New Mojotone Champ build

Discussion in 'Amp Tech Center' started by Heyhey, Jun 9, 2021.

  1. Heyhey

    Heyhey TDPRI Member

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    My dad worked for Honeywell/Raytheon until he retired in the late 90’s. He did work on sonar systems for the Navy.
     
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  2. Heyhey

    Heyhey TDPRI Member

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    She’s alive, and she sounds like a champ should. Nice and quiet too! I’ll post more pics later.

    8D1810D4-F8D6-4BC2-B4D2-0FEB364E9E7B.jpeg
     
  3. Heyhey

    Heyhey TDPRI Member

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    Here’s the final form. I just gave the cabinet three coats of spray shellac, and it’s got a great texture and shine.

    Got the emblem from Etsy, purple lamp (Go Dawgs!), and we used my great-great uncle’s letter die set for the ON/OFF lettering.

    She ran up with no discernible issues, and it has a nice full sound. Input two is thin and lacking with my guitar, but input one sounds fantastic. I might need to figure out how to improve input two without messing with the other one.

    Overall, the kit was pretty easy to accomplish, but the instructions weren’t great and we found an error in the input wiring layout that we caught by using multiple schematics we were researching. I wouldn’t feel comfortable building this if I didn’t have the rudimentary electronic knowledge I have or the expert help I had with my dad.

    Next time, I’ll use different wire. They used tinned stranded wire for the yellow, and it was very brittle. I had to redo more than a few circuits due to brittle wire. The filament wire was copper solid core, and great to work with. We had some solid core that we used in a couple places. With better wire I’d be more confident in organizing the wires more neatly, but I was afraid to manipulate the soldered connections too much with this wire.

    the on/off switch works great, and the amp has a super low noise floor. We have really good power where we are though, so we’ll see how it does when I get home.

    I may put together a new circuit board with better wire and a better thought out ground scheme to replace this one at some point, although that may be my unrealistic perfectionist talking. The clean sound os full and luscious. It works well with my Flint reverb. I’ll need to figure out the more saturated sound with some playing time.

    I’m really happy with it as a first effort.

    Cheers!

    7C620A93-987E-4AF4-AD3B-5AF74D4FB2E9.jpeg EAF0BA26-7DDE-4E13-A903-A9836BD1C7C8.jpeg 4A2C46BE-CCC6-48A2-B676-B69BEA7D7CC5.jpeg E4364E8D-4BE1-4487-9957-EF1519A64DD2.jpeg
     
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  4. 2L man

    2L man Tele-Holic

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    It looks very good!!!

    Lately I have built circuit boards to have all wires on top so there is never a need to take board out in case I need to modify or replace something. Also i don't drill holes to board sides which thru wires go. I use shortes lengths of wire possible between turretts and tube sockets and pot terminals and it looks it improves amp noise ratio. As much as I'd like to build using textile wires I hate when they fray so I use Silicon insulation more often.

    I install bit too long wires to turretts and lay board in and cut wires to a right length, take board out and wire strip wire ends and then screw board in and solder the wires.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Unfortunate about the wire. There is cloth covered stranded tinned hookup wire that performs better than that. Going with a separate on/off switch was a smart move.;)

    The quality time with your dad is priceless. I am sure this amp will be special as time goes on. Good job!

    So... what are you building next.:D
     
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  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Nice build, well executed! And father-son bonding is a nice byproduct. :) The separate switch is smart, and the control panel looks good.

    Mojo may be out of stuff (supply chains are all broken); they usually supply vintage-style solid-core wire. And did they send you that radial bypass cap? (No big.) If you re-do, try to get either the solid "Fender style" cloth wire (workability A+, ends tend to fray) or pre-tinned stranded -- often called topcoat -- plastic (workability A, easy to strip neatly).

    I like Mojo a ton, but I wouldn't follow their build instructions *or* layout on every point. I you re-do yours, I'd just build to Rob's layout. Note power and ground wiring especially.

    5F1_Layout copy.jpg

    You'll see Rob doesn't elevate the heater CT here, and as you found it you may not need to. *But* it is a good way reduce 60Hz hum if needed. OTOH I bet a separate power switch does even more of that in a Champ. If your dad wants the theory, the entire Merlin chapter (post 14 by @Lowerleftcoast ) is a great read about heaters and hum; elevation doesn't really change the AC or DC voltage the heaters or the cathode "see." But as noted, you likely don't need to do it here, and if your dad doesn't buy the EE, well, my dad was stubborn too. :)
     
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  7. 2L man

    2L man Tele-Holic

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    That layout wiring is not good and it should not be used to quide amp builders! There pre amp current flows to chassis thru the input jack which belongs to most sensitive place on the amplifier.

    Chassis should never be used as part of operative circuit to flow any current!!!

    Single solid ground bus is enough for simple tube amps. For more complicated high gain or reverb/tremolo amps a Star ground can be simpler to build.
     
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  8. Heyhey

    Heyhey TDPRI Member

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    I like Mojo a lot as well, and the kit quality is pretty good. The cabinet is fantastic. It did not come with a cathode bypass cap though, so we scrounged a couple reasonable values from his HAM friend locally. I will be experimenting there. That’s a 33uf 50vdc cap in there now.

    We did kind of did a combo of Mojo and Rob’s layout. The power/ground in is from Rob’s, but the circuit board ground scheme is more from Mojo.

    I definitely would lay things out a little differently next time, but that’s what first times are for.

    Maybe I’ll do a Deluxe as a head next time.

    Cheers!
     
  9. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    @2L man, you seem to say this in every new build thread. Yes, a single ground bus, ** if laid out in the right order** and terminated at the input end, is *theoretically* best. Theoretically. I built my first amp that way. But in fact I actually see few 'ideal' grounds built in practice. Why is that, then?

    These are simple circuits. Rob and others, who can cite lots of experience *and* expert sources, have found the split bus may actually be quieter in some amps. I respectfully suggest that repeatedly criticizing new builds and asking builders to distrust one of our best and most helpful advisors does no one a service.
     
    Last edited: Jun 11, 2021
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  10. 2L man

    2L man Tele-Holic

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    After I understood how common it is to build amps which in my knowledge they should not and also read various forums that it works better I temporarily installed a switch to one of my amps and it was possible to switch single ground bus between where the pre amp current did flow thru the input jack and chassis to PS.

    Unfortunately it was a single ended amp and I was not able to hear significant difference because SE amp does hum some by its nature. However if I was not got influenced by placebo I did heard enough so I have not had to change how I wire the ground.

    I promise to test the same using a push pull amp and also try to measure if I can see something using an oscilloscope and signal generator.

    I am a trained electronics technician and its been my work 40 years so when I began building tube amps this has been a suprice how this old tradition has survived so long :)
     
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