Aiken Amplification Invader User Manual download pdf (Page 15)

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 output tubes can be microphonic, particularly if they are at the end of their lifespan.

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Aiken Amplification Invader User Manual Page 15