Ampeg V4 oscillation, long hopeful repair

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by wall_of_sleep, Apr 19, 2016.

  1. wall_of_sleep

    wall_of_sleep Member

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    This has been a long arduous amplifier repair. The previous owner had managed to botch a power transformer swap, dc filament modification, 6an8 to 12ax7 pi conversion, and filter cap job. Past that I'm not sure what else was or was wrong. I may be forgetting a few things at this point.

    Here's what I know.

    -This is the distortion model v4. http://www.ampegv4.com/images/VT22_Schem2.jpg

    -My new filament circuit is good. The power tubes receive AC and the preamp tubes are receiving just shy of 6 volts with a .1 volt ripple.

    -I've returned things to the 6an8 stock style phase inverter

    - When muted (grounded), I get a stable signal with functional controls all the way to the triode grid of the 6an8.

    -If I feed signal into the power amp in, things will wind up for a while and then there will be large clicking spikes (0:35 in) like LFO.


    -If I feed the signal into the normal input things are wildly unstable with the power amp included. They will stay in good shape to a point of volume. Any increase in volume past the point and you get this nightmare waveform. It tries to stay oscillating when I roll things back down.


    It's difficult to isolate anything since it's as though the entire amplifier freaks out when all parties are involved.
     
    Last edited: Apr 19, 2016
  2. drbob1

    drbob1 Silver Supporting Member

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    Worth asking, are you sure the negative feedback is hooked up correctly?
     
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  3. wall_of_sleep

    wall_of_sleep Member

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    I don't see any nfb in the circuit.

    I'll see if I can at least get the power amp behaving today.
     
  4. zenas

    zenas Member

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    Any chance the OT primarys are switched around?
    Just a thought.
     
  5. wall_of_sleep

    wall_of_sleep Member

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    ^ That's definitely a thought. But here's the catch, there's no nfb and it's running into a resistor.

    Big confusing updates.
    I've been doing some cleaning in the amp. I really want the lead dress to be tight around the amp. I threw in a handful of new coupling capacitors just in case it could flush things out. It didn't.

    So I brought the amp up without power tubes... no instability.
    I brought the amp up with two power tubes... no instability.
    With all four power tubes.... it's wild out of control at volume again.

    I then checked both pairs of tubes, ran outside and inside socket pairs, and checked screen resistors. Nothing. Only running a quad of 6l6s is bringing out the gremlin.
     
    Last edited: Apr 21, 2016
  6. amphog

    amphog Silver Supporting Member

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    Check the power tube socket wiring very carefully. Someone may have used a previously unused pin for something, or ungrounded one that should be.
     
  7. BOBSMITH

    BOBSMITH Member

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    If it's unstable with four output tubes it might be in the power amp grounding scheme. Make sure the output tube cathode ground is not the same as the preamp ground. Usually the power transformer H.V. center tap, first filter cap and output tube cathodes are grounded together away from preamp ground.
     
  8. wall_of_sleep

    wall_of_sleep Member

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    Many thanks gentlemen. The grounding scheme was a travesty and picking apart the power tube sockets got me to take a hard look at the non-stock screen supply. I bolstered the screen resistors and brought the voltage down to the schematic's. This made palpable improvements in the performance.

    This thing was stable enough to take a gander through a speaker. The good tones are a blast from the past. Beautiful ampeg tones and mid frequency options. Distortion from the "distortion" control is abysmal, but distortion from lots of volume is outstanding. There's definitely still a lot of high frequency noise and unwanted junk.

    I don't think there's any one answer for this amplifier, but everything I've touched has improved. My next step is to blaze a trail of lead and capacitors across the preamp and control boards. Anything I haven't replaced is suspect.
     

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