Rivera Chubster 40 Owner's Manual - page 27
R Series V1.0 p 27
Checking for microphonic tubes
As tubes wear, some problems can come up. One of the most common symptoms is a
ringing sound. This is usually due to the tube becoming microphonic (like its name
suggests, it's picking up sound and amplifying it).
With the amp unplugged and cooled off, examine the tubes to make sure they're in tight
and straight. Never twist them! Gently grasp the tube and wiggle it into place. Because
some of the tubes are inside of metal sleeves, you will have to remove the sleeves to
check them for microphonics. Grasp the sleeve with your fingers and depress it (it's
spring-loaded) and turn to the left (counterclockwise). Now pull it off; this may require a
little wiggling action.
Preamp tube first aid
If you hear ringing (a feedback-like high-pitched sound) in your amp, it's probably
coming from a preamp tube. Here's a procedure to find which tube is giving you trouble.
With nothing plugged into either the High Gain or Low Gain inputs, and the Master
controls turned down to 5 or below, turn the amp on.
Turn up the Volume on Channel 1 and Channel 2, as well as the Reverb knob. Now use
the tip of a pencil to gently tap the end of each of the small tubes (V1 through V5) and
listen for sustained ringing. Turn up the Volume and Master knobs and keep tapping
until you find the tube that rings (or squeals).
Turn off the amp, and allow the tubes to cool. Now pull out the troublesome tube and
replace it with one of the same value (that is, if you're pulling out a 12AX7, replace it
with a 12AX7).
Make sure that the tube is oriented correctly when pulling it out or putting it back in. If
you look at the end of the tube and the socket, you'll notice that the nine pins are
arranged in an incomplete circle. Always make sure the pins are aligned correctly.
Never force a tube into its socket.
Remember to put the sleeve back on after you check or replace a tube.
Power amp tube first aid - Chubster 40, R30-112
Like preamp tubes, power amp tubes can go bad or wear out. Your Thirty Twelve has
two power amp tubes, and if one goes bad, they should both be replaced. This assures
optimum output and tone.
If a power tube shorts out, the Mains Fuse may be blown. Remove power from the amp
and replace the fuse. If this doesn't remedy the problem, an internal fuse may be
blown(refer this to a qualified repairperson). If there's something wrong with the sound
and it can't be traced to the preamp tubes, it may be a defective power tube. Try the
following:
1. Remove the power tubes. Remember the way the eight pins are arranged, and note
that the center hole on the socket has a keyway that matches the center post on the
tube.