Questions, You said it only produces hum, does this mean it produces no output from your guitar? Is your amp a Silverface with the hum-balance circuit instead of the bias circuit? Have you tried pulling the pre amp tubes individually, one at a time? I don't know you or have any idea of your competence with amps, so if you pull the tubes, please turn off and unplug the amp before you pull a tube, and remember hot tubes are hot, don't burn yourself, if you know all that, please pay me no mind, I just don't want anyone to hurt theirself on my account, ...............