The symptoms you describe point to a not-so-rare condition that we amp techs call “Fenderitis.” In vintage blackface and silverface Fender amplifiers, the component board is made from a type of fish paper fiberboard that was dyed black with a carbon-based dye. In certain cases, the board sometimes becomes conductive. This could be related to the humidity in the environment, but once the board becomes conductive, DC voltage becomes present everywhere on the component board because of the many high-voltage parts mounted on it.
If you suspect that your amp has this problem, take the two leads of a voltage meter and put the black lead to a chassis ground and touch the red lead anywhere on the circuit board to find out just how much DC is present. Measurements of 10 to 40 volts are common. All the components mounted on the board become biased with this positive DC voltage, which throws the preamp tubes’ grid-to-cathode biasing relationship off considerably, resulting in bad tone and scratchy pots.
A sure way to overcome this problem is to transfer all of the electronic components to a new board made from Garolite (epoxy glass laminate). However, an easier solution is to have your tech remove the three tone capacitors of each channel from the component board and mount them directly on the pots. In most cases, this will correct the scratchiness in the pots and the tone will improve dramatically. In extreme cases, the 500pf cap and the 1Meg ž resistor in the grid circuit of the reverb driver tube will need to be lifted from the board and mounted on pin #2 of the reverb driver tube socket.
—Gerald Weber, Kendrick Amplifiers
Have a question for the GP brain trust? Send it to firstname.lastname@example.org.