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knight km-15 conversion

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  • knight km-15 conversion

    I'm doing a conversion on a Knight km-15, turning it into a Vox Cambridge Reverb (minus the the reverb and tremolo). I picked the Vox as a starting point only because the schematics were pretty similar. Maybe it will get changed but I figured why not start simple, as this is the first conversion of a hifi amp i've done. Here's the schematics:

    Vox Amp Heaven Schematics - Berkley Cambridge Pacemaker Foundation Defiant Conqueror Escort Super Midas Busker Vigilante

    Two questions:
    On the Knight schematic there is a .02 cap running from each leg of the PT secondary (B+) to ground via a tie in with the center tap. What is the purpose of this? I can't find any guitar amp schematic that has these caps, and don't think they need to be there, but wanted to be sure.

    Also, I have two of these knights, and my eventual plan (once I get them sounding how I want them to) is to put both in a combo cab with 2 speakers, and have them running in stereo. Is there any reason that this could cause problems? Anything specific to look out for when mounting two separate amps in such close proximity? Any problem with tieing the inputs together, or am I better off using a Y splitter?

    Thanks a lot for any help and/or advise...

  • #2
    The amps both use el84s but the knight is running the tubes at much higher voltages 340 vs. 260 on the plates. If you're absolutely dead set on making a vox copy keep in mind that you'll have to drop those voltages considerably. But there lots of el84 pp amps out there that run at 350v or so. Maybe it's easier to look into those...
    Also there's a member on this forum (ChuckH I believe) who has some great suggestions for making el84s sound really tight at that voltage.


    • #3
      Yes, I realized that the plate voltage was much higher. I figured I could just start with the 360v, and if it didn't sound good go from there. I'm not really trying to make a Vox per se, it was just a schematic I came across that looked like a good place to start. If you get rid of the reverb and tremolo, it's very similar. 2 12ax7's 2 el84's, bass, treble, volume. I would be interested to see any other 2xel84 schematics that are somewhat similar (ie i don't want to drill the chassis or anything just yet.).

      I just fired it up, and i must've missed something as there is almost no volume. Before I got into it, it worked good, just didn't have the correct tone for a guitar amp. Now there is almost no volume. Time to back track!!!


      • #4
        oops. read the schematic wrong and put the wrong cathode resistor in. that was the problem...has volume now, and actually sounds OK, but deffinately needs some tweaking.

        I'd like to play with trying to get the plate voltage down to Vox levels just to hear what it sounds like, but it means I need to drop about 100volts. Any tricks to doing this aside from big ass resistors (yeah yeah I know, a different PT, don't have one right now..)?
        Will using resistors to drop that much voltage make it sound horrible?


        • #5
          dropping 100 volts="big ass resistor"=loss of loads of current.
          There are other ways to drop the voltage using mosfets and zener diodes-- do a search and you'll find something.
          In the end though it's a lot of trouble just to hear what it sounds like. You might be better served figuring out what you would like it to sound like first and then working with what you have to get there. After all, the classic Vox 2x el84 amp is the AC15 which runs at around 315V if I remember right.


          • #6
            yes i know it's a lot of trouble, especially for something I may not like. However, the goal here is more experimentation than having a specific goal. I am trying to a) use only scrap parts I have laying around b)end up with a cool sounding amp "eventually", and c) learn as much as I can along the way. The vox was just the first schematic I came across in books i have that seemed similar. Now that i've looked around on the web, i see there are other options. But since i have the time and the parts and it's already wired to the vox schematic (aside from the plate voltage), I figure why not follow through just to hear it and maybe learn something. And then I'll tear it apart and turn it into an 18 watt marshall type amp. And then.....