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Location Type
Preamp tubes
V1 12AX7
V2 12AX7
Power Tubes
V3 EL-34
V4 EL-34
Here’s a brief description of what each tube does:
V1 Input buffer and tone control driver for Channel 1 & 2
V2 Phase inverter driver tube for the power amp section
V3 & V4 Power amp tubes—for best operation, they should always be changed in pairs
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.
1
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. The tube most susceptible to ringing are V1. Here’s a procedure to find which tube is
giving you trouble.
With nothing plugged into either the High Gain or Low Gain inputs, the amp on.
Turn up the Volume on Channel 1 and Channel 2. Now use the tip of a pencil to gently tap the end
of each of V1 and listen for sustained ringing or rattle.
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).
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