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Why would you do this? Filmosound conversion Filtering

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  • Why would you do this? Filmosound conversion Filtering

    I'm attempting a filmosound amp conversion. Mine is the 621, exactly the same as this walkthrough in Guitar Magazine.

    I've had to gut mine, but I have the circuit diagram so I'm going to try and rebuild it. I've attached an annotated (NOT COMPLETE!) version of a poor copy from the internet

    Ignore almost everything outside the dotted red box, thats for the film soundtrack reading

    Basically the B+ is split and i dont know why. I have 2 x 16uFs and an 8. It looks to me like one of the 16s is left out of the audio circuit, or it will be if i eliminate the part of the circuit around the 3rd 6v6 (V7)

    Why is the configuration so strange?

    Why not just have 1 B+ line with caps coming off as required? like here, in a princeton (or any other fender, I chose the princeton cos i think its similar-ish with the cathodyne PI)

    or am I just over thinking it because it looks funny?

    Tech 621 diagram.PDF
    Attached Files

  • #2
    As you say, the second 16 cap is not used in the audio power amp.
    The additional/separate RC filter (1500R/16) makes sense if the other circuit part requires a higher supply voltage than available at the screen node and/or if it's not desirable to lower the screen supply by additional current.
    A resistor between 2 filter caps has 2 effects: It drops DCV but also considerably improves ripple filtering.
    So the 2 x 16 caps with a 1500R resistor between provide much better filtering than a single 32 cap, but the output voltage will be lower depending on current.
    Last edited by Helmholtz; 02-24-2021, 11:38 PM.
    - Own Opinions Only -


    • #3
      The 16uf on the right is the first reservoir filter cap for all the B+. The second 16uf in the center is the separate B+ supply for the oscillator tube upper right. It is separate because they do not want the audio modulating that supply, they want it real clean. I don't think it is strange at all. Of course it serves no purpose in a guitar amp, but when it was born it wasn't a guitar amp.

      SO remove the resistor between the two 16ufs, and then connect the middle 16uf to the 8uf in parallel.

      But really, make them all 20uf (or 22uf), as 16uf and 8uf have not been standard values in decades.

      Yes, V7 serves no purpose, get rid of it.
      Education is what you're left with after you have forgotten what you have learned.


      • #4
        Who does not know and knows that he does not know - teach him Confucius)
        Who knows and does not know that he knows - wake him Confucius)


        • #5
          Thanks guys!
          i guess it looks strange because I only ever really looked as fender and radio schematics...

          I've already got a new can with 2 x 16uf caps so I’ll leave the values as is.

          Anyway, can I run the B+ in a single line such that:

          T —-16—-1500R—-16—-1500R—- 8—->

          (caps coming off to gnd)

          ie am I changing anything by splitting the 3000r resistor into 2 and moving the position relative to the caps? (I have a few 10w 1500Rs and the original 3000 is done)
          Last edited by gilmo789; 02-26-2021, 08:26 AM.


          • #6
            Yes, can split the 3k into two 1.5k resistors and connected the second 16F cap between for improved screen and preamp filtering.
            - Own Opinions Only -


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