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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. subgenius_one

    subgenius_one TDPRI Member

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    I am trying to document the schematic for an old amp that I picked up a number of years ago. I had the amp repaired by an amp tech years ago and he replaced the defunct field coil speaker and I am guessing added the output transformer. It worked well, but it is no longer working.

    I want to be able to reproduce the circuit as well as maybe modify it to use as a micro amp. I was hoping some of you might be willing to review the circuit I have drawn to see if I have made glaring errors. I also have a few components that I am unsure of the values as well as some of the connections look strange to me.

    V1 12SJ7
    V2 6SN7GTA
    V3 6L6GC
    V4 6L6GC
    V5 5U4GB



    Edit: Update drawing to reflect changes:

    0116211200.jpg 0116211225.jpg
     

    Attached Files:

    Last edited: Jan 16, 2021
  2. NTC

    NTC Tele-Holic

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    Bottom picture is a capacitor. I don't know what the red thing is. Your schematic does not show cathode connections for your 6L6's. First triode is missing an anode resistor - you may have drawn it in the wrong side of the coupling cap.

    I am suspicious about the heater connections - a 12SJ7 has a 12 V heater, but the 6V6's and 6SN7 are 6V. If there is a resistor in series with the 12SJ7 heater on an otherwise 6V secondary, it would not work well. Perhaps the winding is for 12V, the resistor is on the 6SN7, and the 6V6 heaters are in series?
     
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  3. dan40

    dan40 Friend of Leo's

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    The first pic appears to be a "dogbone" ceramic capacitor. Does it have a value marked on it?
     
  4. subgenius_one

    subgenius_one TDPRI Member

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    The component in the first pic does not have any values - just the color code - Red, Black Green I believe. I assumed it was a resistor:

    https://www.radiomuseum.org/forum/replicating_old_dogbone_resistors.html



    The cathode connections on the 6L6's are connected to the heater pin. I do not understand why.

    Any recommendations for a typical value for the capacitor? It connects the Bass Pot to the grid of the first triode in the 6SN7.
     
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  5. Milspec

    Milspec Poster Extraordinaire

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    Not to be a trouble-maker here, but are you certain they are 6L6 tubes and not a pair of 6Y6G tubes? For a '40's amp with that tube complement, I see far more of the 6Y6 tubes since the 6L6 was still pretty new.
     
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  6. printer2

    printer2 Poster Extraordinaire

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    I could be wrong but the 6L6 was introduced in 1936 and the 6Y6 in 1937.
     
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  7. Milspec

    Milspec Poster Extraordinaire

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    I couldn't say for certain either, but always thought the 6L6 came along later in '39. I own a few mid-40's amps and they are all of the 6Y6 variety (precurser to the 6V6) and since amps of that era are normally low watt affairs (12-20w), it just seems less likely for the need of dual 6L6.

    Again, I am no expert at all, just struck me as odd to read 6L6GC in your post both for being a 6L6 and for it being the latest version ("GC") of the tube.
     
  8. subgenius_one

    subgenius_one TDPRI Member

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    The tubes are 6L6, but there is of course no schematic or tube listing for the amp. I have searched endlessly and only found one similar amp - The Guitologist on youtube has one that can be seen in the background of a couple of videos.

    Screen Shot 2020-07-29 at 5.12.24 PM.png
     
  9. printer2

    printer2 Poster Extraordinaire

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    You must have my post mistaken with another. I did not mention the 6L6GC but the 6L6GC is the latest version of the 6L6.
     
  10. David Barnett

    David Barnett Doctor of Teleocity

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    I hope that fiber material isn't asbestos.
     
  11. DADGAD

    DADGAD Friend of Leo's

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    That red thing with the green dot and black end is the 2M ohm screen resistor. Can’t tell the value of the silver mica capacitor because the shadow is obscuring the colors of the dots.

    6L6 tubes came out in the 1930’s.
     
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  12. subgenius_one

    subgenius_one TDPRI Member

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    The 2M on the schematic is just my value based on how I read the color code. Are you saying you concur with the value?
     
  13. Wally

    Wally Telefied Ad Free Member

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  14. Jesco

    Jesco TDPRI Member

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    If it is a cathode biased circuit, I would assume that is an elevated heater supply. Elevated heaters are used to decrease hum.
     
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  15. Milspec

    Milspec Poster Extraordinaire

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    I referred to the details in the OP which stated "6L6GC" power tubes.

    I just thought it was odd, no big deal, but if you don't ask the questions you don't learn anything new. I never ran into a 6L6 amp of that era and didn't think they came along until late '40's to early '50's with Fender. What I have seen a lot of are very early amps without tube complement decals so new owners end up sticking the wrong ones in there....happens a lot which is why I asked the question.

    Got it, the amp in question does rate a 6L6 tube.
     
  16. printer2

    printer2 Poster Extraordinaire

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    The stuff was all over the place. Either encapsulate it or remove it carefully. Dampen it so the fibers do not break off and float away.

    Yes indeed.
     
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  17. subgenius_one

    subgenius_one TDPRI Member

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    0117211335.jpg I powered the chassis and checked voltages with no tubes.

    5v to Rectifier
    690V to Rectifier
    6.3v to heaters

    Wall voltage is ~130v

    Also corrected schematic to move the DC supply to the Cap Can to the correct side of the rectifier.
     
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  18. Wally

    Wally Telefied Ad Free Member

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    Gibson had amps in the late 30s with 6L6s, I think. EH-150???? As used by Charlie Christian??.
     
  19. timfred

    timfred Tele-Meister

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    The 400K resistors connected to the power tube grids are grid leak resistors and either go to a bias supply or ground. You have them connected to the 180K cathode resistor for the cathodyne phase splitter. I think these all need to go to ground.

    The 0.047 cap that you show coming off the plate of the first 6SN7 triode paralleled with the 47K plate resistor is probably instead connected to the grid of the phase splitter (with the 2M leak resistor going to the 15K/180K junction).

    Like others have said, you appear to be missing a cathode resistor and bypass cap for the power tubes.
     
  20. subgenius_one

    subgenius_one TDPRI Member

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    Following up on the questions and comments:

    "The 400K resistors connected to the power tube grids are grid leak resistors and either go to a bias supply or ground. You have them connected to the 180K cathode resistor for the cathodyne phase splitter. I think these all need to go to ground."​

    I have reverified it again and the drawing is correct to the existing wiring. The 400k resistors connect to the 10k resistor, which connects to the 15k Cathode resistor, the 20M resistor connecting to the grid, and the .033MFD Cap.

    "The 0.047 cap that you show coming off the plate of the first 6SN7 triode paralleled with the 47K plate resistor is probably instead connected to the grid of the phase splitter (with the 2M leak resistor going to the 15K/180K junction)."​

    I did find a couple of errors here. The .047 cap and the 470K (updated value) resistor connect to the plate, but are not both connected to the screen of the PI as drawn originally. The resistor goes to B+ only. The .047 cap does go to the grid of the PI. Perhaps it is not easy to read - I will try to redraw.

    "Like others have said, you appear to be missing a cathode resistor and bypass cap for the power tubes."​

    I have reverified again and I believe the original drawing is correct to the actual wiring. I still don't really understand this in depth even after reading some about elevated heaters. Perhaps they are directly heated? I have created a layout drawing of the connections in case this helps any. (I have some photos, but it is hard to get clear sensible photos of it.)

    0119211707.jpg


    I am really hoping to understand at some level the amp so that I can recreate it -and possibly one with 12BH7 or EF80 instead of 6L6 power tubes. I appreciate all the comments and questions - hoping for more input on how this amp worked...
     
    Last edited: Jan 19, 2021
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