Totally beginner magnavox 8608

Discussion in 'Tube Audio' started by Jacques_579, Jun 27, 2018.

  1. Jacques_579

    Jacques_579 Member

    Messages:
    60
    Ok, i found out that on clip on the 12au7 socket is a kinda loose ( the pin of the tube is holding it otherwise it would fell off) could it be the problem?
     

     

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  2. century tek

    century tek Super Member

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    3,138
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    Shorting the inputs just means to take the left and right input wires and ground them to the chassis. It's not an electrical short like a shorted wall socket or power cord so it won't hurt anything but it might tell if you something if that one channel quiets down. The loose pin on the 12AU7 could certainly be the culprit so make sure it is soldered solidly.
     
  3. Jacques_579

    Jacques_579 Member

    Messages:
    60
    So I did try the short but nothing changed. But I’ve bought a new socket as I realize that all tube light up except that 12au7. So the wait game start again. Thanks guys
    Fred
     
  4. century tek

    century tek Super Member

    Messages:
    3,138
    Location:
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    Are you sure? The 12AU7 family of tubes sometimes look like they aren't working when they really are. Look closely at the tube in a darkened room. PS, you wouldn't be getting any sound if the tube were not lit up.
     
  5. Jacques_579

    Jacques_579 Member

    Messages:
    60
    Hmmm. Make sense. Well I did short it and the sound was the same more or less
     
  6. Jacques_579

    Jacques_579 Member

    Messages:
    60
    And I just re power it and I’m still pretty sure it ain’t lighting up at all. I did had a Sherwood a-5500 and the 12au7 were lighting up, I could see them but this one nada
     

     

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  7. FlaCharlie

    FlaCharlie Active Member

    Messages:
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    There is no 12AU7 in this amp. The tube lineup is (1) 6CA4 / EZ81 rectifier, (1) 6EU7 input tube and (2) 6BQ5 output tubes. The 6EU7 has a completely different pinout (pin arrangement) than tubes like the 12AU7, 12AX7, 12AT7 and similar. If you have the wrong tube in there it won't be lighting up because the heaters are on different pins.

    If you wanted to use one of those others you would need to rewire the socket. The 6EU7 is essentially the same as a 12AX7 electrically.
     
  8. Jacques_579

    Jacques_579 Member

    Messages:
    60
    Sorry my bad, it is a 6eu7 and everything was lightning up before but now they all glow except this one and the buzzing sound is only in the right channel
     
  9. FlaCharlie

    FlaCharlie Active Member

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    492
    Location:
    Gainesville, FL
    It looks like you have the 6EU7 wired wrong. The green wires that are coming from the 6BQ5 (pins 4 and 5) should be going to pins 1 and 2 of the 6EU7. The pic seems to show them going to pins 2 and 9?? You need to recheck the wiring on all the pins of the 6EU7. Here's a link to the data sheet which shows the connections of the various pins. The pin numbers are as seen from the bottom. https://tubedata.altanatubes.com.br/sheets/093/6/6EU7.pdf
     
  10. Jacques_579

    Jacques_579 Member

    Messages:
    60
    I used Dave amp as a tamplate and it was wired this way. Could it be that it was wired for a 12ax7 and mine is a 6eu7??
     
  11. Jacques_579

    Jacques_579 Member

    Messages:
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    Here’s the schematic I used and a pics of his amp if it can help understand what I did
     

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  12. FlaCharlie

    FlaCharlie Active Member

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    Yes, if you read Dave's thread you'll see that he rewired his to use 12AX7.
     
  13. Jacques_579

    Jacques_579 Member

    Messages:
    60
    Now I see what i did wrong. Now a newb question maybe, but why do I have sounds on one side if not the good tube ?
     
  14. FlaCharlie

    FlaCharlie Active Member

    Messages:
    492
    Location:
    Gainesville, FL
    His schematic shows the input tube as either 12AX7 or 6EU7 but you'll notice that there are no pin numbers because they would be different, depending on which tube you choose to use.

    Not sure why you'd get sound from one channel. Perhaps the source is very strong and it's able to drive the 6BQ5 just enough to hear it??

    Here's a factory schematic.
     

    Attached Files:

  15. Jacques_579

    Jacques_579 Member

    Messages:
    60
    Ok well I had a 12au7 and I just try it in and the same thing is happening ?!
     
  16. Jacques_579

    Jacques_579 Member

    Messages:
    60
    Ok found the bad guy. I forgot the 2 470k resistor rca to ground. Will try it tomorow.
     

     

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  17. FlaCharlie

    FlaCharlie Active Member

    Messages:
    492
    Location:
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    If you're going to rewire you need to use a 12AX7. The 12AU7 shares the same pinout but its amplification factor is only 20 while the 12AX7 (and 6EU7) is 100. So a 12AU7 will function but not optimally. The signal will be much weaker.

    Also, if you're using a 12AX7 (or 12AU7) how do you have the heaters wired? In order to operate at 6.3v you should have pins 4 and 5 connected with each other and one of your heater wires connected there. The other heater wire connects to the heater's center tap, which is pin 9. If one of your heater wires is going to just pin 4 or pin 5 (not both) and the other is going to pin 9 you are only lighting up one of the two triode sections, which would explain why you're getting sound out of one channel only.

    If you're using a 6EU7, the heaters are only pin 1 and pin 2. There is no center tap on the heater. Of course the other pins are different too so you need to decide which type of tube you want to use and make sure the socket is wired correctly for that type of tube.
     
  18. Jacques_579

    Jacques_579 Member

    Messages:
    60
    Ok so I went to bed to start fresh, if I understand correctly; I’m presently semi wired for a 12ax7 (which I have one at home so I’ll stay this way for now) so I should solder a wire between pins 4-5 but then what ? It’s the heater I’m not familiar with. If you could tell me which wires goes from where to where that would help me
    Thanks
    Fred
     
  19. Jacques_579

    Jacques_579 Member

    Messages:
    60
    I wire them to pin 5 of 6bq5?
     
  20. gadget73

    gadget73 junk junkie Subscriber

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    To use a 12AX7, one heater wire goes to pin 4 and 5, the other goes to pin 9. The 6bq5 heater wires are on pins 4 and 5. I would connect 4-5 on the 12ax7 to pin 4 on the 6bq5, and pin 9 on the 12ax7 to pin 5 on the 6bq5.
     

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