Instructions for converting a Tolex Bassman head so that it will put the feedback into the driver tube’s cathode circuit and for rebuilding the cathode circuit to a stronger, more musically sensitive circuit.
Before Starting:
- Make sure that your amp’s circuits look like the drawing in Figure 1
- Check that the amp is working
- Make a list of all of the parts you will need
- 1 meg Resistor (2)
- 6.8k Resistor (1)
- 4.7k Resistor (1)
- 100k Resistor(1)
- 82k Resistor (1)
- 0.1 Capacitor (1)
- 0.01 Capacitor (1)
- Soldering Equipment
- Unplug amp before starting work
- Discharge filter capacitors before starting work
- Do not work on amp unless you know what you are doing
(Figure 1 on the following page demonstrates how the amp looks before the modifications and Figure 2 shows how the amp will look after.)
- Begin by removing the three 0.1 mfd caps (labeled as C1, C2, and C3 on Figure 1).
- Remove the 330k resistors and replace both with 1 meg resistors (remove R2 and R3 – Figure 1).
- Leave the center resistor as is (R4 – Figures 1 and 2).
- Connect a 6.8k resistor to both new resistors and the old resistor (will look like R2 and R3 – Figure 2).
- Replace the 22k resistor (R1 – Figure 1) with a 4.7k resistor (R1 – Figure 2).
- Replace the 0.1 mfd capacitor with a 0.01 capacitor (C3 – Figures 1 and 2).
- Begin working on the left side of the circuit.
- Remove the 47k resistors (R5 and R6 – Figure 1)
- Replace R5 (Figure 1) with a 100k resistor (R5 – Figure 2).
- Replace R6 (Figure 1) with an 82k resistor (R6 – Figure 2).
- Make sure that the R7 and R8 resistors (Figures 1 and 2) are 100k resistors.
