Mojotone 5f1 Champ

Discussion in 'Amp Tech Center' started by lil scotty, Jul 27, 2019.

  1. lil scotty

    lil scotty Tele-Meister

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    Hi. This is my first build and I have two questions if anyone is up for helping me.

    1) The chassis indicates a 2 AMP fuse. But my kit came with a 1A/250V. The box with kit parts says 3AG/1AMP. My fuse blew after about 2-3 minutes of powering on for the first time. It seems I need a 2 AMP fuse, no?

    2) Instructions don’t say what to do with all of those extra wires (Black with color coding) coming out of PT? I think I might have blown the fuse when moving those wires out of the way. They aren’t capped or “dealt” with at all, currently.

    Thank you for your consideration.
     
    Last edited: Jul 27, 2019
  2. Intubator

    Intubator Tele-Meister

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    You may want to check out this website before powering up your amp again.

    https://robrobinette.com/Tube_Amp_Startup.htm


    Any dangling wires can cause problems and cause a potentially a damaging/dangerous situation. Get the ends well insulated so they won't short.. Sounds like your fuse blew due to a wiring issue, not bc of the amperage rating necessarily... Good Luck going forward!
     
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  3. lil scotty

    lil scotty Tele-Meister

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    Thanks. I will check that out. The manual says nothing about those extra wires. They are just grayed out in all the diagrams. It’s driving me nuts!
     
  4. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Intubator and lil scotty like this.
  5. Wally

    Wally Telefied Ad Free Member

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    Is this the kit with which you are working?
    https://www.mojotone.com/kits/TweedAmpKits_x/Test-Tweed-Champ-Amp-Kit
    Does your power transformer have wires other than shown in that schematic? FWIW, this schematic shows a 1A fuse.
    Re: those ‘extra’ wires you have dangling around.... be aware that there is danger when working with these electronics. Never should there be unused wires that are not taped off or otherwise 8nsulated...maybe with shrink wrap on the ends. The schematic does not show any extra black wires. Black wires usually are used on the primary side of the PT, and there are two shown there.
    If you have a schematic and/or a layout, it would be good to post that IF it is different from what I linked above.

    I also don’t care for the original AC wiring which is also used in that Stew Mac approach. There is a safer way to wire it. The neutral should go straight to a primary wire. The AC Hot would then go through the fuse to the switch and then to the other PT primary connection.
     
    Last edited: Jul 28, 2019
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  6. lil scotty

    lil scotty Tele-Meister

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    image.jpg
    Yes, that’s it Wally. I’m now waiting on a snuffer stick and fuses from StewMac. Won’t be touching amp until then. Step one will be to drain caps. Step two will be to insulate those lose ends.

    I did see the changes you’re talking about on a D-Lab video on YouTube.

    I followed Mojotone instructions to a T. Should have known better than to leave those wires loose. Though, they don’t address them once in instructions. They are grayed out/translucent in this diagram, to say they are “previously connected.” Not true! Wires are for this, I assume: “The multi-tap primary power transformer now comes standard, and works with 110, 120, 220, 230, and 240 wall voltages.”
     
  7. Wally

    Wally Telefied Ad Free Member

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    Take care and good luck. Danger is present even for experienced techs. Leaving AC primary windings bare, presenting wall voltage to that primary winding, and witnessing the results SHOULD be a warning to one’s self that perhaps some preparation for the task at hand is in order. That current could have coursed through your body instead of the fuse. Nothing personal......just concerned. I hold that building kits is something that should be done after some basic understanding is achieved. OMMV...and obviously do.
    I withdraw from this thread. You will get help enough from the forum.
     
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  8. lil scotty

    lil scotty Tele-Meister

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    Wally, I know it. I dodged a bullet and appreciate the severity of the scenario and your warning.
     
    Last edited: Jul 28, 2019
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  9. lil scotty

    lil scotty Tele-Meister

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    I’ve notified Mojotone to get the amp kit manual updated STAT!
     
  10. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Circling back to a question in the first post. I'm not sure if using a 2A fuse presents any real safety problem, but you should be able to use a 1A with no issues in a 5F1 Champ.

    Total 6.3VAC heater draw is 0.45A for the 6V6GT and 0.3A for the 12AX7 = 0.75.
    0.75A * 6.3VAC = 4.725W Let's call it 5W

    Total 5VAC heater draw is 2A for the 5Y3GT = 10W

    Total high voltage circuit draw is very roughly (you can calculate more precisely with your own B+ and tube bias numbers) 15W

    5W + 10W + 15W = 30W on the secondary side of the PT

    120VAC on the primary side of the PT

    30W / 120VAC = 0.25A

    So a 1A fuse has a 4X margin for moments of excessive current demand plus it's a slow-blow type fuse.

    I have used 1A in all my 1@ single-ended and 2@ push-pull 6V6GT powered amps with a greater number of preamp tubes without any issues yet.

    Corrections welcome please if anyone catches a mistake. Cheers
     
  11. jsnwhite619

    jsnwhite619 Friend of Leo's

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    If those PT wires aren't snipped and wrapped, that's probably what shorted and blew the fuse. Most come with all those wires left with bare leads. If they touched each other or the chassis, that would have blown the fuse. You said it was on a few minutes, but one of the wires could have moved and made contact.

    Be sure the tubes are in correctly and line up with the key slot. That can blow a fuse -- if you're lucky.
     
  12. peteb

    peteb Friend of Leo's

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    I think the calculations for expected current are close, but a little low.



    The AA764 champ tube chart shows 60W power consumption at 117 VAC, dividing yields 0.51 Amps, justifying the 1 amp fuse.



    Using a kill a watt power monitor I find champs run between 40-50 Watts of power consumption. The input current is around 0.4 to 0.45 amps.






    Champs run class A, all out all of the time, there is no excessive current demand ever.


    Playing class A and class AB Amps thru a power monitor at high and low out put clearly shows this. That is one of the defining characteristics of class A operation.



    I have used 1/2 amp fuses only, in single and double 6V6 amps for 15 years without a problem.
     
  13. peteb

    peteb Friend of Leo's

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    30 W and .25 amps could be closer than I thought.


    The 5F1 tube chart says 40 W at 110 volts for .36 Amps, making 30 W and .25 amps possible.



    It must be the smaller filter caps in the 5F1 than the AA764 that make it use less power. I just checked my numbers to see if larger filter caps like 40-20-20 draw more current than 20-20-20. It appears to be the case. I have one champ that consumes Watts in the high 40s and one champ that consumes Watts in the low 40s and sure enough the 40-20-20 consumes about 10% more power than the 20-20-20 champ.
     
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