T shirt circuit?

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Oct 18, 2020 at 9:49 AM.

  1. Jewellworks

    Jewellworks Tele-Meister

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    I saw this t shirt on eBay, and I'm really curious about what amp this came from?? Anyone know?

    Screenshot_20201018-094528_eBay.jpg

    it's an interesting circuit. Two channels with pentode preamps with some pretty serious gain, feeding a phase inverter, feeding two 6L6s in push pull, but then each 6L6 has its own transformer and speaker, which is really strange. And that much gain on those pentode preamps is probably going to create a fair amount of noise in the circuit. I can read the circuit, but I'm having a hard time deciphering the reference notes as to what the component values actually are. I really wonder what amplifier this is...
     
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  2. radiocaster

    radiocaster Poster Extraordinaire

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    Microphone and instrument inputs, each with a 6SJ7, 6N7 phase inverter. I'm guessing Gibson.
     
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  3. printer2

    printer2 Poster Extraordinaire

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    There were a number of amplifier companies skinning the cat at the time. I do not think it is a Gibson. There were a number of PA type amplifiers being built at the time, there were books of the schematics. Could have been lifted out of one.
     
  4. elpico

    elpico Tele-Afflicted

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    Hard to make out the exact connections but looks like the OTs are simply wired in parallel. Marshall did that when they built their first 100w amps, just slap on two 50w OTs cause you already have em on hand and let it rip. This would've probably been two 6V6 transformers paralleled to handle the 6L6.
     
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  5. milocj

    milocj Friend of Leo's

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    That may be a Fender Woody Pro.
     
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  6. elpico

    elpico Tele-Afflicted

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    According to this page the source of the schematic is an old Aspen Pitmann book. He refers to it as an old bassman, with the label "5A6" added to one of the drawings. I'm not aware of any such model?
     
  7. elpico

    elpico Tele-Afflicted

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    Some further digging suggests Pittman had it wrong and that schematic is likely for the older dual professional. The dual OTs were mounted on the speakers

    [​IMG]


    [​IMG]

    [​IMG]
     
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  8. awasson

    awasson Poster Extraordinaire Gold Supporter

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    Those aren’t output transformers. Those are step down transformers for a constant voltage output amplifier. They take 70 volts or 100 volts and step it down to work with an 8 ohm or 16 ohm speaker. The amplifier still has an output transformer. They still use constant voltage amplifiers in intercoms where they have enormous long runs from the amp to the speakers.

    I’m leaning towards that Bassman 5A6 amp theory. I’ve got a book with a large Fender section. I’ll see if I can find the 5A6 in it.
     
  9. radiocaster

    radiocaster Poster Extraordinaire

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    [​IMG]
    I was wrong about Gibson, but I knew it looked familiar.
     
  10. elpico

    elpico Tele-Afflicted

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    The number of inputs and controls is wrong for the bassman. It does match the dual professional.

    Placing tube output transformers on the speaker was indeed a thing in the 40s and 50s. Many gibson amps out there with the same layout. I know what awasson is thinking of but that's not the same thing. Here's another dual professional, this time with the standard blue-red-brown OT primary wires visible going from the transformer to the amp. You can see that two of the wires head straight to the rectifer tube, those would be the center taps.

    [​IMG]
     
  11. radiocaster

    radiocaster Poster Extraordinaire

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    I'm not even sure they made a 5A6, maybe only 5B6.
     
    Last edited: Oct 19, 2020 at 4:33 AM
  12. andrewRneumann

    andrewRneumann Tele-Holic

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    bingo.

    Apparently Billy Gibbons has an obsession with them. Could explain the T-shirt.
     
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  13. tubegeek

    tubegeek Friend of Leo's

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    Can't seem to upload this large file here but a very complete Bassman history includes this schematic and ID's it as a 1947 Dual Professional.

    Search for "Bassman history gitec" and you'll find it, it's a pdf.

    Interesting circuit with the dual transformers. Mounting at the speakers was a common layout practice in the '40s. Later 70 Volt distributed PA systems also used this mounting style but it's not the same thing.

    I think the idea originally was to keep the high current wiring very short. In later years the priority shifted to keeping the user protected from the high voltage wiring.
     
  14. printer2

    printer2 Poster Extraordinaire

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    It had been standard practice to have the OT on the speaker in the 30's and 40's in early radios as the early speakers did not have a permanent magnet but a field coil to supply the magnetic flux. The manufacturer supplied the speaker and OT as a unit.

    Now I remember reading about it as the Pro. The style of drawing reminded me of the Aspen Pitmann books, I could not remember the name of them.
     
  15. Peegoo

    Peegoo Friend of Leo's

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    Here ya go:

    [​IMG]
     
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  16. jvin248

    jvin248 Poster Extraordinaire

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    Gibson and Marshall and everyone else were copying Fender amps at the time -- the amps were very successful.
    Only after Leo was pestered to sell a guitar with the amps did he come up with the Tele and Strats, using significant player input into their design.

    .
     
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  17. Paul G.

    Paul G. Friend of Leo's

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    Early Fender amps were pretty much out of the RCA manual, public domain circuits designed to sell their tubes. Gibson amps predated Fenders and also used standard circuits. Marshall didn't make amps until nearly 20 years later. The first Marshall was pretty much a part for part copy of the 5F6A, but went their own way.

    Remember, the first Nikon cameras were exact copies of Contax -- internal parts were actually interchangeable. Canons were Leicas. Everybody starts somewhere.
     
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  18. radiocaster

    radiocaster Poster Extraordinaire

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    What about in the 1930s? Were Gibson copying Fender?
     
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  19. radiocaster

    radiocaster Poster Extraordinaire

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  20. Peegoo

    Peegoo Friend of Leo's

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    It's a '47 Dual Professional. The image I posted is from an article that explains the widely-held misunderstanding that it's an "old Bassman" (link at bottom on image).
     
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