Amplifier Conversion Guide - Blackface/Silverface Normal channel to Bass

Warning: The adjustments and processes described in this article require interacting with dangerous levels of voltage. Even unplugged, an amplifier can retain dangerous levels of charge. We encourage you to take extreme precautions when performing these adjustments or interacting with your amplifier. When in doubt, seek the help of a trained professional.

This guide has been built in place of our K-703 conversion kit which was previously available for purchase. Now, you can simply purchase the required parts below and follow through this guide.

This guide will instruct you on how to convert the Normal channel of a blackface or silverface amp into the classic Bass Instrument channel from Bassman amps. This modification is achieved by altering the tone stack and works for both TMB and TB tone stacks. Following this guide, you will learn:

  • How to locate the tone stack components.
  • How to modify the normal channel tone stack components for a suitable bass guitar tone.

Required Parts

DescriptionSKUItem IDQtyPriceSubtotal
Capacitor - Mallory, 630V, 150s, Axial Lead
  • Capacitance: .047 µF
C-MD047-6301$1.55$1.55 Add to Cart
Capacitor - Mallory, 630V, 150s, Axial Lead
  • Capacitance: .1 µF
C-MD1-6301$1.58$1.58 Add to Cart
Resistors - 0.5 Watt, Carbon Film, 5% tolerance
  • Resistance: 10 kΩ
R-A10K1$1.32$1.32 Add to Cart
Solder Lug - #6 or #8 Hole, Locking
  • Size: #6
S-H1121$1.50$1.50 Add to Cart
Nut - Hex finished, fine thread, zinc
  • Size: #6 diameter, 32 tpi (6-32)
S-HHN6321$0.30$0.30 Add to Cart

Warning: The adjustments and processes described in this article require interacting with dangerous levels of voltage. Even unplugged, an amplifier can retain dangerous levels of charge. We encourage you to take extreme precautions when performing these adjustments or interacting with your amplifier. When in doubt, seek the help of a trained professional.

Required Tools

ToolPurposeWe Recommend
Multi-meterA meter for measuring voltage, current, resistance, and sometimes capacitanceS-Z3220
Soldering IronSoldering iron with a 1/8" screwdriver tip and a variable control capable of producing 25-40Watts. This will allow you to do some precise soldering of circuit components and wires (do not use a soldering gun for soldering of electronic circuits).S-TWE1010
Solder60/40 rosin core solder.S-T9000X
Desoldering pump, bold, or braidThis will assist you in removing solder for circuit modifications and correcting connection errors.S-T7874B
Wire StrippersFor exposing wire leads.S-T105SCGV
Wire CuttersThese are great for cutting leads on resistors, capacitors, etc. before and after soldering.S-T170M
Needlenose Pliers6" long are good for bending component leads and holding components leads while de-soldering.S-TNN7776
ScrewdriversPhilips-head required for disassembly.S-TXP600

Safety

We will take a moment to again stress the importance of putting safety first when working on this type of equipment. Tube amps operate at high voltages which have the potential to kill. Only work on an amp when you are wide awake and attentive.

Please be certain to do the following before opening your amp:

  • Turn the power switch off
  • Turn the standby switch off (down position)
  • Unplug the power cord
  • Give the power tubes 10 minutes to cool down
Turn amplifier off

When the amp is open:

  • Use a multi-meter to measure for high voltage at several test points. (see the "Preliminaries" section for details)
  • Use the one-hand rule. (see the "Preliminaries" section for details)

Soldering Tips

  • Work in a ventilated area with a fan to blow the smoke away from your face.
  • Allow the soldering iron to heat up to the point where the solder melts quickly when touched to the iron's tip.
  • Clean the soldering iron's tip by wiping it across a wet sponge before applying solder to it.
  • Be very careful not to unintentionally burn any wires in the vicinity of the soldering iron.
  1. Solder tip step 1 Bend the component lead or wire ending and wrap it around the connection point.
    • Make sure it is not too close to a neighboring component which could cause an unintended connection.
  2. Solder tip step 2 Wrap the component lead so that it can hold itself to the connection point.
  3. Solder tip step 3 Touch the soldering iron to both the component lead and the connection point allowing both to warm up just before applying the solder to them.
  4. Solder tip step 4 Be sure to adequately cover both component lead and connection point with melted solder.
    • Remove the soldering iron from your work and allow the solder joint to cool. (The solder joint should be shiny and smooth after solidifying.)
    • Cut off any excess wire or component leads with cutting pliers.
    • Clean the soldering iron's tip by wiping it across the wet sponge again after making the solder joint.

De-Soldering Tips

  1. De-solder tip step 1 Heat up the old solder joint with the soldering iron
  2. De-solder tip step 2 Apply fresh solder to mix in with old solder joint
  3. De-solder tip step 3 Use a de-soldering tool to remove the old solder joint while it is heated.

Preliminaries

Please be certain to do the following before opening your amp:

  • Turn the power switch off
  • Turn the standby switch off (down position)
  • Unplug the power cord
  • Give the power tubes 10 minutes to cool down
Preliminary step 1
  • Make sure the power tubes have cooled down enough to touch.
  • Remove the power tubes from the amp and store them someplace safe.
  • Remove the amplifier chassis from its cabinet and set it on a safe workplace.

When the amp is open, follow the one hand rule and use a multi-meter to measure for high voltage at several test points. (see test points below)

Use the one hand rule

The one hand rule (pictured above) is a safety precaution for working on an amp that is plugged in or could potentially have high voltages present. Using your multi-meter, clip the ground side to the chassis and use the other side to probe at various test points with one hand. This prevents a fatal shock which can result from current passing through the heart. (Many people even put their other hand in their pocket or behind their back to ensure it is not used).

Test Points

Inside view of amp chassis - not all components are shown

Inside view of amp chassis - not all components are shown

Warning: If there is a high voltage present at any of these test points, there may be something wrong with your amp. Please have it checked by a qualified repair shop.

  • TP(1), TP(2): These test points are both on the standby switch. When the amp is in operation, these two points have a very high voltage present. By 10 minutes after turning off the standby and power switches, all the voltage should have been drained.
  • TP(3): This is the anode side of a solid state diode.
  • TP(4), TP(5): These test points are the cathode pins of the preamp tubes. It is not a bad idea to check these two pins on each one of the preamp tubes.
  • (Tube Rectifier TP: If your amp has a GZ34/5AR4 tube and no standby switch, then check the GZ34 pins 2 and 8 for voltage.)

Tone Stack Modification

Here is a schematic representation of this mod. It is not necessary to be able to read the schematic in order to complete the mod.

Before Tone Stack Modification

Before Tone Stack Modification

After Tone Stack Modification

After Tone Stack Modification

Preamp Tube Pin-out (12AX7)

Preamp Tube Pin-out (12AX7)

The modification is the same for amps with a middle tone control.

Before Tone Stack Modification with middle control

Before Tone Stack Modification with middle control

After Tone Stack Modification with middle control

After Tone Stack Modification with middle control

Tone Stack Component Location

Depending on your amp model and which channel you want to modify, the inside of your amp chassis will look slightly different than this example. Follow these steps to find the tone stack components you want to modify.

Tone stack modification - detail of inside chassis

Step 1: Choose which channel you would like to modify and find the input jacks related to that channel. (They should be labeled like one of the drawings below).

Tone stack modification - locating components step 1

Step 2: Follow that channels input signal wire to its preamp tube pin 2.

Tone stack modification - locating components step 2

Step 3: On that same tube socket, find pin 1 and follow its wire to the circuit board.

Tone stack modification - locating components step 3

Step 4: On the circuit board, identify the two capacitors to be modified.

Tone stack modification - locating components step 4

Step 5: If your amp does not have a middle control, find the 6.8kΩ resistor connected from the bass control to ground for the channel you choose to modify.

Tone stack modification - locating components step 5

Tone Stack Component Modification

Tone stack modification - detail of inside chassis

Step 1: De-solder and remove the .047µF cap.

Tone stack modification - modifying components step 1

Step 2: Insert and solder the 0.1µF cap in its place.

Tone stack modification - modifying components step 2

Step 3: De-solder and remove the original 0.1µF cap.

Tone stack modification - modifying components step 3

Step 4: Insert and solder the .047µF cap in its place. If your amp has a middle control, do not implement the following steps 5 and 6.

Tone stack modification - modifying components step 4

Step 5: If your amp has no middle control, de-solder the 6.8kΩ resistor connection at the bass pot.

Tone stack modification - modifying components step 5

Step 6: Insert and solder the 10kΩ resistor.

If there is a grounded screw nearby, use the locking solder lug and nut to connect the grounded lead of the resistor.

Tone stack modification - modifying components step 6

You may choose to remove the 6.8kΩ resistor completely and solder the 10kΩ resistor in its place instead.

Tone stack modification - modifying components step 6

Step 7: Close the amp back up, re-insert the tubes and you are done.