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• Re: Look at Herbert Keroes original circuits. - Henry Pasternack 08:00:59 12/04/04 (2)
  In Reply to: Look at Herbert Keroes original circuits. posted by grhughes on December 4, 2004 at 07:23:16:

  The special output transformers used in the McIntosh amplifiers, aside from being unavailable commercially, impose a daunting requirement on the output voltage available from the driver circuit. These amplifiers aren't generally regarded as the ne plus ultra in subjective performance, so it's unclear what the advantage would be of trying to copy them. Even if you were determined to clone the MC-30, this would be a project well beyond your current skill level.

  In my humble opinion, you are coming at this from a backwards, or at least sideways, perspective. This is not the time to be worrying about the types of resistors and capacitors you will use, even before you've got a schematic. What you need to do is complete your top-down requirements analysis.

  I believe what you want is a quality implementation of a classic 1950's negative feedback amplifier. I would strongly suggest a variation of the Williamson driver circuit and a single pair of KT88/6550 output tubes with a 450-475V B+ supply giving you 50-60W output power. This type of circuit is time-tested, bullet-proof, and easy to set up. I think one of the Triode Electronics or MagneQuest Dynaco Mk-3 replacement output transformers would be an excellent choice. I would recommend a choke-input power supply with tube rectification and polypropylene filter capacitors. Use decent quality resistors and capacitors and you will get state of the art performance for this type of amplifier.

  The main challenges you will face are sourcing the right power supply components to give you your desired voltages, learning how to do a proper physical and wiring layout of the amplifier, tracking down hum and noise in the working prototype, and stabilizing the feedback loop. The latter two challenges may be particularly daunting as they get into the black art of audio design and construction.

  I encourage you to undertake this project, but I also think there is a lot to learn. I hope you view it as the starting point, not necessarily the ending point, of what may become a years-long process of experimentation and discovery.

  -Henry
  
  

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  • Re: Look at Herbert Keroes original circuits. - 7591 14:50:25 12/04/04 (1)
    In Reply to: Re: Look at Herbert Keroes original circuits. posted by Henry Pasternack on December 4, 2004 at 08:00:59:

    Henry,

    I don't think your opinion needs to be humble at all-you're absolutely correct; I'm coming at this bass-akwards and I know it!

    I started with the restorations about a year and a half ago and the only thing I knew was which end of the soldering iron was the hot end. Now I can read a schematic and I have a pretty good idea of how pre-amp and power tubes work, where they get operating voltages from, where the voltages go and what they do and why, how, why and where it's filtered and how bias voltage with an AC signal imposed moves audio signals between each stage through the coupling caps. That's a very general overview, but what I'm getting at is if I have a relatively simple schematic, I think I could build it. Making mistakes along the way is part of what I'm looking for.

    Now I at least have a somewhat more than limited knowledge of what's going on in the amp and why. But I think the next step which will link what I've learned during restoration and what I've read will be building an amp. I also should've been more clear in my post.

    I'm not looking to design my own amp or redesign the wheel; I'm looking for a simple line amp circuit that I can understand and build, not looking to design one from scratch. The MC30s were an idea only because it's a relativly simple circuit.

    "I believe what you want is a quality implementation of a classic 1950's negative feedback amplifier. I would strongly suggest a variation of the Williamson driver circuit and a single pair of KT88/6550 output tubes with a 450-475V B+ supply giving you 50-60W output power. This type of circuit is time-tested, bullet-proof, and easy to set up."

    That right there is exactly the type of info I was looking for. I know some of the earlier Heathkit amps used the Williamson circuit-should I look for a pair of those to start from?

    I also appreciate your taking the time to help a newbie out.

    Best,
    7591

    
    
    
    

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    • Re: Look at Herbert Keroes original circuits. - Henry Pasternack 15:18:38 12/04/04 (0)
      In Reply to: Re: Look at Herbert Keroes original circuits. posted by 7591 on December 4, 2004 at 14:50:25:

      The MC-30 is actually a relatively complicated amplifier on account of having four stages in the driver, positive feedback in the driver, cathode negative feedback in the output stage, and overall global negative feedback. The Williamson isn't the simplest design, but it's a good one. The devil is in the details.

      If you look on my "website", you'll see three files starting with "5881". This amplifier worked out well in practice, and I think you would be pleased to do something similar. As shown, it made about 35W per channel. I sold it to a fellow who made minor adjustments and ran it with KT-88s, producing around 60W.

      -Henry
      

      • http://users.rcn.com/hpasternack/   (Open in New Window)

      
      

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