5F1 Start-up Issues

Discussion in 'Shock Brother's DIY Amps' started by MrGibbly, Aug 3, 2019.

  1. King Fan

    King Fan Friend of Leo's

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    That booklet does contain an impressive amount of info, though I can't help but notice the oh-so-StewMac list of 18 required and suggested tools they can sell you.

    But even this subsection of their layout is like one of those "What's wrong with this drawing?" puzzles. Even if the novice builder sees the subtle gray wire linking the lugs on the tag strips, I see five or six things that aren't right, or at least aren't best practice, and that's before you count the wire colors, which sorta scream "built by noob."

    The amp would work wired this way. That's about all I can say on the plus side. For the benefit of the OP, Rob's layout is better in all regards.

    upload_2019-8-4_7-42-3.png
     
    Last edited: Aug 4, 2019
  2. King Fan

    King Fan Friend of Leo's

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    Here's Rob's.

    5F1_Layout.png

    Note especially the switch is in the hot limb of the AC power, the household (mains) safety ground (earth) wire is longer than the black or white and is anchored to a totally separate bolt, and the PT grounds share the power amp ground.

    In the detail department, Rob's split ground on the filter caps is 'better' electronically, though in a small amp like this you'll get away with grounding all three caps with the power amp. Also, by UL standards and best practice, it's best if the household safety ground is on its own, separate bolt, not a PT bolt. Either way, this anchor is critical. Keps nuts and loctite above and below a swage-on ring terminal are ideal.

    Finally, if you don't have a 6.3V heater CT (as Rob's PT does) you *will* add the two 100R resistors to the lamp socket as StewMac shows as step 52 and 53.

    On a quick read, I'm impressed with all the details StewMac includes. But I'm not sure I trust all their details, and even though they hold your hand every step of the way, I'm not sure you actually learn how your amp works.
     
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  3. Jorgen83

    Jorgen83 TDPRI Member

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    Well I don’t know the intricacies of what would be best practice (I mean yellow wire for high voltage is unwise obv), so was only commenting on if it would work this way. Rob also uses a power tranformer with a center tap for the heater wires, so that’s def different. Other than that though, the power section is the same right?
     
  4. corliss1

    corliss1 Friend of Leo's

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    I personally would have never noticed the gray wires on the terminal strips, and I stare at this stuff on a daily basis.
     
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  5. King Fan

    King Fan Friend of Leo's

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    Heh, Jorgen, part of this is US wiring, which is different (Europeans may say outdated) compared to EU. I only recently learned (from smart people here) *why* the switch shouldn't be in the neutral limb of the power wiring. It is subtle. I had to make myself a drawing. Compare to the StewMac drawing which has the white household wire (neutral) going to the switch.

    Power switch options.png

    US standard is on the left, EU standard (even safer) on the right. I notice Rob's newer drawings have gone to switching both hot and neutral, and it is easy and elegant.
     
    Last edited: Aug 4, 2019
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  6. King Fan

    King Fan Friend of Leo's

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    As long as I've dragged this poor thread so far off the road, :) I might as well add a note about the subtle problem of switching the neutral. I just learned about this recently.

    It's easy enough to do, and as @corliss1 pointed out in my 'duh?' thread, many old amps end up this way when converted to modern wiring. And StewMac isn't the only small shop showing this in their drawings. Plus, some others draw both hot and neutral AC as black lines, which doesn't help.

    With a switched neutral, the amp will work, but it leaves AC power going into one PT primary and coming out the other to the switch. And that means, as @Blrfl noted in my 'duh?' thread, that if the insulation gets chewed off your primaries, or the splicing / soldering of the white wire on the switch fails (maybe more likely), you have household current exposed in the primaries, which are waiting to wander to the chassis, or to a handy conductor like your finger, which on a bad day may be grounded through your heart...

    At least I think that's how it works. I'm a student here myself. :D
     
    Last edited: Aug 4, 2019
  7. MrGibbly

    MrGibbly Tele-Afflicted

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    Yup, I've since found it. I did use a safer wiring scheme for the fuse/volume/transformer leads. Thanks!
     
  8. Jorgen83

    Jorgen83 TDPRI Member

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    Not in the layout, but it’s clearly explained in the step by step explanations.
     
  9. dunehunter

    dunehunter Tele-Holic

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    Nope. That wiring was standard practice back in 1957 (StewMac opted for period accuracy rather than modern standard). Modern standard has a separate (isolated) lug for the power cord ground, said ground should be twice as long as either the hot or neutral wires, and the neutral connects directly to the tranny lead rather than the power switch.

    Picked all that up right here from folks that know a lot more than I do :D. Compare with @robrob s graphic. His is right, StewMacs is wrong. It'll still work, just not modern standard.
     
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