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Help with 1940s ? era amp schematic

Discussion in 'Shock Brother's DIY Amps' started by subgenius_one, Jan 16, 2021.

  1. elpico

    elpico Tele-Afflicted

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    Getting there. Some work left to do sorting out the filament supply and cathode connections.

    12SJ7 needs 12.6Vac between pin 2 and pin 7 to function. You're showing pin 7 as having no connection so either there's an error in the drawing or 12SJ7 is the wrong tube for that position.

    The heater connections for the other tubes look functional, but the center tap of your 12V heater winding has to be connected to something else or this wouldn't work. Take a resistance measurement from pin 8 of the 6L6 to ground and see what you get.
     
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  2. subgenius_one

    subgenius_one TDPRI Member

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    Thanks elpico, and yes I was careless with that last layout. The heater line goes to pin 7 on the 12SJ7 not pin 8. Not sure it would be 12.6v as wired though, as the other heater connection goes to ground. It did work as currently wired.


    I assume the center tap is the line connected to the pilot light? I drew it as such because I assumed it would be since there were 2 other heater wires going to the 2 6l6 on pin 2. I will check continuity as suggested.
     
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  3. subgenius_one

    subgenius_one TDPRI Member

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    No continuity between either 6L6 pin 8 to ground.

    Pin 8 is connected to the other terminal of the pilot light, but that connection terminal is isolated from the grounded case of the pilot light.
     
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  4. elpico

    elpico Tele-Afflicted

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    Okay check 6L6 pin 8 to the center tap of the high voltage winding. If that's open circuit as well then you've found a problem.
     
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  5. timfred

    timfred Tele-Meister

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    Oh that heater wiring is super interesting. It looks to me like the 6L6s are cathode biased using the combination of the 10R resistor and the resistance from the heater filament of the 12SJ7, which both heats the 12SJ7 with “waste” DC current from the 6L6’s AND elevates the heaters. I guess that would work???? but it must be hard on those 12SJ7 heater filaments.


    Can you check resistance between the junction of the 400/400/180k resistors and ground? I’m pretty confident this has to be a ground.
     
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  6. elpico

    elpico Tele-Afflicted

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    Good call, if that 10R is connected to pin 7 then that's probably the situation. That was a popular trick once to provide "free" bias and a dc heated input section all in one go.

    So that points the question of why you found "No continuity between either 6L6 pin 8 to ground" at the 12SJ7. Was it in it's socket when you tried the measurement? Probably should check if it's filament is open.
     
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  7. subgenius_one

    subgenius_one TDPRI Member

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    The tube was no in the socket for the continuity measurement.

    At this point, I think I am going to replace the electrolytics and a couple of wires that are in bad shape and see if I can get it functioning again so that I can take voltage readings.

    I will check the resistors to ground question shortly.
     
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  8. subgenius_one

    subgenius_one TDPRI Member

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    I was correct that the 400k/180k did not go to ground, BUT that is because I cut the ground wire to be able to get at the underside of the board. Before I began tracing the circuit it WAS connected to ground... smh.

    Sorry for the confusion. So I suppose that clears up all of the questions regarding the operation of the circuit? How would you label/name this type of circuit for discussion purposes? I know it is an Octal push/pull circuit. The PI is Cathodyne? How would you label the bias?


    Thanks for your patience!
     
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  9. ArcticWhite

    ArcticWhite Tele-Afflicted

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    Just want to suggest/confirm that you are powering up using a light bulb limiter..
     
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  10. elpico

    elpico Tele-Afflicted

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    Right name for the PI. The power amp is cathode biased. You have the wrinkle that another tube's filament is being used for a cathode resistor, but it's still cathode biased and I don't think there's a name for that particular trick.

    I'd still ohm out the 12SJ7's filament. A burnt out tube there would explain your amp not working and be a pretty easy fix.
     
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  11. subgenius_one

    subgenius_one TDPRI Member

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    I replaced the damaged wires and fired up with the current limiters. Everything looked good so I moved on to regular wall voltage. Everything till looked ok. The amp plays and sounds good, except it's a little noisy - has significant hum and the volume pot is scratchy - but it is pretty much like it was last time it was working.


    B+ is 340v with all tubes installed. I guess now I need to figure out what the PT,OT and Pots specs are.
     
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  12. timfred

    timfred Tele-Meister

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    Assuming the filter caps are old, that's probably the cause of the hum. If it were me I'd take a closer look at the power supply and probably do something like:
    5UGB---40uF (OT)---1K---40uF (screens)---51K ---10uF (new, V2)---5K (new)---10uF (V1)

    Where the middle 10uF and 5K are new. Or leave those out and maybe the hum isn't too bad. Hope that makes sense.
     
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  13. crazydave911

    crazydave911 Doctor of Teleocity

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    That thing reminds me of my old ham transmitter years ago. Two 6L6s push/pull key down 40 watts clean, no chirp ;)
     
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