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Birth of a Champ Micro 5F1M-EF80

Discussion in 'Shock Brother's DIY Amps' started by Hyakuya, Mar 3, 2021.

  1. Hyakuya

    Hyakuya TDPRI Member

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    Hello....

    My first attempt at a guitar amp, thanks to @robrob design, is born....

    Added a DPDT switch and the Double Boost Mod but otherwise standard apart from the PT which I already had.

    2354.jpg

    just need volume knobs (in the post) and labels on the switches.

    Powered on for the first time today and happy to report no magic smoke :D

    B+1 @ 376Vdc, B+2 @ 372Vdc and B+3 @ 302Vdc

    2353.jpg

    One question..... I measured the voltage on pins 4 and 5 of the EZ81 and they read 12V which surprised me. The PT secondary filament winding is labelled 0 - CT - 6.3 and I've connected the CT (yellow wire) to the EF80 cathode bypass so it is elevated. I've read that when the CT is elevated the filament voltage effectively sits ontop of the elevated voltage but I don't fully understand this. Would this account for the 12V reading I get on the pins though ?

    Need to build a cab now. I have the speaker and some wood so ready to begin.....
     
    Last edited: Mar 3, 2021
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  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I take it you mean 12VAC, measured pin-to-pin?
     
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  3. ThePanda

    ThePanda TDPRI Member

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    subscribed!

    Your wiring is really neat
     
  4. Len058

    Len058 TDPRI Member

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    Wow! Your B+* voltages are very high! You know the max plate voltage for the EF80 is 300? The 12auy version has B+2 (which is B+3 in the EF version) at 240V.

    I've built one and with B+1 at 301, my EF80 was biased too hot. Did you check your bias?
     
    Last edited: Mar 3, 2021
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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    @Hyakuya, congrats on getting the amp up and running. Yes, your plate voltage is very high for an EF80. At 376v the tubes may have a short life. Dim the lights and check for red plating too.

    I would recommend you insert a dropping resistor between the rectifier tube and the B+1 filter cap to get the plate voltage down to at least 320v.
     
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  6. Hyakuya

    Hyakuya TDPRI Member

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    I think I had a brain fart with the 12v heater measurements.... checked again and they're fine. Probably over excited.

    I definitely need to get those voltages down.... If the max plate for the EF80 is 300v where should I be aiming for ?
     
  7. Len058

    Len058 TDPRI Member

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    I would aim between 250 and 295. Lower plate current will result in less headroom but at 289V I experience more than enough headroom.
    You would need to check your bias after getting the plate current down. In my case the plate voltage was ok, but the bias was still too hot.
     
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  8. Hyakuya

    Hyakuya TDPRI Member

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    The PT I'm using has 0-190-275 secondary and I had it on the 275-0 so switched it over to 190-0 and it's pretty much on the money.

    B+ now 260, 255 and 208.

    I think I've badgered the EF80 though.... makes a little squealing noise now and very bright. I'll have to find a replacement.
     
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  9. Len058

    Len058 TDPRI Member

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    Sorry you lost the tube :cry:

    Those values are low but that might not be a problem. Get the new tube and try it first without further changes.

    You're definitely in the safe non tube burning zone. I'd anticipate some tweaking and order some lower value resistors for the EF cathode and the second dropping resistor.
     
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  10. dan40

    dan40 Friend of Leo's

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    If the amp begins squealing as soon as the tubes heat up, it may be positive feedback caused by the NFB circuit. If you installed a switch to disconnect the nfb you can simply switch it off and see if the squeal stops. If you don't have a switch, you can unsoldered the wire leading from the output jack to the turret board nfb resistor. If this stops the squealing you have positive feedback and need to switch the OT wires.
     
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  11. Hyakuya

    Hyakuya TDPRI Member

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    Thank's for the advice, I didn't know the NFB circuit could cause that.

    I did fit a switch and I'm pretty sure it was in the off position but I'll double check to be sure.
     
  12. Hyakuya

    Hyakuya TDPRI Member

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    @dan40 i just checked and you're right. The NFB was on and if I flip the switch the noise stops.

    It's the OT wires to the output jack you're talking about switching, right ?
     
    Last edited: Mar 4, 2021
  13. Hyakuya

    Hyakuya TDPRI Member

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    I've gone wrong somewhere. I've never wired up 1/4 inch jacks before, only RCA on stereo amps, so can I have some verification that I've got the jacks wired up correctly.

    Here's the output (right) and headphone (left). The OT wires (green and black) are labelled along with the wires from the feedback switch (green and yellow).

    OT NFB.jpg

    Wired up like this

    Output wiring.png


    I used the diagram @robrob has on his website

    Screenshot 2021-03-04 085915.png

    along with the specs for each jack.

    Speaker output

    mono.png

    Headphones output

    stereo.png
    And the OT wiring diagram

    Screenshot 2021-03-04 092201.png


    Have I got this right....
     
    Last edited: Mar 4, 2021
  14. Hyakuya

    Hyakuya TDPRI Member

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    And the Input

    2360.jpg

    Like this

    Input Wiring Diagram.png
     
  15. Len058

    Len058 TDPRI Member

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  16. Hyakuya

    Hyakuya TDPRI Member

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    Haven't had a guitar on it yet.

    Powered up and the EF80 starts to squeel once it's heated up.

    So, from that link I need to swap the primary wires which run from the output transformer to the power tubes, i.e. the brown and blue on the OT primary which connect to B+ and the EF80 plate.
     
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  17. Len058

    Len058 TDPRI Member

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    You use another OT than Rob recommended but used the same color wires as in the design pictures.


    From the Champmicro page
    Remember 25K:8Ohm and look at the schematic of your OT. What speaker are you using?
    I'm no expert but I think you need to choose other wires from the OT. Choose those that match your speaker impedance.
     
    Last edited: Mar 4, 2021
  18. Hyakuya

    Hyakuya TDPRI Member

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    Hi @Len058, You're looking at the spec for the standard champ micro but I'm on the EF80.

    Further down the same web page:

    The Champ Micro EF80 uses a Hammond 269EX (USA) or Hammond 369EX (International) power transformer and 125ASE 3 watt output transformer wired for 10K:8 ohms using the green and black wires.
     
  19. Hyakuya

    Hyakuya TDPRI Member

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    Right, Primary OT wires swapped round and squeel gone....

    Thanks again. I'm going to read through the troubleshooting guide I think :)
     
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  20. Len058

    Len058 TDPRI Member

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    You're right!
    You're right! My bad!
     
    Last edited: Mar 4, 2021
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