I know its probably been asked before, but can you just replace stock 12AX7A's with regular 12AX7's. I wanted to retube my Fryette Sig X, and I have a bunch of TungSol 12AX7's, would this be a problem?
Historically, there was a significant difference between where the 12AX7 could be used (only circuits with parallel-wired filaments) and where the 12AX7A could be used (circuits with both parallel and series filament wiring). This is a moot point in modern-production tubes, as the standard now is 12AX7A - whether the tubes are called "12AX7A" or just the usual "12AX7"
Your Freyette probably has parallel-wired filaments anyway - most modern (post 1950s) circuits do.
There can be significant sonic character differences between historical 12AX7 and 12AX7A versions of tubes (think RCA!). There is no generalization, and the difference cannot be attributed to the the series/parallel capacity of the tube's design.