1959/87 Preamp To 2204/3 Conversion Instructions

Discussion in 'The Workbench' started by RevJamie, Mar 11, 2018.

  1. RevJamie

    RevJamie Active Member

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    I'm trying to do some 1959 to 2203 preamp conversions. I couldn't really find any step by step instructions on the net. So I took a look at these three images and wrote the steps I though needed to happen. I must be missing something as I haven't been successful yet. I know I haven't done the instructions on how to wire the input jacks yet, I'm not exactly sure how to write that out.

    Anyone want to look them over and see if I've done anything wrong or missed something?

    ##### 1959/87 Preamp to 2204/3 conversion #####

    *Normal Volume pot becomes the Preamp Gain pot
    *Bright Volume pot becomes the Master Volume pot

    Remove V1A Cathode Capacitor and resistor. (Usually a 330uf or 220uf cap and a 820 resistor)

    Replace the now removed V1A Cathode resistor with 10K resistor.

    Wire from the V1B Plate Bypass Cap (.022 cap) which is currently connected to the far left lug on “Bright Volume” pot, gets moved to one of the input jacks

    Disconnect wires from all 4 68K input resistors and heat shrink them to cap off.

    Replace green wire on V1pin2 with shielded wire going to the middle leg of “Normal Volume” pot. Connect the shield from that cable to the far right lug on the “Bright Volume” pot

    Disconnect V1 pin 7 wire. Replace it with a shielded cable. On the end connecting to V1 pin 7 connect a 68K resistor. Run the other end of the shielded cable to the input jacks.

    Move wire from Normal channel mix resistor (470k on the left) to the bus wire.

    Disconnect and cap off the wire from V1A Plate Bypass capacitor which is currently connected to far left lug of the “Normal Volume” pot. Then move wire from Bright channel mix resistor (470k on the right, currently connected to the middle lug of the “Bright Volume” pot). Connect it to the V1A Plate Bypass capacitor (Usually the most left .022 cap).

    If installed, disconnect the .68U V2A cathode cap. Leaving the 820 resistor.

    Move wire from the far right .022UF tone stack capacitor next to the 33u+33U cap FROM middle leg on the treble pot TO middle leg of “Bright Volume” pot.

    Install a wire from middle leg of treble pot to far left of “Bright Volume” pot

    Put a 1000pf radial cap on “Normal Volume” pot, on the middle and far left legs.

    Piggy back a 470K resistor and 470pf cap together. Connect one end to the far left leg of the “Normal Volume” pot. Connect the other end to the input jacks.

    Like I said, I haven’t yet figured out exactly how to write the docs for how to wire up the input jacks.

    [​IMG] [​IMG][​IMG]
     
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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Yeah, I think you got the right idea.
     
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  3. ampmadscientist

    ampmadscientist Well-Known Member

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    This is one of the better schematics, because it shows both low gain and high gain preamps side by side:
    making it easier for you to see the differences and similarities.

    (click image for full sized)

    [​IMG]

    So if you can read it, you will see that the difference between the 2 is very small.

    It's very cool that you are tackling this, it's going to help you learn a lot about how tube amps are designed.
     
    Last edited: Jan 16, 2020
  4. NewReligion

    NewReligion Well-Known Member VIP Member

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  5. lespaul6

    lespaul6 Active Member

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    Just bumping this thread to ask if there are any additional changes to the power section when doing a conversion from 1959 to 2203.. other than moving the negative feedback wire (purple) to the 4ohm tap??? thanks gents
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Not really any changes to power section other than the option PPIMV control, in place of the regular master volume control.
    You can move the feedback or disconnect it entirely. I just disconnect it because feedback spoils the tone...
     
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  7. ampmadscientist

    ampmadscientist Well-Known Member

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    A. Does the low input work?

    The most common mistake is wiring the input jacks with the connections reversed.
     

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