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A Rare one: Wallace AC5075-XT

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  • A Rare one: Wallace AC5075-XT

    On my bench today...

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    PS: Even with the ally chassis it weighs a ton!
    Last edited by nickb; 01-17-2020, 11:06 AM.
    Experience is something you get, just after you really needed it.

  • #2
    sweet !! what is it in for ?? is the gut shot after repairs ?? If not looks to be a respectable repair from the past, easy to notice TAD caps . Impressive features from the 60's , hats off to Ted.
    Last edited by shortcircuit; 01-16-2020, 11:46 PM.

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    • #3
      The ticket says "Buzzing and grinding". Two pots, the range switch and several electrolytics bad. I replaced all the electrolytics, which is something I hestiate to do, but it was warranted.

      Here is the schematic, drawn by yours truly. May contain errors. Note unique tone control arrangement, sag resistor in HT supply. Also I don't recall ever seeing a tremolo arrangement like that. I found that with the tremolo off, the depth control still varies the DC operating point of the first stage and you can get huge amounts of even order harmonics. In fact it was hard to set the amp to get away from that.

      Wallace AC5075-XT.pdf

      Here is an article about Ted Wallace:
      GIT277.oldies_LPDF(1).pdf
      Last edited by nickb; 01-17-2020, 11:05 AM.
      Experience is something you get, just after you really needed it.

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      • #4
        I've never seen one of these first-hand, despite having some really unusual brands through the door over the years. That article makes for very interesting reading - I'm really interested in all things British when it comes to amps and guitars. Many British amps used catalogue parts from RS and I look out for NOS stuff all the time and they made so much it's still about at boot fairs and swap-meets. Interestingly Central Eyelets and Pressings who made all the tag boards and terminal strips are still going. As a customer's amp we have to do what the economics dictate, but if it was my personal amp and was a 'keeper' I'd have to match up the pots and re-stuff the caps.

        That tone stack is very close to the design I arrived at when experimenting to get a low-loss usable stack, except my bass pot had a 47k tail resistor and the pot needed to be C taper to get the sweep right. I had it in my main amp for about 18 years but eventually wanted more mid scoop than it offers.

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        • #5
          The output stage has bias feed resistors (220k) on the high side for KT88s. Many HiFi amps with KT88s in fixed bias don't go over 100k.

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