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Next Build - RobRob Champ Micro

Discussion in 'Shock Brother's DIY Amps' started by Lavely, Jan 23, 2021.

  1. Lavely

    Lavely TDPRI Member

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    I'm having a ball trying to get my 5E3 working...not there yet, but that build should be done after I accomplish some troubleshooting. I have most of what I need to get started on the Champ Micro, so starting this thread!

    For those of you that don't know, here is a page on the Champ Micro, with build details:

    https://robrobinette.com/Champ_Micro.htm

    It's basically a lower-volume 0.75 watt Champ with a Master Volume. Very simple circuit. I'm going to house in a Priceton Reverb chassis and head cab.

    First basic question: the circuit calls for a voltage dropper resistor of 2W, 39k. I couldn't find 39k, so I bought 33k & 47k - what are the implications of using one of these instead?

    Also, any tribal knowledge to be aware of while building this one? I'm going with the tube recto instead of SS.
     
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  2. Lavochkin

    Lavochkin TDPRI Member

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    I built a Champ Micro about a year ago and also couldn't find a 39K 2W. Instead I used a 12K and 27K in series (circled in red). It made the cap leads a little crooked but I managed to make it fit. series resistors.jpg
     
  3. Lavely

    Lavely TDPRI Member

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    @Lavochkin - how do you like the sounds of the amp?
     
  4. Lavochkin

    Lavochkin TDPRI Member

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    I like it, but I'm a new guitarist and my only prior amp experience is with a Blackstar Fly practice amp, so I don't much to compare it to.
     
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If you can't find a 39k I'd go for the 33k instead.
     
  6. Lavely

    Lavely TDPRI Member

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  7. Lavely

    Lavely TDPRI Member

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    A couple more questions:

    1 - I'm building in a Princeton Reverb chassis (widening a few holes, etc.). TONS of open space, which is a nice change from the 5E3! However, given the slant to the front control panel, the pilot light assembly touches the bottom of the chassis and flexes to fit. Is this OK, or will I have electrical issues? It LOOKS like the leads touch pieces of metal that are not the same as the "body" of the assembly that touches the chassis, but wanted to confirm. Given I'm using a standard Fender piece, I assume this is OK?

    2 - The layout for the Micro Champ (below) shows four wires coming from the OT - blue, red, yellow, orange. HOWEVER, the OT actually has only blue, red & orange wires (no yellow). HOWEVER, it also has tabs 1, 2, 3, 4, 5, 6 on the other side of the OT. I'm guessing I attach a yellow wire to one of these tabs and that goes to the Speaker Jack (as shown in the layout). Looking at the specs for the transformer, this is pretty unclear...my guess, from the specs, is that I connect the yellow wire to BOTH tabs 1 & 5, then to the speaker jack, and this gives me an 8 ohm tap. Am I reading correctly? The table I'm referencing is shown below as well (last 2 images have data on OT).



    [​IMG]Screenshot (9) by joelavely, on Flickr



    [​IMG]Screenshot (13) by joelavely, on Flickr

    [​IMG]Screenshot (11) by joelavely, on Flickr
     
  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I'll need to see a pic of the fuse holder.

    It looks like I specified the wrong output transformer. The 125A is a push-pull OT and the Champ Micro needs a single-ended OT. A standard 25K:8 ohm Fender style reverb transformer is what is needed.

    I don't know how I did that and I apologize profusely. I must have edited the layout when I was doing the EF80 version of the Champ Micro.

    The good news is the 125A you have in hand will work in all the other push-pull micro amps so maybe you won't have to send it back. I'll edit the Champ Micro webpage immediately. Again, I'm sorry about the mix up.
     
    Last edited: Feb 5, 2021
  9. Lavely

    Lavely TDPRI Member

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  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    No, that is not good. If you put an additional bolt or shim on the outside it may give you clearance.

    The Hammond 1750A output transformer makes a very nice output transformer for the Champ Micro.
     
  11. Lavely

    Lavely TDPRI Member

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    @robrob - looks like both the Hammond 1750A and the Classic Tone 40-18034 are unavailable right now. Hammond is out of stock everywhere (maybe issues with components from China? Just a guess) and the Classic Tone is no longer made. I could wait for the Hammond, but feels like a couple months. The other Hammond you list, the 125ASE, is around $50 instead of the $20 for the (unavailable) 1750A.

    Any thoughts on what other OT will work? I am building BOTH a Deluxe Micro and a Champ Micro, but I assume they are interchangeable OTs. Mercury Magnetics?
     
  12. Paul-T

    Paul-T Tele-Holic

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    Last edited: Feb 13, 2021
  13. 2L man

    2L man Tele-Meister

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    If you have an 8 ohm loudspeaker you could use Hammond 1750C or 1760C. They are 7000 ohms and have 3,2 ohm output and when 8 ohm speaker is connected the impedance comes 17500 ohms. If you use two parallel 12AU7 (or any paralelel tube) one tube load comes half 35000 ohms. If operative voltage is 370VDC loadline looks about this. Using lower operative voltage and more bias current operation point would get centered better.

    If you find other OPT you can easily test how it will operate using Universal Loadline Calculator.

    PHOTO-2021-02-13-22-32-29.jpg
     
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  14. Lavely

    Lavely TDPRI Member

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    @Paul-T - Amplified Parts has the Hammond 1750A, but the specs there say it is 22.8k:8 ohms, while different places on the Hammond site say the same thing in some places, but 25k:8 ohms in others!! Anyone know (1) does it matter and (2) has there been a change in the model and this now needs to be avoided?



    Can anyone tell me the proper Heyboer model #? Or, is that just the Mojo Reverb Driver above? Weird, as it doesn't have the 25K:8 ratio...

    How about this Weber?

    https://tedweber.com/w022921/
     
  15. Paul-T

    Paul-T Tele-Holic

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    @L-Man, the OP is using the EF80.

    The Weber is like the other reverb transformers, but I think I was wrong to link as I'd forgotten you have a single ended power section which needs 10k:8Ohms according to Rob. That seems slightly more esoteric. I can't see many in stock anywhere, outsid of VVT in the UK.

    Have you bought a speaker yet? Someone with more knowledge than me might be able to tell us if a 22k/8ohms transformer would give the correct reflected impedance to the output tube with a 4 ohm speaker?
     
    Last edited: Feb 14, 2021
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Lavely, 22.5k to 25k will work just fine.
     
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  17. Lavely

    Lavely TDPRI Member

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  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The tubedepot should be a standard Fender so it should work. 29k is higher than I would want to go with a 12AU7 or EF80.
     
  19. Lavely

    Lavely TDPRI Member

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    I'm waiting on Tube Depot to confirm their transformer is 25k:8...any idea WHY I can't find a Hammond 1750A? I have tried everywhere I can, no one has them in stock. Is there a Heyboer that would work that someone actually has in stock? Feels wrong to pay $50 for a Mercury Magnetics version...

    Is the Weber going to leave me wanting something better?
    https://tedweber.com/w022921/
     
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