Dating fender bandmaster amps
The reason is that 12AX7 tube has a voltage gain factor = 100, while 12AT7 = 60 and 12AY7 = 45.Pull out the V1 normal channel preamp tube – More preamp gain in vibrato channel We’ll start with saying that this is a mod we personally do to all dual channel blackface, silverface and reissue amps.This makes it possible to adapt to small and big stages and gigs just by configuring the speakers.Most of the maintenance and mod tips below will make the amp break up earlier, reaching the sweet spot at a lower volume.The C12n sounds very punchy, wide and deep in a 2×12″ cabinet.See page How to select speakers for speaker recommendations. If you want cleaner and spankier preamp distortion charcteristics, you may replace the V1 or V2 12AX7 preamp tube with 12AT7 and 12AY7.The silverface Bandmaster amps got a 5U4GB tube rectifier and the vibrato channel got reverb. Due to this the silverface Bandmaster Reverb is less powerful, has less attack and breaks up earlier than the blackface Bandmaster.
They are similar in the way that both are clean sounding with just one 12ax7 tube in the preamp stage (vibrato ch in bandmaster and normal channel in bassman).
These tubes have different frequency responses than 12AX7, particurlarly when distorting.
People describe these tubes to have less harsh and buzzy distortion.
The vibrato channel in the Bandmaster is even more clean than the Bassman because of the vibrato circuitry loading the signal chain and reducing the gain level in the premp section.
Hence, the AA864 Bassman normal channel has more preamp “juice” and reaches the sweet spot at an earlier volume knob setting.