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still struggling with reverse engineering schematic

Discussion in 'Tube Audio' started by brud, Oct 9, 2018.

  1. brud

    brud AK Subscriber Subscriber

    Messages:
    21
    I wonder if someone could start me off with drawing the schematic from the valve pin outs.

    I have drawn some but get stuck when components divide off.

    If I get an example I feel I can proceed on - Thanks in advance !
     

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  2. trobbins

    trobbins Active Member

    Messages:
    492
    Location:
    Melb, Australia
    The L63 triode is in a cathodyne circuit - there should be a 0.1uF coupling cap to each KT66 connecting to the 47k at the anode and cathode of the cathodyne. The 2k2 provides bias to the input grid via the 1Meg grid leak, and the previous stage couples to pin 5 via the 0.05uF.

    The 6SJ7 is set up as a simple pentode amplifier, but with cathode going to ground via a bypassed 1k5, and then an unbypassed 100 to which the feedback is connected (as a resistor divider). The screen pin 6 is RC filtered from B+. The suppressor pin 3 is connected to cathode. The input grid comes from the treble pot and has a 220k grid leak to cathode.
     
  3. brud

    brud AK Subscriber Subscriber

    Messages:
    21
    Thankyou - I will draw out tomorrow !!
     
  4. brud

    brud AK Subscriber Subscriber

    Messages:
    21
    Thankyou - I have attempted to draw this out - diag enclosed.

    So putting all these valves together - here is my sequence -

    Pre -amp : EF41
    6SN7 ?? use
    6SJ7 - amplifier
    L63 -Cathodyne
    KT66 - Output tubes

    Im still struggling where to place the 2.2k off L63

    I really struggle at pin 6 of the L63 when there is 3 connections - serial /parallel .

    Possibly you could draw me a rough sketch ??

    I really appreciate your input.
     

    Attached Files:

  5. kward

    kward AK Subscriber Subscriber

    Messages:
    3,512
    Location:
    Utah
    This is most likely the layout of the L63 stage. It's a self-biased "split load" or also called Cathodyne phase inverter, which also acts as the driver for the output stage. The 2.2K resistor provides the self bias voltage by dropping several volts from the cathode, so the junction of the 2.2K resistor, 1M resistor, and 47K resistor sits at something like 3 to 4 volts lower than the cathode. The 1M resistor impresses this voltage onto the grid. Hence it uses its own cathode current to bias itself.

    upload_2018-10-9_20-5-50.png
     
    Last edited: Oct 9, 2018
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  6. brud

    brud AK Subscriber Subscriber

    Messages:
    21
    Thankyou - goit it !!!

    So what do the 6sJ7 (??amplification) and the 6SN7 do and The "Radio" and "Gram" inputs go into the G2 of the EF41 - so is this the Pre-amp tube. Just trying to get the signal path organised
     

     

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  7. kward

    kward AK Subscriber Subscriber

    Messages:
    3,512
    Location:
    Utah
    According to your diagram, the 6SJ7 is a pentode voltage amplifier for the power amp. Like this (using the values you provided in your sketch):

    upload_2018-10-9_22-44-13.png

    I don't yet know what the other tubes do, as you haven't sketched them out yet.
     
    Last edited: Oct 9, 2018
  8. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,036
    Location:
    Southern NJ
    The 6SN7 might be used for the tone controls. Two triodes with the tone controls between them is a fairly typical setup.
     
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  9. brud

    brud AK Subscriber Subscriber

    Messages:
    21
    Thankyou !

    Here are the final 2 tubes ( except the 5V4G rectifier that I have been able to draw the Power supply !) 6SN7 and EF41

    If you could add to diagram I would be so grateful .
     

    Attached Files:

  10. kward

    kward AK Subscriber Subscriber

    Messages:
    3,512
    Location:
    Utah
    The 6SN7 stages appear to be the preamp section (with tone controls). I don't have much experience with tone controls to recognize the topology used, so I'm not sure I can draw them correctly. Bass and treble controls might be separated, one in betwee the 6SN7s and the other in between the second 6SN7 and the 6SJ7. As far as the 6SN7 stages themselves, they are biased identically as near as I can tell, with a 33K plate resistor and a 1K cathode resistor on both triodes. There's a volume pot feeding the grid of the first triode, the output of the first triode goes into the tone control network, and then into the grid of the second triode. I assume the output of the second 6SN7 triode connects to the power amp section probably through a coupling capacitor and grid leak resistor on the 6SJ7 input.

    So at a high level you have this:

    upload_2018-10-10_22-19-23.png


    Maybe someone else can fill in how the tone controls are wired. I assume it will be a common tone control topology. Also I assume the EF41 is part of an RF circuit that sits in front of the volume pot, but that is also out of my comfort zone. Perhaps the volume pot is hooked up through a selector switch to switch between RF reception and an Aux type line level input.
     
  11. kward

    kward AK Subscriber Subscriber

    Messages:
    3,512
    Location:
    Utah
    I'm trying to remember your original photos of the units from the other thread. Was there also a balance control on this unit you are tracing out? I thought I remembered three controls on the front, I assume bass, treble, and balance.
     

     

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  12. brud

    brud AK Subscriber Subscriber

    Messages:
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    I checked my photos there is a Bass , Treble, Volume controls - No balance control ?? so what is the EF41 for it has the "Radio" and the "Gram" input connections on the front of the panel with these tone controls. Thankyou for your input really appreciate it - I am learning sooo much . I find reversing schematics quite hard .
     
  13. brud

    brud AK Subscriber Subscriber

    Messages:
    21
    Any thoughts on my last posts ? Cheers
     
  14. brud

    brud AK Subscriber Subscriber

    Messages:
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    Any thoughts on kwards comments re possible tone controls on 6SN7
     
  15. brud

    brud AK Subscriber Subscriber

    Messages:
    21
    The 2.2M resistor from Plate of Triode 1 goes to pin 4 ( G1) of the 6SJ7
     
  16. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    37,036
    Location:
    Southern NJ
    Is it stereo? If not, you won't have balance.

    There have been so many types of tone control over the years that its hard to guess what exactly it might be honestly.
     

     

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  17. trobbins

    trobbins Active Member

    Messages:
    492
    Location:
    Melb, Australia
    The EF41 shows radio coming in to the g2 grid, and g1 used as a screen (with 1.5M supply from B+ and C decoupling to ground). The cathode has 2k2 bias with bypass cap, and the anode has a 330k load, with capacitor coupling to ?? Maybe the EF41 is not the correct valve model, given that pin 6 is set up as a pentode screen.

    It looks like the first triode in the 6SN7 has the anode capacitively coupled to a resistive divider comprising 220k, then 1M5 pot, then 22k to ground. The pot wiper is likely to be directly connected to the grid of the second tube (pot acting as the grid leak), and there is a small cap bypassing the top of the pot to the wiper to provide some treble lift at lower volume pot setting.

    It looks like the second triode of the 6SN7 has the anode output capacitively coupled to a simple RC shunt filter (ie. treble control) to ground, where the R is the 1.5M pot, and the C connects to ground - the sketch doesn't show what happens to the wiper (it is likely connected to one of the pot arms).
     

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