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Can anybody translate this schematic?

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  • Can anybody translate this schematic?

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    Trying to build a small cigar box tube amp, came across this schematic, but I'm not exactly trained in reading them, can anyone help translate?

  • #2
    The 12U7 and 12K5 tubes appear to be special tubes intended for low voltage operation, in this case 12V. They are rated at 30V maximum plate voltage. There is no indication what the output transformer impedance needs to be.

    The schematic shows two preamp stages and a power amp stage with volume controls between each stage. There appears to be a mistake on the power amp stage. The signal is applied to the screen grid and the control grid connected to B+. Probably won't work like that. My guess would be that whoever drew the schematic never got it working or published it as a joke.

    It would be easy enough to cobble something together similar to a Champ with a 12A?7 and a small power tube like 6AQ5. The hard part is finding transformers that will fit. If you are in a country with 100 to 125V mains, I'll show you a little trick to find a small transformer that will work for a B+ of about 150V or maybe 300V with a doubler.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      Hmm well first off how difficult are we talking? I'm in Michigan so I imagine I have access to whatever I may need in the country. I'm not exactly well versed in anything with amps.
      To give an idea of whats going on in my head, I've built a few cigar box guitars and am now trying to make a really nice one. 3 string, aluminum pickups I'm making, zebra wood bridge and fretboard etc... and want a nice little tube amp to go with it. Normally I just cram a cheap 2.5w headphone amp attached to a pot in them w/ a input led and speaker. They sound great in the most atrocious of ways however not fitting for my goal. Now I would imagine lower voltage dc would work better for temps, not sure how much heat a old wood box can handle. Preferably something in the 2-8w range. My goal is to find a prohibition era large cigar box, they could be found upwards of 17x10x6 which leaves a bit of room including 3-6" speaker. I'm a machinist so I could fabricate a steel plate to guard the inner wood from temps if required, but would prefer to keep things somewhat "rustic".

      Also making something from scratch isn't really a necessity too, I'm willing to put in the work and very handy with a soldering iron, but if theres "cheap" like these tube pre-amps and such I find on ebay that can be combined in line with something else I wouldn't be against that either.

      http://amps.zugster.net/projects/micro-champ
      I did come across this.
      Last edited by R.Madgwick; 04-14-2013, 04:36 AM.

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      • #4
        Wow, that's a big cigar box. Just about any tube amp will need ventalation. Think about what a 25 or 40 Watt light bulb would be like in the box. A good plan is to have small rubber feet under the box, perhaps 1/4 inch or more and slots to allow air to enter. More slots on the top so heat can escape. With just 2 tubes, 4 to 5 Watts is a practical limit. Going higher means more tubes, bigger transformers and more heat.

        Look over the Micro-Champ you linked above. Think about how you can construct a chassis to mount the transformers, tubes, circuit board, controls and switches. Keep in mind the airflow issues. Think about what controls you want, volume, tone etc. In general keep the preamp tube, input jack and controls as far form the transformers as you can.
        WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
        REMEMBER: Everybody knows that smokin' ain't allowed in school !

        Comment

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