Bassman Master Volume Mod

Discussion in 'Amp Tech Center' started by spoontex, Oct 10, 2019.

  1. spoontex

    spoontex TDPRI Member

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    Hello,

    I like to add a Master Volume to my Bassman aa864 amp. I found this video on youtube:



    I understand the circuit, but I don't know because he add a 33uf cap to the bias pot.

    Anyone can explain me?

    Thanks!!
     
  2. RottenTheCat

    RottenTheCat Tele-Meister

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    Oh boy... now the bias is dependent on going thru three pots, even though its adjusted by one. I like the way he removed the accessory outlet for the install.

    Be aware - there are many variations of the Blackface Bassman. There are transitional models out there, as well as transitional to the several Silverface versions. Most of those were the way they used negative feedback as a "compression" device, both on the output and interstage.

    One way I've seen MV on BF/SF amps is to leave everything in the bias circuit alone, use a 1m pot, fed thru some .047uf (or .1uf) caps, and connect that right to the grids of the output. The idea is to phase cancel the AC component of the grid signal.

    THe AC (signal) component is not "perfectly symmetrical", as the phase inverter is not perfect, but its damn close, so the circuit works.

    Another simple way is to use dual pots, again fed thru .1uf capacitors, to ground out the AC component, either to chassis (which may hum, gotta try it), or to the negative supply (which may also hum). If the negative supply hums, add another reverse electrolytic to the existing bias pot's output, about 25@100v will do.

    That last one will work perfectly as a master volume.

    Both of those schemes are easy to do.

    The idea in both cases - block the negative bias DC with capacitors, either phase cancel or ground out the AC (signal). Blocking the DC means you don't need to screw with anything in the existing bias circuit, except maybe add filtration as noted.

    The biggish DC blocking (and AC/signal passing) capacitors (200v is fine for voltage) can be mounted to a little terminal if you like. If you have a big enough soldering iron, you can solder a terminal right to the chassis of the amp, close to each output tube. No connection to ground, just soldered on as a way to affix it, without making a hole. And regular pushback wire works, but like the video shows, use that AC outlet space, keep the wire runs short. Shielded wire is not crucial if you do that.

    Hope that helps!
     
  3. Paul G.

    Paul G. Friend of Leo's

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    Bassman heads work well through attenuators. I'd do that.
     
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  4. Wally

    Wally Telefied Ad Free Member

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    Attenuators are superior to any MV, imho/ime, since the attenuator doesn’t interfere with the way the various sections of the amp work together. That is, an attenuator allows one to involve the whole circuit rather than decreasing the output section’s involvement in the equation with a MV.
     
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  5. spoontex

    spoontex TDPRI Member

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    I think I go for an attenuator.

    thanks!!
     
  6. schmee

    schmee Poster Extraordinaire

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    Volume pedal first in line.
     
  7. avspecialist

    avspecialist TDPRI Member

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    7D191424-8B2C-4DE4-ACAC-5EE37597EFB5.jpeg I love the trainwreck ppi master volume in my bassman 50. A dual 250k works great.
     
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