Sunn Spectrum I - Schematic and question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by joelster, Feb 24, 2012.

  1. joelster

    joelster Silver Supporting Member

    Messages:
    2,442
    Joined:
    Feb 19, 2009
    Location:
    Portland, OR
    I recently opened up my Sunn Spectrum I amp. As an exercise and because I can't find a schematic anywhere, I decided to trace the circuit and see how it compares to the Spectrum II schematic. Here is the resulting schematic:

    [​IMG]

    There was one thing in the circuit that I hadn't seen before so I though I'd ask about it here. On the pentode of the 7199 tube the cathode is connected back to B+ through a 330k resistor. What's that about? Cathode lift?

    Thanks,

    Joelster
     
    Last edited: Feb 24, 2012
  2. joelster

    joelster Silver Supporting Member

    Messages:
    2,442
    Joined:
    Feb 19, 2009
    Location:
    Portland, OR
    For comparison's sake, here's the Spectrum II schematic:

    [​IMG]
     
  3. Chrome Dinette

    Chrome Dinette Member

    Messages:
    14,435
    Joined:
    Aug 4, 2005
    Location:
    Pittsburgh, PA
    Are you sure you aren't misinterpreting something that is part of the NFB circuit?

    The second schematic has nfb and the first one doesn't.
     
  4. Jerry Glass

    Jerry Glass Member

    Messages:
    880
    Joined:
    Feb 11, 2010
    Location:
    Fort Wayne, IN
    In that type of configuration, it would add a little power supply noise rejection.
     
  5. joelster

    joelster Silver Supporting Member

    Messages:
    2,442
    Joined:
    Feb 19, 2009
    Location:
    Portland, OR
    It's definitely not part of an NFB circuit. It exists in the Dynaco ST70 circuit from which this design was heavily copied (but without NFB).
    http://www.tuberadios.com/dynaco-st70/st70schematic.jpg

    So, Jerry, does that work by feeding PS voltage back into the circuit but out of phase because it's going to the cathode?

    Thanks.
     
  6. guitarcapo

    guitarcapo Senior Member

    Messages:
    2,337
    Joined:
    Mar 16, 2010
    Why does the first circuit have two resistors in series (680 and 47 ohms) off the cathode to ground? I'm thinking something was connected there between them that has been removed or modded (like in the second one). Why wouldn't the manufacturer just use a 727 ohm resistor (or something close to that)?
     
  7. joelster

    joelster Silver Supporting Member

    Messages:
    2,442
    Joined:
    Feb 19, 2009
    Location:
    Portland, OR
    Yeah, that is weird. I think it's a carry over from the ST70. It was part of the NFB and that was omitted in this amp. Maybe Conrad changed his mind after he ordered the parts. Maybe mine was modded but I really don't think so. When it reappears in the Spectrum II it is identical to the ST70.

    I would love to find the original schematic if any has any ideas...
     
  8. Ronsonic

    Ronsonic Member

    Messages:
    3,313
    Joined:
    May 14, 2006
    Location:
    Sunny Tampa, Florida
    Guitar capo, has a good point, it looks like a point to return NFB.

    As for the voltage divider off the B+ which the drawing would indicate, that would tend to lock in the bias on that tube element. More a fixed bias than cathode bias arrangement. Just speculation.
     
  9. VacuumVoodoo

    VacuumVoodoo Member

    Messages:
    1,567
    Joined:
    Jan 9, 2005
    Location:
    Gothenburg, Sweden
    That is correct, it's called combination bias. But cathode potential should than be around 1.5V, not 0.75V as is indicated on both schematics. This in turn lowers the plate current which makes plate potential higher and also affects the DC coupled split load PI. But all potentials are identical on bothe schematics.
    Some vital evidence is definitely missing or the presented material has been tampered with.:confused:
     
  10. joelster

    joelster Silver Supporting Member

    Messages:
    2,442
    Joined:
    Feb 19, 2009
    Location:
    Portland, OR
    Combination bias. Thank you!

    Yes, VacuumVoodoo, you are correct. The voltages on my schematic are left over from the Spectrum II schematic that I modified and should be ignored. Didn't mean to muddy things up.
     
    Last edited: Feb 25, 2012
  11. Keyser Soze

    Keyser Soze Member

    Messages:
    1,477
    Joined:
    Dec 14, 2008
    Location:
    Johnson city, TN
    The power supply is DC, so (hopefully) should not have any phase.
     
  12. joelster

    joelster Silver Supporting Member

    Messages:
    2,442
    Joined:
    Feb 19, 2009
    Location:
    Portland, OR
    Of course. Thanks.
     
  13. donnyjaguar

    donnyjaguar Member

    Messages:
    4,243
    Joined:
    Nov 4, 2005
    Location:
    Canada
    The 330k completes a circuit in the power supply that drains the capacitors when the amplifier is turned off for safety reasons. This circuit is the Dynaco Stereo 70 that was sold as a kit. :)
     

Share This Page