find any fault...

    The readings with intensity and speed on 5 are:

    V1
    pin1 211.6
    pin2 1.72
    pin6 215
    pin8 1.7

    V2
    no oscillation at all
    pin1+6 355
    pin3 3.46
    pin7 243.2
    pin8 246.8

    V3
    pin3 346.3
    pin4 355.4
    pin5 0.036
     
  2. Jerry Bee

    Jerry Bee TDPRI Member

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    IMG_20200324_170015.jpg IMG_20200324_170039.jpg IMG_20200324_170028.jpg
     
  3. Ten Over

    Ten Over Tele-Holic

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    V2 is wired wrong.

    Change the 470K configuration on the socket and move the blue wire that goes to the caps to pin 1.
     
    Last edited: Mar 24, 2020
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  4. schmee

    schmee Doctor of Teleocity

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    Who is "Trem"? and tell him to get back to work!
     
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  5. Jerry Bee

    Jerry Bee TDPRI Member

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    Ah, now I see!
    A little detail can screw up everything...
    Thank you a lot!
     
  6. Jerry Bee

    Jerry Bee TDPRI Member

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    But there's still a hum issue, when the intensity is turned off. Above 2 on the dial it's becoming less.
    With the amp volume off I can here the oscillation of the trem quite strong like a dull click. When turned up it's quieter but still there...
    Could be a grounding problem, what do you think?
    The only thing I did is an additional nfb resistor of 10k.
    First I will elevate the heater supply with two 100 ohm resistors.
     
    Last edited: Mar 24, 2020
  7. BigDaddy23

    BigDaddy23 Tele-Holic

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    Starting with the heater artificial center tap is a good idea. Also review your input jack and volume pot wiring.
     
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  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Trem click or thump can be a 'normal' thing on these amps; I might work on that after the hum. (If the hum is loud, abnormal, bothersome?) Hum has a lot of potential causes. As a starting point, is it 60Hz or 120? Buzzy or smooth? Does it occur without a cable plugged in? What if you pull the tubes (one at a time)?

    You say the hum varies with the intensity -- maybe inversely? We hear that lead dress on the trem circuit can be critical. Your board is tidy but your leads in front take separate paths. You might try twisting them in two specific bundles -- here's my factory '67 for example. (They head off to their various pots once they reach the front.)

    upload_2020-3-25_8-18-11.jpeg

    For sure I'd take out the extra NFB resistor during testing. No need to artificially increase noise right now.

    Finally, just to get us all on the same page:

    champ_vibro_aa764_layout clean.png
    vibro_champ_aa764_schematic.png
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    ^^^^^
    This.

    It's funny, @King Fan and I were just discussing grounding the HT center tap. I think your hum is from a different source, but to help optimize your ground scheme move the center tap red/yellow wire from the transformer bolt to the negative terminal of the cap can. Doing so will reduce hum from ripple current.
     
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  10. Jerry Bee

    Jerry Bee TDPRI Member

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    Thank you for the good thoughts guys!

    The grounding of the heater center tap did the trick. There is only that kind of hum left, that I hear from my tweed champ and Princeton builds. Although I did it with these before, I forgot it in this build. The layout I used is without this feature.

    Also the trem klick became less and is only hearable, wenn the volume is turned off. And who is playing with the volume on 1 :) It sounds kinda cool if you play with speed and intensity...

    The overall hiss is absolutely okay, because the grounding of each position with its own wire to the star ground is very effective..

    I'm very pleased with the result. I added a 25k mid control with a fixed 6k8 mid resistor for lots of versatility in sounds. The extra 10k nfb resistor is still there, which gives me a more open sound with more overtones and responsiveness.
    I run the amp in a frontman 15R enclosure with a Jupiter 10sc, an amazing speaker! Really nice warmth and clarity comes out of this, contributing its own for the ultimate blackface sound.
    This amp is really sounding bigger than its original design.
     
  11. Jerry Bee

    Jerry Bee TDPRI Member

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    Lowerleftcoast, I will try your advice too,
    but I have to play this beast right now!
     
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