A.F. DOUBLE TRIODE5751 can be used in most 12AX7 circuits to achieve more headroom. It has a lower gain than the standard 12AX7. Good to rope the gain of a high gain amplifier.
A.F. DOUBLE TRIODEECC82 is 12AU7 medium gain pre-amplifying double triode. Provides a very smooth sound with well-defined tones suitable for audio and guitar amplifiers.
R.F. DOUBLE TRIODEECC81 is 12AT7 medium gain pre-amplifying double triode. Can be also used as replacement for 12AX7 in many applications. Very smooth and warm sound. Very low microphonic. Good for phase inverters or reverb drivers.