Installing Pro Jr. Mod Kit( JUNE 2022)

Written By: John Fromel (and one other contributor)
  • Comments: 6
  • Favorites: 0

Difficulty

Moderate

Steps

15

Time Required

03:00:00

Sections

1

Flags

0

Introduction

Thanks for purchasing the Pro Jr. Modification Kit. After these simple changes, your amp will sound so much better. Please read through this entire guide prior to starting the mod. If you have any questions please let us know.

This is an interactive guide and we encourage you to leave comments (good and bad) as well as any tips or tricks you have that may help everyone!!!

Words of Caution:

If your amp is currently under factory warranty performing these mods will void it.

We guarantee the quality of components and completeness of the kit. However, we cannot guarantee the quality of your work. We are always happy to help troubleshoot via email support.

Tube amps operate at very high temperatures and deadly voltages. Special care must be taken when working on an amp. Wear Safety Goggles at all times, make sure filter caps are drained, only use tools with insulated handles, wear rubber-soled shoes. Finally any time the amp is plugged in keep your left hand in your pocket and only work with your right hand.

If you are unsure of your ability to perform these mods get some help!!! Feel free to reach out to us with any questions if you are on the fence. A qualified local tech should be able to do the mods in 2-3 hours. We also offer installation service for this kit if you want to send the amp into our shop, we charge 2 hours of labor ($190) plus actual shipping both ways.

Optional Tool/Supplies:
Silicone Glue/Caulk
9/16” nut driver or socket wrench
Fine point sharpie
Masking Tape

  1. It’s a good idea to have all the parts and tools you will need to be laid out before starting in a well-lit area.

    • It’s a good idea to have all the parts and tools you will need to be laid out before starting in a well-lit area.

    • Unplug the Amp and put on your safety goggles.

    • Remove the chicken head control knobs (they pull off) and the nut holding the input jack with a 9/16” nut driver or socket wrench.

    • Remove the 6 screws holding on the back panel and remove the panel. If the back panel does not come off easily you can loosen the two chassis screws on the side of the amp about a half turn each.

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  2. Drain the Filter Caps: The filter caps on your amp may be drained already but you must follow these steps to ensure that any deadly voltage is removed from the amp before you begin work. Test the voltage across the filter caps. Set your multimeter to DC and measure the voltage across the positive and negative sides of any of the filter caps. If the voltage reads higher than 10 VDC, then you need to drain the caps.

    • Drain the Filter Caps: The filter caps on your amp may be drained already but you must follow these steps to ensure that any deadly voltage is removed from the amp before you begin work.

    • Test the voltage across the filter caps. Set your multimeter to DC and measure the voltage across the positive and negative sides of any of the filter caps. If the voltage reads higher than 10 VDC, then you need to drain the caps.

    • Using insulated pliers, hold the power resistor touch one lead of the resistor to the negative side of the filter cap and the other to the positive side, leave the leads in contact with the power resistor for 30 seconds, and test the voltage again. Keep repeating this until the voltage reads below 10V.

    • CAUTION: Capacitors can recharge even when the amp is not plugged in!!! After the filter caps are drained take an alligator lead and clip one end to the positive lead of the filter cap and the other end to the chassis and leave it there until the caps are replaced. This will prevent them from re-charging.

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  3. Clip all of the Zip ties (all of them). When putting the amp back together we will be re-routing most of the wires for proper lead dress. Unplug all the transformer leads by pulling firmly with steady pressure gently rocking the lead terminal from side to side.

    • Clip all of the Zip ties (all of them). When putting the amp back together we will be re-routing most of the wires for proper lead dress.

    • Unplug all the transformer leads by pulling firmly with steady pressure gently rocking the lead terminal from side to side.

    • As you remove each lead mark the wire with masking tape and a Sharpie so you will know where to plug it back in when you are putting the amp back together. I write the terminal number on the lead.

    Edit

    • Be careful that your soldering iron is not too hot. The solder pads on the PCB are prone to curling and separating from the PCB, remove the components with care, never using force. We recommend using de-soldering braid as solder suckers can rip the pads off the PCB.

    • For axial components (leads on each side), it is best to clip the leads on the front of the PCB then remove the remainder of the lead and solder from the backside carefully with de-soldering braid. For radial components (both leads on bottom) it is best to remove the solder first.

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  5. Clip the leads and remove the axial mount components noted below and highlighted in Blue. The large filter caps may be glued down with silicone. After the lead is clipped, using pliers gently twist the cap off the board.

    • Clip the leads and remove the axial mount components noted below and highlighted in Blue. The large filter caps may be glued down with silicone. After the lead is clipped, using pliers gently twist the cap off the board.

    • RESISTORS: R1 - R2 - R3 - R5 - R6 - R9 - R10 - R27 - R29( If you have the newest version Pro Jr IV with a black circuit board DO NOT remove R29! - it doesn't get swapped)

    • SUPREME KIT ONLY: CAPACITORS: C15 - (C16 If present) - C17 - C18 - C22 (some versions may say C23.

    • NOTE: Early versions of the Pro Jr have two 22uf filter caps at C15 and C16, later versions have a single 47uf cap at C15.

    Edit

  6. Pull gently on the board to make sure that it is not stuck to any of the standoffs that were holding the circuit board screws.

    • Pull gently on the board to make sure that it is not stuck to any of the standoffs that were holding the circuit board screws.

    • Press the wires from the output transformer as far away from the board and as close to the chassis as possible. The board needs to come down toward the bottom of the amp before the pots will clear the top of the chassis as shown.

    • Using gentle pressure on the top of the circuit board press the board down making sure it is not getting hung up on any wires until the pots have cleared the top of the chassis then pull the board to you to expose the underside.

    • DO NOT PUT PRESSURE ON ANY COMPONENTS INCLUDING THE FILTER CAPS WHEN MOVING THE BOARD.

    Edit

  7. Use de-soldering braid to carefully remove the solder joint and remaining lead on all the components you removed in the previous step (highlighted in blue)

    • Use de-soldering braid to carefully remove the solder joint and remaining lead on all the components you removed in the previous step (highlighted in blue)

    • If you have the newest version Pro Jr IV with a black circuit board do not remove R29!

    • De-solder the following components highlighted in purple. C1 - C2 - C3 - C6 - Input Jack.

    • Do not use force to remove a component! De-soldering braid is your friend, solder suckers are an enemy. On the input jack it may help to de-solder, clip the pins flush with the board, and de-solder again.

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  8. solder the pre-tinned leads to the new jack as shown. Note the switch (middle terminal), and ground (right terminal) are connected.

    • solder the pre-tinned leads to the new jack as shown. Note the switch (middle terminal), and ground (right terminal) are connected.

    Edit

  9. Some of the resistors in this kit may be a different size than others. Specifically some of the 1/2W resistors may be a smaller size, depending on what our suppliers have available. We assure you these are indeed 1/2W resistors

    • Some of the resistors in this kit may be a different size than others. Specifically some of the 1/2W resistors may be a smaller size, depending on what our suppliers have available. We assure you these are indeed 1/2W resistors

    Edit

  10. Insert all the components to the top side of the board before soldering. This prevents repeated bending of the ribbon cables which can cause them to break.

    • Insert all the components to the top side of the board before soldering. This prevents repeated bending of the ribbon cables which can cause them to break.

    • R1 = 10K

    • R2 = 1M

    • R3 = 100K

    • R6 - This resistor controls the overall gain (breakup). For more clean headroom install the 22k, for earlier breakup (more gain) install the 82k. To keep the gain structure the same install the 56k.

    • R9 = 100 Ohm

    • R10 = 100k

    • R27 = 82k

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  11. R5 - 470K

    • R5 - 470K

    • R29 = 20k ( If you have the newest version Pro Jr IV with a black circuit board do not change R29)

    • C1 = Wima 10mm .022 Cap

    • C2 = 100pf MLCC (the little blue one)

    • C3 = Wima 7.5mm 4700pf cap

    • C6 = Wima 7.5mm .015 Cap

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  12. The following parts are polarized and must be inserted correctly. Positive to positive, negative to negative. On axial caps the dent indicates the positive side. On radial caps the positive lead is longer and the negative side has a black stripe. The PCB has indicators for +.

    • The following parts are polarized and must be inserted correctly. Positive to positive, negative to negative. On axial caps the dent indicates the positive side. On radial caps the positive lead is longer and the negative side has a black stripe. The PCB has indicators for +.

    • SUPREME KIT ONLY: C23 or C22 = 22uf/35v radial cap. Depending on the version you have this may say C23 or C22.

    • SUPREME KIT ONLY: C15 = 100uf/450v axial cap

    • SUPREME KIT ONLY: C17 = 22uf/500v axial cap

    • SUPREME KIT ONLY: C18 = 22uf/500v axial cap

    • Install input jack, the ground wire goes in the hole closest to the top of the board and the tip goes to the hole closest to the tubes.

    Edit

    • Carefully put the PCB back into place making sure that the pots and input jack all line up with the control panel.

    • Replace all of the PCB mounting screws

    • Make sure all the ribbon cables have a smooth path with no kinks or sharp turns in the cable

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  14. Proper lead dress is critical to reducing noise and oscillation in an amp. AC lines carry 60 cycle hum. This is reduced when the lead pairs are twisted together creating a shield. Avoid running lead pairs parallel to each other, if possible lines should cross at a 90 degree angle.

    • Proper lead dress is critical to reducing noise and oscillation in an amp. AC lines carry 60 cycle hum. This is reduced when the lead pairs are twisted together creating a shield. Avoid running lead pairs parallel to each other, if possible lines should cross at a 90 degree angle.

    • The most important leads to twist are the Green heater wires. Also twist the red pair from the power transformer to the PCB (opposite of what you see in the pic.)

    • Plug the speaker back into the speaker jack

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    • Re-install the control knobs and install the jack with the isolation washers and nut.

    • Replace the back panel and make sure to tighten the chassis screws on the sides if you loosened them. The silver screw is for the top center back panel screw. It's just a bit shorter than the stock screws and will reduce hum in early models of the Pro Jr

    • Enjoy your modded amp!

    Edit
Finish Line

Author

with 1 other contributor

John Fromel

Member since: 06/03/2020

4,273 Reputation

23 Guides authored

One Comment

While you've got the amp undressed, replace the crap stock speaker. You won't believe how good this amp sounds until you put a decent speaker in it.

Lee Downey - Reply

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