A bad Fetron can be the cause of scratchiness in guitar volume control. Usually you can
only get a replacement Fetron from us. We use a special type and we select and install
load resistors to fine tune each Fetron. If you ever do need a replacement, notify us.
Installing a new Fetron with its particular resistors should take anyone with any soldering
skills only 10 minutes or so and is not difficult.
Use a 12AX7 in place of the Fetron. They are direct replacements for each other and
either device can be plugged into the socket and will work perfectly with the surrounding
circuitry. Tube performance is not substantially altered by the load resistors selected for
the Fetron, but you should select a tube which does not rattle or feed back when an
instrument is plugged into INPUT 1. A rubber band stretched tightly around the tube will
reduce noise. Replace the shield. Always tap on new pre-amp tubes to check for
noise and stability.
If the amp experiences a cutting out when in the heavy distortion, loud driving mode after
a 12AX7 has been substituted for the Fetron, then install a 250 pf 1000v disc capacitor
across the plates of the driver tube (12AX7) next to the 6L6's. The plates are pins
1 and 6. If an oscilloscope is available and the Fetron shows a lopsided wave form or
uneven clipping, it can sometimes be corrected by changing the value of the plate load
resistor. Values should fall in the range of 33K to 100K. Check Channel 2 first with
controls set as follows:
V1:0
V2: adjust for clipping with 1 Khz 1 volt input.
MASTER: 10
TREBLE: 10
OTHER CONTROLS: 0
Then, leaving V2 at clipping, plug signal into INPUT 1. Changing plate resistor value
will shift the operating point of the Fetron.
• Check Filter capacitors. A shorted filter will cause smoke and blow the fuse; an open or
intermittent-open filter cap will cause hum, distortion and weak tone. To test, merely clip-
lead a cap of sufficient voltage across each filter, one at a time. (Filter caps are large
yellow, orange, or blue cylinders mounted on heavy eyeletted power supply board.
Any significant improvement of tone or wave shape or any reduction in hum indicates an
open cap. To replace, merely clip out bad cap - it cannot be unsoldered because its leads
extend underneath the board. Make sure you note the proper polarity and replace with the
+ at the same end as the original, note especially the 2 marked 60 mf 350 volt, since they
face opposite directions. A subharmonic note audible below the note you are playing
(when fairly loud, lower to midrange) indicates an open main input capacitor, either one of
the 60 mf 350 volt or the capacitor connected to the output side of the standby switch
(located next to power transformer in models having the Graphic Equalizer.)
• Check the bias. Should have -52 Volts across 50mf 75 volt bias filter (small white cap on
power supply board). An open cap will cause low bias with lots of hum. The bias voltage
is permanently set when amp is first tested and shouldn't need adjustment; however a
malfunctioning Graphic Equalizer can sometimes cause a change in bias. Problem would
then be a blown transistor on the EQ board. See section on Graphic Equalizer.
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