VHT Special 6 Mod thread

Discussion in 'Shock Brother's DIY Amps' started by TNO, Apr 9, 2010.

  1. TequilaCaster

    TequilaCaster Tele-Holic

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    A couple of thoughts...
    1. ) check the filament voltage with the 6CA7 plugged in to make certain you are getting enough volts, as the 6CA7 draws more filament current.
    2.) Try increasing the load on the tube by connecting an 8-ohm speaker to the 16-ohm tap. (The amp designer says this is OK to do.) Use your same bias. Also verify your bias voltage.
    3.) Don't shock yourself.
     
  2. KP Porter

    KP Porter TDPRI Member

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    More filament current with 6CA7 is an interesting catch, honestly, I didn't think of it. Now checked and got a bit of shock (no, not literally...) since for 6V6 it is 0.45A, for 6L6 it is 0.9A and for EL34/6CA7 it is... 1.5A, i. e. 3 times as much!

    In fact, I've received a replacement tube, and it worked, yes, with both speaker configurations (normal and 8 Ohm into 16 Ohm tap). An interesting question is if the power tranny will be able to sustain the increased filament current draw...

    Now, if I get back to my previous bias calculation:

    It shows that if I follow the recommendations on @robrob's page to add 560 or 480 Ohm resistor in parallel to R39, with 230-250 Ohm of effective bias resistance, it will throw the tube way above plate dissipation. Using a 2.2K resistor instead will give 387 Ohms (i. e. smth like 63mA of current) which seems to be a safe bet. However this differs from what is suggested by a good order of magnitude, which confuses me...
     
  3. TequilaCaster

    TequilaCaster Tele-Holic

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    Glad a new tube sorted things out for you. You should try your own calculated bias resistors first. Then try RobRobs, and watch out for red plate. Maybe it will be OK.

    I recall the Special 6 designer saying that running the bigger power tubes was OK as long as you turned the amp off every couple hours to let the transformers cool down.
     
    Last edited: Nov 6, 2019
  4. Even Harmonics

    Even Harmonics TDPRI Member

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    That description sounds amazing:

    "Very good JCM800 crunches when maxed and fluid sparkly, glassy cleans when you roll back the guitar volume."


    I just ordered the VHT Special 6 Ultra head based on the reviews. Delivery is scheduled for tomorrow. Very excited.


    Are you still happy with the mods?
     
  5. Even Harmonics

    Even Harmonics TDPRI Member

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    The VHT Special 6 Ultra head arrived today. The build quality seems solid. The head cabinet is made from birch. The stock preamp tubes weren’t horrible but the overall tone was a little dull. I put a JJ12AT7 in V1 and a JJ12AX7 in V2. Huge difference. It really brought this thing to life. Sounds awesome! It came with an Electro Harmonix 6V6EH power tube. I left it in for now but will probably try other types at some point. The variety of tones I’m getting from the amp at this price point is really amazing.
     
    Last edited: Dec 4, 2019
  6. Even Harmonics

    Even Harmonics TDPRI Member

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    Unfortunately only 2 days in and the amp started to short out. The signal started dropping in and out on the Ultra channel. I could also hear a low crackle on the Clean channel. I tried changing out all of the tubes but that did not resolve the issue. Maybe a bad solder joint somewhere. The signal would kick in when I played harder.

    Anyway... I returned the amp and ordered another one. The new production batch will not be available until the end of January. I hope I have better luck with the next one as I really do like the variety of tones I experienced with this head.

    Bummer...
     
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