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First Build - Rob Robinette Micro Champ

Discussion in 'Shock Brother's DIY Amps' started by SJK45, Aug 14, 2020.

  1. SJK45

    SJK45 TDPRI Member

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    I’m a bit nervous about this since I know very little about electronics but I thought I’d give it a go.

    I’ve almost populated the board (one capacitor left to find) and need to source a chassis.

    Other than that I have all the parts including the recommended 270ac transformer (although i wasn’t sure that it was ok for the UK?). Anyway hopefully I can educate myself along the way!
     
    Last edited: Aug 14, 2020
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  2. andrewRneumann

    andrewRneumann Tele-Holic

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    Welcome aboard my friend! You will find a lot of helpful people here if you run into trouble. Let’s start with that transformer you’ve got. What make and model is it? Does it have any labels?
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    If you are referring to the Hammond 270 cax, unfortunately that is for 120V wall voltage. Do you have a 120v to 240v converter available???

    Welcome to TDPRI.
     
  4. SJK45

    SJK45 TDPRI Member

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    image.jpg Thanks guys it’s the 270ax which is the one that Rob recommends on his site.
     
  5. andrewRneumann

    andrewRneumann Tele-Holic

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    As @Lowerleftcoast notes, that particular transformer will not work unless you have something to step your wall voltage down to 115-125vac. Sorry. You can check out the Universal series from Hammond but there’s probably something else around that others on this forum will lead you to.
     
  6. SJK45

    SJK45 TDPRI Member

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    Ahh, ok. Do I need to change the output transformer too? image.jpg
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    ^^^^
    What Andy said.

    The OT will work.
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    So you are looking for a PT with secondary near 480V with a center tap. Around 50mA. With a 6.3V heater winding. Let's see if @Jon Snell from London can give a suggestion that won't break the bank.

    (Jon, this Champ micro is just a 12ay7 preamp valve and a 12AU7 power valve. Probably does not need 50mA.)
     
  9. Paul-T

    Paul-T Tele-Meister

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    Ah, annoying, sorry to see that.

    I'm building a Deluxe Micro. Looked around for a while and picked on this tube-town toroidal. I love Hammonds but this was much cheaper. 240V secondary which I think is in the right zone for you.

    The classic budget-ish Hammond with European primaries is the 290CAX. Look out for Mouser as they claim it costs £71 but then charge VAT on top. I

    Key question though: are you using valve rectification or diode? The tube-town would work for diode, the Hammond for tube I think (wait for other opinions, it might be a tad high).

    IN a hurry and haven't had time to look for the ideal secondary voltage for the Micro Champ, but if you're using the diode rectifier (which I'm doing, bridge configuration), this ampmaker transformer has a 190V secondary which might work well too.

    Best till last: this little toroidal is made by Carnhill who make transformers for high end mixing desks etc. 30mA which should be good with a 12AU7. I think it will have enough power for you and is tiny. Voltages look good for diode rectification. Would love to see how it works out. £33!

    Ampmaker look to be a great little company, they were on furlough when I got my own Micro parts, I bet they'd offer good advice whether that little toroidal works for the Champ Micro too (he states he's tested an amp with the 12Ax7/12AU7 combination)
     
  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Rob mentioned on the sight that 190v is a little low but with using a lower value dropping resistor it would be OK.

    How about the Hammond 370 ax?
     
  11. SJK45

    SJK45 TDPRI Member

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    I’m think I’m using diode rectification.

    I’ve got my other parts from Amp Maker so I‘ll get in touch with him tomorrow and see what he says. Thanks so much for your help guys.
     
  12. andrewRneumann

    andrewRneumann Tele-Holic

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    Does your plan call for a 25k:8 (impedance ratio) output transformer? If so, I think you might also have trouble with the Hammond 125ASE. We will find out.
     
  13. SJK45

    SJK45 TDPRI Member

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    The schematic does say 25k:8ohm for the output transformer. Some more research required I think!
     
  14. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Hold your horses. The Hammond 125ASE is a 3W multi tap single ended OT. All those taps mean *options*. The transformer is just coils of wire and there is a *winding ratio*.
    This means the 10k, 5k, 2500 primary taps have a relation to the 4, 8, 16, and 32ohm taps.
    Now the 125ASE is not going to be able to get to the magic numbers 25k:8ohm but it can get to 20k:8ohm which IMO is close enough. (Remember speaker impedance changes with frequency so there is a fudge factor involved.)(Windings aren't always the same either.)

    Consider a 5k:4ohm OT. When a 8ohm load is plugged in, the primary is no longer 5k. The tube(s) sees 10k impedance. So the ratio is maintained. A 5k:4ohm OT = a 10k:8ohm OT in that respect.

    With the 125ASE a 20k:8ohm ratio can be had by using the 10k primary and using a 8ohm load on the 4ohm tap. The wattage rating of 3w is sufficient for the 12au7 micro champ.

    Also, I would feel more comfortable with a OT built for a SE amp, rather than a reverb driver. OMMV.

    ...now if we could just get SJK45 to build a second amp so we could have an A-B test. Hmm.
     
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  15. Mexitele Blues

    Mexitele Blues Tele-Afflicted

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    You got me wondering, is the micro power amp that much different from a fender reverb driver circuit with its 8 ohm reverb pan?
     
  16. andrewRneumann

    andrewRneumann Tele-Holic

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    That’s all I wanted to check. The impedance ratio in the design is 3.1k:1 and the closest that OT has is 2.5k:1. All else being equal does this just mean the power tube doesn’t deliver quite as much power to the speaker?
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    From my internet research (not hands on experience), the 12au7 would want to operate on a much lower primary impedance. (7k7). There is talk that it is sonically more pleasing at 2 to 3 times that impedance. Rob's PP micro bassman suggests a OT primary of 22k5. The SE micro champ, with the parallel 12au7, suggests the 25k primary. The 25k seems it would be too high with the parallel 12au7. Maybe it should be half that. In any case, according to the Valve Wizard, the higher primary impedance would deliver less headroom. So I would think the lower impedance ratio would be more efficient, but honestly I doubt I could hear a difference between the 25k and 20k primaries.
    Of course different voltages and frequencies will skew the numbers. The frequencies the 125ASE and reverb driver are designed to handle would probably have to be factored in as well.
    @Mexitele Blues the reverb driver is very similar to the micro. You can put an 8ohm speaker on the output and have a listen.
     
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  18. andrewRneumann

    andrewRneumann Tele-Holic

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    @SJK45 what @Lowerleftcoast is saying is that when you get around to hooking the 125ASE up, use the orange wire (4 Ohm : 10,000 Ohm) for the speaker jack. :)

    EDIT: The black wire also goes to the speaker jack. And still use an 8 Ohm speaker.
     
    Last edited: Aug 15, 2020
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  19. andrewRneumann

    andrewRneumann Tele-Holic

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    Yeah that’s going to look like 50k when you parallel 2 tubes. I wonder why the design shows 25k.
     
  20. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I apologize SJK45, I thought I had a suitable international power transformer listed on the Champ Micro webpage but I do not. I recommend the Hammond 369HX which is 225-0-225V HT and supports international mains voltage. I will add this to the webpage.
     
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