5F1 Start-up Issues

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

  1. MrGibbly

    MrGibbly Tele-Afflicted

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    As some of you may have seen in my previous posts, I am in the process of building my first amp. It's a 5F1 based on the Boothill kit with a few minor substitutions from on recommendations here (e.g., different pilot light) and for parts that I happened to have on hand from recent repair projects. I've "completed assembly" and have everything together in the chassis. I'm following this start-up procedure:
    • Without any tubes installed turn power switch to on and about 3 then plug in amp - pilot light illuminates and the fuse doesn't blow so that's a positive sign. Let it idle for a few minutes...no smoke, no smells. No shorts to the chassis, etc.
    • Still no tubes installed. Measured voltage across pins 4/5 and 9 on V1 filaments at 7.21v. A little high but doesn't seem like anything to worry about at this point.
    • Unplug amp, turn switch off, drain caps using snuffer stick, and installed new TAD 5Y3GT. Plug amp in, power on. Rectifier tube glows a pleasant orange. No smells, no smoke. Voltage at pins 4 and 6 measures 349v and voltage at pin 2 measures 5.37v.
    Here's where the problem comes in... When I then move to measure the B+ at the positive lead of the first cap (C3) before the voltage dropping resistors (R10) I get essentially nothing. I thought it might be a problem with the lead from pin 8 of the rectifier tube (V3) to that post but I get the same bleh reading on pin 8 directly (B+ DC out).

    Thoughts?
     
    Last edited: Aug 3, 2019
  2. RottenTheCat

    RottenTheCat Tele-Holic

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    Hows your B+ ground doin?
     
  3. MrGibbly

    MrGibbly Tele-Afflicted

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    Not sure specifically what B+ ground means but I used my multimeter to check continuity from the negative leg of each of the B+ filter caps (C3, C4, and C5) to the chassis and they all make it beep happily. Am I checking the right thing the right way? Thanks!
     
  4. Jorgen83

    Jorgen83 TDPRI Member

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    Pin 3 on the rectifier? That’s wrong... right?

    Did you connect the 5V to pins 2 and 3 instead of 2 and 8?
     
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  5. King Fan

    King Fan Friend of Leo's

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    Jorgen nailed that. Should look like this:

    upload_2019-8-3_13-41-16.jpeg

    BTW, 'the fuse didn't blow' is a kinda crude way to do startups. A simple light-bulb limiter would be preferred.
     

    Attached Files:

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  6. MrGibbly

    MrGibbly Tele-Afflicted

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    Good catch on my typo! It's wired as follows: Yellow 5v runs to 2 and 8. Red HV runs to 4 and 6. I will correct my original post for clarity. The reading off pin 8 and on the lead into C3 is what doesn't make sense...
     
  7. MrGibbly

    MrGibbly Tele-Afflicted

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    Sorry, typo! It's wired exactly as this diagram is depicted. I've checked against the Boothill diagram and robrob's.
     
  8. Jorgen83

    Jorgen83 TDPRI Member

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    Well if everything measures up as you say I’d guess it could be a bad rectifier tube.

    Any pictures of the build we can check out?
     
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  9. King Fan

    King Fan Friend of Leo's

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    Yep, at this point we really need to see a close-up of the wiring on the socket and connections from there to the board.

    Also, how are you measuring voltage at pin 8? Note the 5Y3 (and 5U4) combine the heater and cathode. So you need to measure pin 8 in both VDC and VAC.

    upload_2019-8-3_14-10-41.jpeg
     
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  10. Jorgen83

    Jorgen83 TDPRI Member

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    Wait, another stupid question: did you set your multimeter to DC for the B+ measurement?

    Edit: Great minds...
     
  11. lowatter

    lowatter Tele-Meister

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    Basically...POST CLOSEUP PICS!
     
  12. MrGibbly

    MrGibbly Tele-Afflicted

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    [​IMG]
    [​IMG]
     
  13. Jorgen83

    Jorgen83 TDPRI Member

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    You’re not big on color coding your wires huh ;)

    Right, I don’t see anything wrong immediately. You could try to whip up a solid state rectifier with some diodes and see if the thing works that way. If so, the rectifier tube is toast I’d say.

    I’m assuming you don’t have another rectifier tube, and of course assuming you measured the high voltage in DC mode on your meter.
     
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  14. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yeah, what's the DC volts right at pin 8 of the rectifier tube? That's the first number we'll need to know.
     
  15. MrGibbly

    MrGibbly Tele-Afflicted

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    Okay, King Fan called it. In my haste to start poking at things and get readings I did NOT set my multimeter correctly. Now showing B+ of 472v at pin 8 on V3 and at C3. Every other measurement checks out in range as I add V1 and then moved on to V2 + speaker. Sounds glorious! Off to play test a little more. Thanks for helping!
     
    Last edited: Aug 3, 2019
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  16. King Fan

    King Fan Friend of Leo's

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    Heh, you aren't the first and you won't be the last to measure a DC voltage in AC.

    In aviation, they say, "There's two kinds of pilots. Those who've made a gear-up landing... and those who will."
     
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  17. MrGibbly

    MrGibbly Tele-Afflicted

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    Yeah, on the color coding, there's two wires I really wish I had color coded...the two that run under the board, exiting through the same hole to V1. Had to do use my multimeter to figure out which was which. <sigh> The others are easy to trace on this simple little guy. Next build!
     
  18. MrGibbly

    MrGibbly Tele-Afflicted

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    Thanks. I'll look into making one. I was following the start-up procedure in the StewMac kit instructions as it seemed to be the most thorough, step-by-step reference that included where to take all the measurements and what values to expect (more or less).
     
  19. dunehunter

    dunehunter Tele-Holic

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    The StewMac guide is okay for some stuff but also got me into a bit of trouble on my first build; their treatment of wiring the fuse block/volume-switch/transformer leads to the power cord was just flat out wrong.

    Check out robrobinette.com for a better start-up procedure. It's packed with good info.:)
     
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  20. Jorgen83

    Jorgen83 TDPRI Member

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    They must have fixed it, because page 10 looks correct to me:
    https://www.stewmac.com/freeinfo/i-10730/10730.doc.0718.pdf
     
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