hi folks, i've got a 1963 Calbest 8211 amplifier. for each channel, there are 2 6BQ5 power tubes and 3 12AX7 preamp tubes. they share a 5U4G rectifier tube. i know that when i replaced the power tubes, they had to be properly biased. I recently had a preamp tube go bad. do i have to have the replacement biased or can i just pop in another 12AX7?