Anders' '64 Fender Showman
Updated: 2002-10-16
Amp is a '64 Fender Showman (model AB763). The amp, which seems to be in original condition, is reported to be rather weak and to suffer from significant hum. The work includes a thorough check-up of voltages, capacitors, tube sockets and potentiometers. To get rid of the annoying hum, the original filter capacitors need to be replaced (they have large "blisters" on them already). The amp is equipped with the infamous Hagström power transformer and, as a consequence, the DC voltages in the amp are way too high (B+ = 500 V -- approximately 50 V over the intended value). In addition, the heater voltage is way too high: 7 V.
To lower the heater voltages I have opted to remove one winding of the heater
coil. With a smooth cut in the paper (left picture) I got access
to the heater copper wire. After removing one winding from each half of the
heater coil (right picture) the voltage became 6.2 V. Quite acceptable.
To lower the B+ voltage to a more reasonable value, I have used a zener
circuit. In fact, I was able to
bring the B+ voltage down to the original value of
450 V when wall power is 230 VAC and the plate current is 36 mA per power tube.
Old filter capacitors needed changing and the power resistors are tired (drifted values).
Note the large "blisters" on the rightmost caps (upper left picture). No wonder
this amp is a serious hummer. Cap board was emptied and cleaned (upper right
picture). Finally, new Sprague Atoms and wire wound resistors were installed (lower
picture).
On the Vibrato channel, a new tremolo on/off pot (#1 from left) and a 25kOhm
middle pot (#4 from left) have been installed. On the Normal channel, a middle
boost switch has been installed in place of the bright switch (right picture).