Scott Stereomaster distortion Type 340 Tuner/Amp

Discussion in 'Tube Audio' started by LUKE RANDALL, Jun 2, 2018.

  1. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    35,509
    Location:
    Southern NJ
    Low current draw in the triode. Basically anything off at pin 9 will seriously mess with how that stage balances and performs. Yours seems to be drawing so little current that I'm not surprised its not working well.

    Check the voltage at pin 3. The pentode is direct coupled to the triode, and the voltages at the pentode plate are strongly influenced by the screen voltage at pin 3. Verify the 270K and 100K resistors are on value. If those are good, one option would be to adjust them to get the voltage at pin 9 where it needs to be. If pin 9 is close to right, the rest of it should fall in line nicely.

    Trying other 6U8 or 6GH8 tubes may also yield some joy. The annoying part about those pentode/triode tubes is they are often pretty variable in terms of performance. Luckily yours happens to use fairly available and inexpensive ones so it won't be the end of the world to try others.
     

     

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  2. LUKE RANDALL

    LUKE RANDALL New Member

    Messages:
    18
    Location:
    Rhode Island
    I did check that voltage on pin 3...it is at 51 volts. I will check those resistors. Could that resolve the very high voltage I found on pin 1? Thanks.
     
  3. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    35,509
    Location:
    Southern NJ
    Voltage at pin 3 is the screen. Screen voltage strongly influences voltage at the plate of the pentode which is connected to the grid of the triode (pins 6 and 9). Higher voltage at pin 3 will make for lower voltage at the plate of the pentode. Grid voltage at pin 9 will determine voltages at pin 1 and 8. Basically the pentode biases the triode, and if the triode is way off the phase inverter behaves badly. It doesn't have to be dead nuts on the schematic values, but it should at least be in the neighborhood and yours are not.



    Actually just for S&G, would you mind measuring all of the pins on the 6U8 and posting the readings? Wouldn't hurt to measure both channels too. I'm curious how it compares overall to the schematic.
     
  4. GordonW

    GordonW Speakerfixer Subscriber

    Messages:
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    It's not common- but it's not impossible that the 100pf cap between pins 6 and 9, and ground, is bad. If that's leaking, that would pull the voltage at pin 9 down to a bad level.

    Also, check the tube socket- make sure that there's not a bad connection on pin 6. That could block current from flowing properly in the pentode, which would upset the voltage.

    Once you make sure that's not an issue- I would then do what it takes, to get the voltage on pin 3 down somewhat. That will help get pins 6 and 9 up to their proper level (about 60v), which should then get pin 8 down to its proper level (about 185v).

    Regards,
    Gordon.
     

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