||The ubiquitous guitar preamp tube
||Military 12AX7 with just slightly lower gain than a 12AX7. Less highs too. Great for cleaning up a harsh or overly bright sounding amp channel.
||Fender reverb driver and phase inverter tube. Sometimes used to replace 12AX7s where lower gain and more note definition/clarity is desired.
||First gain stage tube in Fender tweed amps from the 1950s. More of the same effect as the 5751 above. Good for cleaner blues tones. Harp players like these to reduce feedback.
||Phase inverter in Vox amps. Sometimes used by harp players to reduce feedback in 12AX7 stages. Also a great substitution in 12AX7 type guitar distortion pedals. Allows for more usable range on the gain controls with less fuzzy/more bluesy distortion.