These are just basic guidelines, and we’ll need to dig deeper to find out what we really have, and if it will suit our purposes.Keep this in mind as well; starting about 1977, output power was increased on many Bassman heads from about 40-watts RMS to about 60-watts RMS.But the ‘Top 40’ player needs a little more, and using the cathode follower as a plate-loaded gain stage is definitely the ticket.So, you must decide, are you a collector or a ‘working’ player?There is more to tweak, beyond the input and tone control stages.Above we see a partial schematic to the seldom-encountered AA165.Of course there are accepted mods (or ‘upgrades’) that are absolutely not going to ruin the vintage value of your Bassman head.Changing to a 3-prong AC cord is pretty much an expected upgrade or mod, as is replacing the filter capacitors.
Should you decide to fleece some other sucker via e Bay with your ‘original’ Bassman head, it’s easier to do if the ‘old’ parts are still around.
If the cathode follower is not to your liking, you could have the one-half of that 12AX7 to use as you desire.
For the ‘modern’ guitar player interested in a ‘raw’ tone for classic Rock ‘n’ Roll or Blues, these heads are still hard to beat.
There is no real ‘trick’ in determining which circuit you have without taking the chassis out.
The tube chart seen inside the ‘box’ is a good indicator, but not 100% accurate.