The hi-fi-style bias adjustment for the power tubes.

The hi-fi-style bias adjustment for the power tubes.

When you change your power tubes (they do wear out) it’s best to re-bias them. “Bias” is the negative voltage applied to the power tubes’ grid to control the number of electrons passing between the plate and the cathode. For the tubes on The Twin you place the probes from a voltmeter in the two bias test points and turn the bias potentiometer until the meter reads 0.04 VDC. Once you set the bias you must set the Balance between the push-pull sections of the amp. Again by using the test points and a voltmeter adjust the balance to 0 VDC. As tubes age their bias/balance can drift, so it can be worthwhile to make these adjustments more than once over the life of the tubes.

If you aren’t comfortable with using a voltmeter or performing this kind of maintenance thenĀ don’t touch this stuff! You can do a lot of harm to your amp if these adjustments are done incorrectly. Take your amp somewhere to be serviced.

Also in this section of the back panel is the footswitch jack. Nothing special – a two-switch pedal can be used to switch channel two on and off in channel-switching mode, and to switch the reverb on and off.

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