On the CL the master is right before the preamp out. So the master is acting like a pad. Turn it all the way up and the full preamp signal is going to the power amp. Turn it down and you are attenuating the preamp output. If you want to minimize the extent of the pad affecting your signal, turn up the master. It is a linear pot so the position of the knob is proportional to the amount of attenuation.
The gain is located at the input stage of the amp. Turn it up and you are increasing the amplification. When cranked you are pushing the preamp tubes into distortion. The master regulates the overall volume level by padding and therefore lowering the signal level. The two work together to balance dirt and volume.
Using the gain and master is about getting the sound that you want. If you want maximal clean, crank the master and use the gain to regulate the overall volume level. If you want dirt, crank the gain and use the master to control how loud the rig is.
By cranking the gain, you are putting more strain on the preamp tubes. This will wear them down faster than if you were to turn up the master and control the level with the gain. But this is a minor issue. You aren't going to kill your preamp tubes by cranking the gain. The amp was designed to this. Besides, preamp tubes are relatively inexpensive. I'd be more concerned about running the amp so loud that you wear down the power tubes which cost more to replace. Having said all this, tubes last quite a while.
Official Ampeg Portaflex Club #89
Last edited by beans-on-toast : 01-28-2014 at 07:19 AM.