Modding Valco/Wards GIM 9151A

Discussion in 'Amp Tech Center' started by ArcticWhite, Dec 10, 2016.

  1. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    R18 in combination with R19 form a voltage divider that is the same as having a volume control set at a fixed level...a standard 10% audio pot would be just below the half-way setting (11 O'clock).
    BUT....more to the point is that the R19 100k has a 500pF cap parallel to it, which, coming after the 1M R18, will be bleeding all of your high end away.
    The volume shift of the two resistors might well be needed to keep the P.I. from distorting to much...but one could just try replacing the whole R18, R19 and C10 combination with a 1M pot as a pre P.I. master volume.
     
    Last edited: May 17, 2019
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  2. ArcticWhite

    ArcticWhite Tele-Meister

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    Great idea!

    I took a look at how Fender arranged a master volume on the SF Twin Reverb. They included a 120 pf cap on the wiper, and a. 01 cap after the pot.

    Would you recommend the same arrangement?
     
  3. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    The SF twin reverb has a LTP P.I, whereas you have a Paraphase P.I...if you do not intend changing yours (which I wouldn't...(at first)) then the following .01 cap is not needed to block the high DC bias voltage at the LTP P.I's grids from the pots wiper. The Paraphase P.I has its grid at ground potential.
    upload_2019-5-17_23-22-27.png upload_2019-5-17_23-25-15.png

    That 120pF that you think is on the wiper, is infact conected to a seperate tap on the specially designed pot, it is there to help compensate for any treble loss at lower volumes. One sees these pots in many Fender amps, but they are not needed, and one can get away with the slightly less ideal, but quite adaquate, bright-cap connected to the wiper like you supposed.
     
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  4. ArcticWhite

    ArcticWhite Tele-Meister

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    Is this correct? 20190517_152217.jpg

    Or this way?

    20190517_152535.jpg

    Edit:

    I just realized those are both wrong.
    Like this one maybe?

    20190517_155231.jpg
     
    Last edited: May 17, 2019
  5. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    upload_2019-5-18_8-43-29.png
     
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  6. ArcticWhite

    ArcticWhite Tele-Meister

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    Perfect, thank you.
    What's a good value for the bright cap in that scheme?
     
  7. Bendyha

    Bendyha Tele-Afflicted Silver Supporter

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    There is no realistic way of seeing, what the amp will finally sound like, unless something is obviously out on the limits of normality, which is not the case here.

    The final decision as to the value of a bright cap...if one is desired at all, or, switchable for selective use only, can only be ascertained by empirical observation after you have the amp up and running. Start with no bright cap, then add someting in the 120 -220pF range later if you find it necessary.
     
  8. ArcticWhite

    ArcticWhite Tele-Meister

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    Master Volume is in. I used a 1 Meg audio taper pot. It Works well, and I am quite pleased. Most of the effective range, however, is from 1 to 4ish, at that point volume is probably 90%.

    I've been messing around with filter cap values, and clipping in cathode bypass caps. Learning by doing, with plentyoof trial and error.

    I'm aiming for a clean channel with tons of headroom, and a second that is beginning to break up pretty nicely.

    Trying to make it happen earlier.

    The grounds in this thing are all over the place so it hums pretty bad. Will try organizing them soon.
     
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  9. ArcticWhite

    ArcticWhite Tele-Meister

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    This thing hums like a substation.
    I started by chop sticking the grounds and quickly discovered that the entire amp is microphonic as hell. I mean everything..
    Yeah it's worse around V1, and V3, but really just about anything I can tap sounds like a ball bearing in a metal drum.
    I swapped in some different preamp tubes and got no changes.
    I don't even know where to begin.
     
  10. BobbyZ

    BobbyZ Doctor of Teleocity

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    Had octal preamp tubes get microphonic like that, everywhere you tap the amp it gets amplified.
    I'm thinking it's a tube here as well, at least I'd start by swapping them all out, to eliminate that possibility.
     
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  11. ArcticWhite

    ArcticWhite Tele-Meister

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    Well, I replaced all three 12AX7s, put in grounded input jacks, shielded cable on the inputs, tweaked some cap values, and it runs great now. Two of the tubes were microphonic, and the ungrounded inputs were bringing in radio.

    But now I'm thinking about ditching Channel One entirely, and putting in a tremelo circuit. I'd have V3, the tone controls would be converted to speed and intensity, and plenty of room for the circuitry.

    Anyone recommend a simple tremolo circuit that would drop in?
     
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