1969 with low plate voltage. Help needed

Discussion in 'Building the Classics' started by Chris4189, Nov 5, 2019.

  1. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    No. I said voltage AC, VAC.

    Tubes in and play. We are measuring incoming signal level to pins #5 or the power tubes.
     
  2. Chris4189

    Chris4189 Well-Known Member

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    V4:13.6 VAC
    V5: 13.4 VAC
    V6: 14.3 VAC
    V7: 14.9 VAC

    VOL I on 10
    VOL II is dead.
     
    Last edited: Nov 8, 2019 at 2:39 PM
  3. Chris4189

    Chris4189 Well-Known Member

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    May have figured this out a bit more. When I bought the amp it had a master volume installed. I removed the MV completely. Well I went back and looked at a diagram and noticed there should be a wire running thru the center treble pot to the .022 cap. Can I use 22 AWG top coat wire for this connection?

    After a double shot of Crown, just discovered V2 was bad. Think I found the missing link in sound.
     
    Last edited: Nov 8, 2019 at 3:34 PM
  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    That 22 AWG works fine.
    I have to ask, how did the power tubes get signal without a connection from the TREBLE center lug to the .022uF coupler to the phase inverter?

    And why is VOLUME II dead? Was that used for the MASTER VOLUME?
    Was the first two tube stages modified, maybe cascaded?
     
  5. Chris4189

    Chris4189 Well-Known Member

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    That I don’t know. I’m still learning. I’d have to defer to you guys for that answer.

    literally as soon as I soldered the wire from PI input cap to the treble middle lug, VOL II worked as normal. They had the master on the back panel. Once I discovered the bad V2 tube and replaced it, everything was back to normal and loud.

    cascade I’m unsure of what that is or how to tell.
     
  6. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    A stock model 1959 has a Normal and Bright channel both of which tie in before V2. V2 drives the tone stack and form the TREBLE potentiometer the signal is passed to the PI (phase inverter).

    But you said Volume I worked, Volume II did not untill connecting TREBLE to PI. So that would mean something would be wired differently.
    Does the tone stack work for both channels?
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    "...everything was back to normal and loud...."

    Progress.....progress.....
     
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  8. Chris4189

    Chris4189 Well-Known Member

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    The MV they had in this was wired like this: one wire from the MV pot to the the PI cap and then a wire from the MV pot to the treble wiper lug in question. That’s it.

    All the other wiper lugs still have the original red lay out fluid on them. There are no added resistors, wires, pre amp sockets or anything that I see that what indicate any mod being done other then the MV as described above.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Keep going you're going to get it.
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    Let's see some pictures of the input jacks and input side circuit board.
     
  11. neikeel

    neikeel Well-Known Member

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    He said it is working now.
     
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  12. ampmadscientist

    ampmadscientist Well-Known Member

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    I thought he said vol ii was not working?
     
  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Everything so far points to working normally. If all the controls work as they should, it is biased correctly and is loud then its done. :fingersx:
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Last thing from me for now is see the resistor lead in this picture, upper left capacitor. Make sure it is not to close to the chassis. You do not want shorting. It would be best to have leads as short as possible there.

    Check around for stuff like that. :thumb:


    [​IMG]
     
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  15. ampmadscientist

    ampmadscientist Well-Known Member

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    He could put some tubing over that lead...
     
  16. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I think short and sweet would be best. He has enough wire jackets in there.
     
  17. Chris4189

    Chris4189 Well-Known Member

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    The resistors were/are no where near touching the chassis. The photo makes it appear that way.

    Since the OT is original and original to the amp I’m thinking of daisy chaining three 1N4007 diodes together as fly backs. Shouldn’t they go on tubes V4 and V7? From the diagram it looks like the go to pin 1 or 7 and then simply to ground.
     
    Last edited: Nov 11, 2019 at 6:41 PM
  18. ampmadscientist

    ampmadscientist Well-Known Member

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    What diagram are you talking about?
    This amp should not need those diodes.
     
  19. Chris4189

    Chris4189 Well-Known Member

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    I can’t seem to find it now but it was a generic diagram for adding fly back diodes to protect the OT in the event something wonky happens.
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

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    I have never seen anybody install diodes like that in this type amp.
    Doesn't need it.
     
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