Set the "Channel 1" mode switch to "Clean." You'll get a balanced, clean sound, unlike with the “Pushed” setting, which introduces a higher gain early on. This latter setting is great for solos but not recommended otherwise.
Set the "Channel 2" mode to "Raw" to enhance the sound with the least amount of gain. Use the “Modern” setting for aggressive and speedy low-end reactions.
Set the "Gain" control to the low 7:00 to 11:00 range to produce bright, clean, three-dimensional upper harmonics. Alternatively, set this control to the middle range between 11:15 and 2:00 range to give some of the upper harmonics a warm, rich tone and fuller bottom-end reaction. Setting the gain beyond the 2:00 level puts it in the high range, in which microphonic squealing is more likely to occur.
Set the "Treble" and "Presence" controls to the low range between 7:00 and 11:00. The Treble control, in particular, compounds the amount of gain and may cause microphonic squealing if it's set in the higher range. The Presence control affects higher-end frequencies and provides overall global tone control.
Set the "Mid" control to the low to mid 7:00 to 1:00 range, as higher settings allow it to act as an additional gain control.
Confirm that while the rectifier was powered on, an instrument connected using the Slave Out jack wasn't removed and then inserted into the FX Loop Return jack. Doing this will produce a high-pitched squeal.
Replace the pre-amp tubes if squealing still occurs. Position the rectifier on a table so you can clearly see the pre-amp tubes.
Set the rectifier to "Standby" and then wait at least 30 minutes or until the rectifier and pre-amp tubes have cooled.
Locate the "V1" pre-amp tube; it's typically the closest to the Input jack.
Gently twist the "V1" pre-amp tube away from its socket. Replace it and then test the rectifier for squealing.
Repeat the procedure just described and remove the next pre-amp tube in the set if squealing still occurs. Replace the old tube with a new one. Check each tube, one at a time, until the bad tube is identified and replaced.