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Issue with a Clone Hybrid Hiwatt/Ampeg

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by blackwhaleamp, Dec 7, 2017.

  1. blackwhaleamp

    blackwhaleamp Member

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    Hi,
    I am building an sort of hybrid 200W Hiwatt amplifier including the midrange circuit from Ampeg SVT. The amp has two channels with Hi/Low (You can find the circuit here https://photos.app.goo.gl/NBn0L5slkjQS2lM62)
    I just assembled the whole amplifier, and for some reason I have two issues
    .- There is an oscillation, like something like a chopper sound, which appears when the volume/treble/master increases. Also, when I play with the midrange rocker switch, the same chopper sound appears
    .- There is a dependency between the volume at each channel. I was expecting as I introduced a valve V3 to isolate both channel.
    Apart from that, the sound is very clean and there is almost no hum.
    Does anyone has an idea about how to solve this?
     
  2. pdf64

    pdf64 Member

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    I think dc blocking caps are needed between the vol control wipers and V3 grids.
    The V4 grids have no dc reference (grid leaks).

    The schematic is very faint, component references can't be read.
     
    Jeff Gehring likes this.
  3. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Maybe I'm getting a higher res view than pdf64 is; I think he meant to say caps are needed between the volume control wipers and the V2 a and b grids, and that grid leak resistors are needed on both sides of V3....
     
    pdf64 likes this.
  4. pdf64

    pdf64 Member

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    Doh, there's a (+) button to magnify the image, which makes the font rather more legible:oops:
    Yes, Jeff has interpreted my meaning correctly.
    Also, the same HT node feeds 3 stages of cascading voltage amplification, which may be poor mitigation for the motorboating (positive feedback oscillation via the HT) symptom described. I've seen a guideline of 2 cascading stages per node.
    Note that the HT node R58-57 seems to be unused.
     
    Last edited: Dec 7, 2017
    Pete Cage and Jeff Gehring like this.
  5. blackwhaleamp

    blackwhaleamp Member

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    I made a mistake on the schematics. Actually, I connected the first pre-amp stage after R57, and the rest after R58

    The original Hiwatt circuit does not use the leak resistor for the Master

    [​IMG]

    But to be fair, they use it for the Brilliant and Normal Volumes. As for the decoupling capacitor, this is not really needed. The C20 and C21 are already decoupling HT from the 12AU7 grid.

    I disconnected the midrange circuit, and the oscillation is gone. So, for some reason there is a feedback introduced at that stage.

    Also, I noticed that the Presence does not affect the audio signal at all. Any suggestion about that?

    Thank everyone for the help
     
  6. pdf64

    pdf64 Member

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    All tubes require a dc path between their grid and cathode.
    The max resistance of this path is specified as a limiting value in their manufacturer's info.
    Whereas your V3 grids have no dc path and their Vdc will 'float', and so the stages will have an undefined operating point.

    In the Hiwatt, the channel and master volume controls provide this dc path and so function as grid leak resistors.

    The CC stages of your V2 12AU7 have their cathodes dc elevated by their 13k tail resistors R19 and 27 and HT dc that flows through them.
    As their grids are Vdc referenced to the 'top' of those resistors, that is their intended Vdc point; whereas the channel volume controls are trying to pull them down to an 0Vdc reference, thereby messing up the operating point of the stage. Hence the requirement for dc blocking caps between the channel volume controls and the V2 grids.
    Note the 10nF cap at the V4 pin 2 grid http://schems.com/schematicheaven.net/manu/ampeg/ampeg_v9.pdf

    I suggest to get the operating points of the V2 and 3 stages properly defined, then see how it performs.

    Are you sure that the feedback loop is negative, ie is the open loop gain higher or lower than the closed loop gain?
     
    Last edited: Dec 8, 2017
  7. blackwhaleamp

    blackwhaleamp Member

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    I rearranged the circuit like this

    The motorboating (aka put-put-put) is gone [​IMG]. The midrange circuit works (with some noise). However, I still have some issues

    1.- The amplifier is not as loud as it should. I am not sure if I am missing one additional pre-amp stage, or simple there is another thing wrong with the amp
    2.- After the modifications, the pre-amp was not working. So, I connected the scope to C28 and C30 and suddenly it works [​IMG].
    3.- The PRESENCE is subtle, almost unnoticeable.

    Someone recommended me to add a 1M resistors to V3 grid to ground. I will do that tomorrow, and see the effect

    Thank for the help [​IMG]
     
    pdf64 likes this.

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