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  • Bogen PA *battery powered model*

    Has anyone ever converted one of these? The power transformer runs a strange heater arrangement with two secondaries and they connect in series partially to come up with the 6.3vac. Here is a schematic I edited to remove the confusing "vibrators" that generated the AC for the primary.Click image for larger version

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    The amp plays really nice, but I was thinking of using the transformers in another project, but only if I can get the "Right" filament voltages with a "Standard" arrangement of tubes. Thanks
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  • #2
    OK, I did some AC voltage measurements and came up with 6.8v across R43, V6, V7 & V8 and 6.4v across R44, V4 & V5. R45 & R46 drop .58v each and bring down V1, V2 & V3 to 5.22v. This is with 123v AC input line voltage. With the pre-amp tubes drawing 1A those .47 ohm resistors should drop .47v each but even then that would leave 5.46v to a 6.3v tube. I may change those out, or just omit them entirely. I guess if I keep four 1.2A filament tubes in the amp I can use it the way it is now. It's a balancing act with the filament currents through four series parallel tubes, and when I first got it, one of the rectifiers used a slightly different type of heater wire and it threw the whole thing off balance.

    It looks like KT66 may do as a replacement for the odd 6av5ga. I'm not sure if it will draw more supply current than the 6av5ga. The schematic shows the transformer as only .080A, but that may just be the idle current tested by Sams when they did the schematic. It's heater is 1.2A and that matches the 6ax5gt.
    Last edited by guitician; 02-12-2014, 06:09 PM. Reason: additions
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    • #3
      NOS 6av5 ga tubes can be purchased at AES for $5.50 each. They will match them if you want.
      They were often used as "horizontal sweep tubes " in TV sets and are quite rugged.
      Bogen used them in several audio amps were they worked quite well. DB130 is an example
      The plates handle high voltages well but the screens do not .
      SG

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      • #4
        I wonder if in Battery Mode the heaters are just connected to 12VDC and the otherwise unused heater windings on the PT are used as the primary for Vibrator AC.
        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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        • #5
          Yeah, the 6V battery(who knows what they used in 1959) would just run all the 6.3v filaments directly, and when 12v was used the series parallel arrangement was used. The smaller transformer powered the phonograph motor and the HV transformer, which only loaded the filament windings in 120VAC mode.

          The stock outputs are fine and cheap I know, but I was thinking of a simpler circuit to use the salvaged transformers for. I only paid $35 for the whole amp, with tubes.
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