The output tu es will also age unequally (much like car tires), so some may adjustment may
e needed through out the tu es' life to keep them in alance. Although it's not necessary to
keep the tu es perfectly alanced, it will keep the amp putting out maximum power with the
least amount of hum.
If adjustment is necessary, first loosen the outer lock-nuts on the ias adjustments. Make
move the control with a flat-head screwdriver (a guitar pick works well too), while monitoring
the num er on the multimeter. You may have to adjust oth controls ack and forth until they
are alanced ecause they are somewhat interactive.
Tu e life and trou leshooting
Output tu es (V4 and V5 – See tu e chart on pg. 10) generally produce a good strong sound
for 6 months to a year when played regularly, then they may ecome dull sounding, and/or
the amplifier may egin to loose some power. Output tu es may last much longer depending
on how hard the amp is played. Sometimes they die gently, sometimes a ruptly causing a fuse
to low, which in turn protects the amplifier from further damage. Output tu es may also
ecome microphonic like preamp tu es.
It's sometimes possi le to see which power tu e(s) is damaged. If necessary, remove the
output tu es to inspect them (Remove power cord, allow tu es to cool, then gra y the
plastic ase, and gently pulling up in a circular motion). Here are things to look for:
•Shiny silver area on top of tu e turns white.
•Burned spot on the large gray structure inside the tu e.
Sometimes a output tu e or preamp tu e will ecome “microphonic” when something
ecomes physically loose inside the tu e's glass envelope. In turn, this noise then gets
amplified through the speaker. The noise can e anything from static, a high-pitched ringing,
or intermittent sputtering noises. This is usually exacer ated y vi ration from the speaker
ca inet. If you think a tu e may e microphonic, try isolating the amplifier from the speaker
ca inet.
Tip: If you think you have a microphonic tube, you can very gently tap on each tube with a pencil's
eraser to see if the sound becomes worse or changes. All tubes will amplify the sound a little,
especially preamp tubes. But for example, if you hear an intermittent ringing sound and tap each
tube to find V1 is causing the ringing to start and stop, you can be sure it's the problematic tube!
In general preamp tu es (V1, V2, and V3 see tu e chart on pg. 10) can last many years, and
usually ecome microphonic efore wearing out or failing.
Changing preamp tu es
6
