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Blackface Champ AA764 with a couple modifications

Discussion in 'Shock Brother's DIY Amps' started by stinga11, May 29, 2018.

  1. stinga11

    stinga11 TDPRI Member

    Age:
    31
    27
    Apr 14, 2018
    Puerto Rico
    Well I finished my project. A Champ black face AA764. I modified the original schematic with some improvements. I added a middle pot and bright switch. The two bypass capacitor are 25uf and the 6v6 has a gridstopper and screen resistor with a 820ohms cathode bias resistor. The power transformer is a Classictone 40-18019 and the output transformer is a Classictone 40-18031 15W, Large Sized Upgrade with up to 15 watt.

    Blackface Champ.png
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    The sound was definitely what I was looking for. I wanted to have a Blackface sound in a small size since I want it to record.

    Here is a demonstration of the sound. (Sorry for the grimaces I make with my face hahaha)

     
    Last edited: May 30, 2018
    Nickfl, Snfoilhat, D'tar and 5 others like this.

  2. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    28
    Feb 22, 2009
    New York
    Pretty sweet! Seems biased really cold for a single ended cathode bias, though.
     
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  3. stinga11

    stinga11 TDPRI Member

    Age:
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    Apr 14, 2018
    Puerto Rico
    Thank you. Could you explain what you mean by the cold Bias? Because when I did the bias I got 12 watt on the 6v6.
     

  4. jimgchord

    jimgchord Tele-Meister

    Age:
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    Apr 3, 2018
    victor,ny
    How did you determine that with no voltage measurements?
     
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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Is the 470k screen resistor on the schematic a typo?

    A 470 ohm cathode resistor for a single ended 6V6 is pretty standard so an 820 is a cool bias. Be sure and alligator clip in a parallel 820 so you can sample a more typical bias.
     
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  6. stinga11

    stinga11 TDPRI Member

    Age:
    31
    27
    Apr 14, 2018
    Puerto Rico
    Wow, It's an honor that you answered my thread. Most of the things that I have learned have been on your website. The Screen resistor value was a typo error. I did the Bias following a video of Uncle Dough and the result gave me 12 watts in the 6V6.
     
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  7. Bristlehound

    Bristlehound Tele-Afflicted

    Age:
    57
    Jan 4, 2017
    Wales
    Great sound... and great playing, too.
     
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  8. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    What is your plate voltage on the 6V6?
     

  9. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    28
    Feb 22, 2009
    New York
    You can guestimate B+ by the PT, and even with a very optimistic plate voltage, an 820ohm cathode bias resistor will be fairly cold on the bias.
     

  10. jimgchord

    jimgchord Tele-Meister

    Age:
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    Apr 3, 2018
    victor,ny
    I get that, but I didn't see that he posted any specs on the pt. Maybe I just missed it
     

  11. JD0x0

    JD0x0 Poster Extraordinaire

    Age:
    28
    Feb 22, 2009
    New York
    Classictone PT is pictured, you can use the model number and get specs online ;)
     

  12. stinga11

    stinga11 TDPRI Member

    Age:
    31
    27
    Apr 14, 2018
    Puerto Rico
    350V and 24mA
     

  13. VintageSG

    VintageSG Tele-Afflicted Ad Free Member

    Mar 31, 2016
    Yorkshire
    Aim for 34 - 38 mA
    34-36 is a sweet spot. Valves are ( relatively ) cheap. Tone is priceless.
     
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  14. stinga11

    stinga11 TDPRI Member

    Age:
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    27
    Apr 14, 2018
    Puerto Rico
    Thanks!!!
     

  15. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

    The playing is excellent. the amp may sound cold in that it doesn't sound open. the playing sounded to me kind of like jazz and I didn't hear any cranked power chords.



    350v and 24 mA is cold.


    subtract 25 v for the cathode from the plate voltage to get the difference:


    325V X 0.024 Amps = 8 Watts
     

  16. jimgchord

    jimgchord Tele-Meister

    Age:
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    Apr 3, 2018
    victor,ny
    Yea so that is a bit cold.mine come in around 35ish and sound pretty dang good.
     

  17. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Something that hasn't been said is why. Since this is a Class A amp the most power the tube will be dissipating is at idle. So when you put a signal through it the tube is dissipating less. Not so with push pull amps biased in Class AB. As you increase the signal more power is dissipated in the tubes. Still some people will bias a 18 Watt Marshall for example, close to 100% of the rating. What happens as you clip the amp the signal gets rectified at the cathode and it makes the voltage more negative which reduces the dissipation. Basically it causes a Class AB amp to behave more like a Class B amp.

    Now the Uncle Doug vid most likely was a fixed bias amp, no cathode resistor. You bias those amps cooler, 70% is a roundabout figure to keep the tubes from melting. Rather than the cathode resistor and capacitor increasing the bias the electrons on the grid start mucking things up on the fixed bias amp. The bias could be increased on the fixed bias amp but to do that you have to reduce the grid resistor to allow more electrons to leave the grid. But that brings down the signal voltage from the phase inverter. Much easier to just use a grid resistor value above the recommendation on the spec sheet and bias not much more than 70%.
     
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  18. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Silver Supporter

    Dec 11, 2009
    Bloomington, MN
    Stinga, your playing is great! And the amp does indeed sound very nice.
    Now for the bias - I'd follow the Wally method from where you are at now. Most folks agree here to possibly bias a little hotter, and they may be right.
    So perhaps try to get it to the stock (470 R) that Rob suggested, then try different values in between, until you get what sounds right to your ears, rather than what "works" theoretically.
     
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  19. stinga11

    stinga11 TDPRI Member

    Age:
    31
    27
    Apr 14, 2018
    Puerto Rico
    OK guys, I replaced the 820 ohm resistor with a 470 ohm resistor and added a 500 ohm potentiometer. Now I was able to get 12.07 of plate disipation.
     

  20. Nickfl

    Nickfl Tele-Holic

    923
    May 24, 2016
    Florida
    Be careful about using a pot to adjust the bias on a cathode biased power tube, most common pots are only rated to dissipate about 1/2 watt, and if you turn that pot up too much you may end up exceeding that rating. A rule of thumb is that you want a component to be rated to at least twice what it is going to be dissipating (preferably more), so with a 1/2 watt pot and around 35ma you would not want to turn the resistance on the pot up past about 200 ohm. Even then you should probably plan to use it to find the bias point you like and replace it with a fixed resistor.
     

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