Aiken Amplification Invader MKII User Manual page 16

Ringing noise or feedback that occurs with no guitar plugged in:
If the amplifier exhibits a "ringing" noise, especially when tapping on the chassis, or if it
makes a howling or whistling feedback noise when the volume is turned up but no guitar
is plugged in, there is probably a microphonic tube in the amplifier, likely in the first
position (the small tube closest to the input jacks), or in the reverb recovery position.
Remove the shield and lightly tap on the tube with the volume turned up. A small amount
of noise is normal, but a loud, ringing noise indicates a microphonic tube, which should
be replaced. In a emergency, you can swap the other ECC83/12AX7 preamp tubes
around until you find the least microphonic one to put in the first position. Typically, the
phase inverter tube is the least susceptible to microphonics, so the most microphonic
tube should be used in this position. The least microphonic tube should be used in the
first position, and the next least microphonic tube should be used in the reverb recovery
position. When purchasing tubes, be sure to purchase tubes that have been tested for
microphonics. Many vendors do not test for microphonics, and you could end up wasting
your money on tubes that are unsuitable for use. Combo amps are more susceptible to
microphonics than heads, because the sound field coming from the back of the speaker
can excite the tube into oscillation. In rare cases, the EL84 or EL34 output tubes can be
microphonic, particularly if they are at the end of their lifespan.
InvaderMKIIManual.doc 12/08/11 Page 16 of 17
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