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  1. Thread: PCB grounding

    by R.G.
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    5
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    355

    Ah, grounding. :) For your limited case, it...

    Ah, grounding. :)

    For your limited case, it will probably not matter.

    At audio frequencies, what matters in grounding is what >ground offset voltages< are generated by >ground currents<...
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    Do what the high power radio guys used to do: put...

    Do what the high power radio guys used to do: put a load resistor into a can of oil. Drive it with the output signal and the measure the temperature rise of the can of oil. This is probably best done...
  3. Good advice, all. It's all about gluing sawdust...

    Good advice, all. It's all about gluing sawdust together with an adhesive. There are better and worse adhesives for a variety of conditions.

    For wood in dry conditions, ordinary carpenter's glue...
  4. Yes, what he said on stacking. Plus - there are a...

    Yes, what he said on stacking. Plus - there are a number of different stacking patterns that were (and maybe still are) used in transformers, for different effects. It's not just a binary decision to...
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    There aren't any pictures of that board. I can...

    There aren't any pictures of that board. I can put up some pics of the bare board, but my amp tech friend coaxed me out of the first-article assembled item. I'm have to assemble another one to get a...
  6. Stacking the laminations is fairly trivial....

    Stacking the laminations is fairly trivial. Obtaining new laminations in the first place is the trouble. If you can get new lams, you can stack a core in a few minutes. You just start inserting the...
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    I did a little poking through the archives. No...

    I did a little poking through the archives. No luck. It must be the bass 50 I remember.

    But I did come up with some thoughts. Both the US and UK Vox folks liked to do mix and match designs. I...
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    477

    That's reasonable. Another approach would be to...

    That's reasonable. Another approach would be to make and use a light bulb limiter, which is a simpler way from keeping the whole amplifier on a power budget.

    Removing the outputs to eliminate the...
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    I'll go pore through my archive of Vox...

    I'll go pore through my archive of Vox schematics. It sure seems like there was an Escort Lead 50 in there.

    As a side possibility, I made up a PCB for the preamp section of the UK Vox 700 series -...
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    477

    Next step. Once I was sure that the caps were...

    Next step.
    Once I was sure that the caps were not leaking me into problems, I'd remove Q306,7,8,9,11,12,13,and 14 temporarily. If a solid state amp is not loaded it should be stable, and since the...
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    477

    I'm going to sound like a broken record, but how...

    I'm going to sound like a broken record, but how old are the electrolytic caps? If they're not less than 10 years old, replace them all before going any further. A little leakage in C301, 304, 305,...
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    Good call, OTM!! I've seen fuses where the inside...

    Good call, OTM!! I've seen fuses where the inside was mottled-silvered from what I think was the filament vaporizing.

    @ eschertron
    I can't think of a good way to prevent the internal thermal fuse...
  13. First of all, kudos to you for trying to keep...

    First of all, kudos to you for trying to keep these old timers out of landfills. They're unique amps and I'm very fond of them.

    You're getting good advice. Fixing these amps requires some...
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    Good question. I suspect that it's a result of...

    Good question. I suspect that it's a result of the problems with ordering effects.

    If you distort a signal, t generates a stew of higher frequency stuff based on but not the same as the original...
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    It's a sad commentary that many governments "fix"...

    It's a sad commentary that many governments "fix" problems by simply making something illegal, and saying to the public "There, I fixed the underlying problem by making some minor aspect of it...
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    As a step sideways - look at cabinet scrapers....

    As a step sideways - look at cabinet scrapers. These are not the handled putty-knife sort of thing. They are a square of thin steel, hardened and spring tempered. You use a whet stone to grind one...
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    50 millivolts is a good target. In addition,...

    50 millivolts is a good target.

    In addition, JMF is right - there is likely something else going on if your meter is producing that answer. It's always worth investigating why your instruments...
  18. There is a great discussion of how to drive...

    There is a great discussion of how to drive reverb coils in the Floobydust (sic) section of the old National Semiconductor Audio Handbook. Very worth finding and reading.

    Coils obey the law that...
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    Some theoretical, as opposed to practical,...

    Some theoretical, as opposed to practical, considerations follows.

    Shielding an enclosure is an attempt to form a Faraday cage. A Faraday cage is a conductive enclosure that surrounds the...
  20. You're welcome. I forgot a big one -...

    You're welcome. I forgot a big one - counterfeiting. Toshiba output transistors are very good - and widely counterfeited. The On-Semi devices less so than Toshiba, but the chances of getting a fake...
  21. There are some "gotta have enough" specs, and...

    There are some "gotta have enough" specs, and then there are some subtle ones.

    The ones you mention are largely "gotta have enough" types. If a transistor doesn't have enough breakdown voltage,...
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    The circuit shown is a self-split push-pull...

    The circuit shown is a self-split push-pull circuit. It was invented to get an active cathode follower "push pull" output to get lower output impedance from a tube stage as well as high gain. It was...
  23. No. The path from the rectifier to the filter cap...

    No. The path from the rectifier to the filter cap does not include the field coil.

    (1) Maybe, if inrush current was the actual problem; (2) no, as the real problem is the constant high current...
  24. Good advice Chuck! That's the way to deal with...

    Good advice Chuck! That's the way to deal with an unknown amp that you can't just run and measure. Say, one that hasn't been built yet. And I especially like the advice to add safety margin....
  25. If you want low output impedance and are willing...

    If you want low output impedance and are willing to switch out devices to do it, consider a high voltage MOSFET source follower. A source follower has dramatically higher gain - and hence lower...
  26. No content on original question, but a comment on...

    No content on original question, but a comment on winding machines.

    Change gears, advance screws, and the like on winding machines are very time consuming to set up, as JM notes. However, in...
  27. Well, they do, but neither "normal" transformers...

    Well, they do, but neither "normal" transformers nor pulse transformers act like we commonly think they do. Both kinds act like a series leakage inductance into a parallel combination of the primary...
  28. It's hard to drive an OT too far inside a...

    It's hard to drive an OT too far inside a push-pull amp that's already working. Not impossible, but rare.

    To the amplifier driving it, an OT looks like a big inductor in parallel with the...
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