Tube gurus: help me diagnose my SVT-II issue

Discussion in 'Amps and Cabs [BG]' started by takeout, Mar 16, 2006.

  1. takeout

    takeout Supporting Member

    Dec 27, 2002
    Kansas City area
    Patient: Ampeg SVT-II, '86 or so.

    - Distortion
    - Loss of lows
    - A weird hum not related to shielding/grounding/etc. (oscillation?)
    - The freakin' power and EQ lights stay on even when I unplug it from the wall.

    Is a filter cap going bad and discharging even when it shouldn't? Tube failing (they're about due)? Other?

    Your thoughts?
  2. Thor

    Thor Moderator Staff Member Gold Supporting Member

    You may have to run over to the DB side and ask for the
    community magic spell book. Sounds like it's haunted, for

  3. tadawson


    Aug 24, 2005
    Lewisville, TX
    If this is a true 60 cycle hum, then I suspect you are correct, and a filter cap is going out, or possibly a rectifier in the power supply may have failed. Oscillation is typically much higher in frequency than a typical hum, and can pretty easily be differentiated. You say the lights stay on - for how long? (And are they LED indicators, incandescent light bulbs, neon lamps, or ???) If indefinitely, then don't do a thing - patent it! You will have discovered the first perpetual source of energy! Can't really come up with anything to explain the lights, at least not without a set of schematics for this beast.

    - Tim
  4. Thor

    Thor Moderator Staff Member Gold Supporting Member

  5. takeout

    takeout Supporting Member

    Dec 27, 2002
    Kansas City area

    The hum is above 60hZ, but not too much. It's a low-mid "mooing" king of sound. And by lights, I mean the LEDs for power/standby and the graphic EQ in/out. They aren't fully lit - they're about half-lit, like in a brown-out or something.

    They may or may not have come on right around the time I plugged in my cabs, before I plugged in the amp or tried to power up.
  6. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    OK, loss of lows (are tehy REALLY lost lows......or "kinda bad" lows) + distortion + lights stay on (is the door closed? :D )

    In no particular order.......

    1) bad tube, probably in preamp

    2) some kind of open in the signal path. Could be as simple as the ultra-low switch.

    3) some kind of short, possibly in the midrange switch or coil etc....

    4) Tube filament voltage problem in at least part of the unit, likely in part of the power amp..

    5) something else.....

    Opens usually give only a leakage of highs, no lows or much mids.... and they often hum because the open input of next stage picks up hum from filaments. Distortion from overdriving the prior stages attempting to get a signal thru.

    Shorts can make hum, and have obvious distortion potential.

    The power light is triggered off the +300 to preamp, so if it stays up for a long time, as it might if the filament supply were weak and tubes never heated up right, the light may stay on a long time.
    Then hum, due to 1 or more tubes not being "on" ..... and distortion, because of 1 or more tubes not being "on", turning up to compensate, plus low emission, and possible bias issues due to "weak" tubes.

    All-in-all, I am inclined to think of a filament supply problem as being the most likely to explain everything.

    But it would not be the first time I was totally surprised by a completely different problem coming from left field.
  7. takeout

    takeout Supporting Member

    Dec 27, 2002
    Kansas City area
    ...but I don't play a "fridge"...

    I'll have my guy check that stuff out. Thanks for the input.