PI and Reverb drivers

[gregarious]

Been cobbling a few designs together and wanted to know what the experienced hands opinion is to driver tubes for PI / reverb.

Fender changed the PI & reverb drivers from 12AX7's to 12AT7's when the Brown faced amps gave way to the Black faced amps. I'm guessing this was done to make the Black faced amps more polite (less aggressive).

Is there an appreciable difference between the 12AX7's to 12AT7's in the PI and Reverb positions, or did Leo just come across an available supply of 12AT7 at a smoking price?

All responses appreciated.

[Fischerman]

My guess is that the Reverb driver tube (when driving that Reverb transformer) stresses the tube and a 12AT7 is WAY more robust than a 12AX7. It has way more current capability and much higher max plate dissipation. Then they tried the 12AT7 in the PI and thought it sounded cleaner (and clean was their design goal) so they went with that. It seems to me that the PI circuit was designed for a 12AX7 (just look at the values...they scream 'designed for a 12AX7!').

Many times I prefer a 12AX7 in a BF Fender PI but not always. This is just my guess.

[rfgordon]

Driving a verb tranny is about power (or current) not about gain. A 12AX7 has lots of gain, but produces almost no power. On the other hand, both sides of a 12AT7 wired together give at least 1 watt of output, which drives that verb nicely. (A cool trick you can do with verb amps, particularly Fender style, is connect a speaker to the verb send. Pull out the PI tube, and you've got an instant 1 watt practice amp.)

The 12AU7 also works well as a reverb driver tube, since it handles more current than the 12AX7. The 12AU7 can also make really boingy Fender verbs a bit more smooth sounding.