In certain applications, with minor socket re-wire, and re-rating things a bit, many tubes can sub for each other.
This is also good to let you quick-check an existing circuit (if same base code tube is used) and for those who like to tinker, building a circuit from scratch using available tubes.
The 3 lists below will be “all compatible” IF:
Frequency below 30MC is used
Pins 2&7 are joined and treated as cathode
B+ not over 110VDC
Rk (cathode resistor) 220ohms minimum
(7BD)
6AG5	6Aj5	6AK5	6AN5	6BC5
6CE5	408A	5590	5591	5654
6028	6096	6186	6968	9001
9002 	“A” suffix versions too
(7BK)
6AH6	6AK6	6AU6	6BA6	6BD6
6CG6	6FD6	6HR6	5749	6136
6485	6660	7543	8425	8426 	“A” suffix versions too
(7CM)
6AS6	6BH6	6BJ6	6CF6	6DB6	6BZ6
6DC6	6DE6	6DK6	6EW6	6GM6	6CB6
6HQ6	6JH6	6JK6	6JL6	6LU6	5725
6187	6265	6661	6662	6676	6954
7056	7732	8084	8136 		“A” suffix versions too
The tubes above are a mix of sharp cut-off and remote cut-off types. This may not matter in some circuits and for quick checks.
A few of 4 digit number tubes may include 12-13V heater types. Consult http://www.nj7p.org/Tube.php
There are some “3” “4” and “12” prefix versions of the =others= which indicate heater volt class.
Note:	(this includes the other shown later)
There are circuits that will not work with some tubes (especially due to 2 & 7 for base codes 7BD 7BK 7CM in this family) or will work poorly due to reasons below:
In general, sharp cut-off tubes are best as detectors, mixers, oscillators, multipliers, and non AGC’ed amps (RF IF).
Remote cut-off tubes are used in AGC’ed (AVC’ed) RF and IF amps but will work ok as non-AGC’ed amps – but NOT as detectors, mixer, multiplier and maybe poorly as oscillator.
Most oscillators will prefer base code 7BK or 7CM, not 7BD, ini this family you ground 3rd grid, which ever pin it goes to.


All tubes listed below are inter-compatible with no re-wire close enough IF:
Frequency below 30MC
B+ not over 110V
Rk 220ohms for each cathode, minimum
(9AE)
6AX8	6BL6	6EA8	6GH8	6GJ8	6HL8	6KD8
6LM8	6LN8	6MQ8	6MU8	6U8	6JW8	6KE8
6LX8	6MG8	6678	7059	7687	7731	8445 	and “A” suffix versions
The triode will work as oscillator or amp (included grounded grid input-amp)
The pentode will work as amp or mixer or multiplier being sharp cut-off
Some of 4 digit number ones may include 12-13V heater types. Consult: http://www.nj7p.org/Tube.php
In cascade circuits triode should “drive” pentode, not the other way around.


The tubes listed below are all inter-compatible close enough IF:
Frequency below 30MC
B+ not over 110V (160V for pentode plate)
Rk 220ohms for each section’s cathode
(9AD)
6AU8	6AW8	6BA8	6BH8	6CX8	6EB8
6GN8	6HF8	6HZ8	6JE8	6JL8	6JT8
6JV8	6KR8	6KS8	6KV8	6LB8	6LF8
6LQ8	6LY8	6MV8	12AU8	7716
The triode is good as crystal oscillator or amp.
The pentode is good as audio or RF power amp with up to 1.5W DC input and ample output as audio for receivers or for QRP in transmitters.


The 2 lists shown are all inter-compatible enough IF:
Frequency is below 30MC
B+ is not over 110V
Rk is 220ohms minimum
(9PM)
6HM6	6HT6	6JC6	6JD6	6KT6
(9AQ)
6EH7	6EJ7	6GK7
Pins 6 and 9 and central “pin” on all above to go to ground and can support a shield which crosses the socket to hide pins 7 and 8 from pin 3, when used as RF or IF amplifier.
These are high gain tubes and will try to self oscillate if pin 2 can see pin 7.
Note: base codes 9PM and 9AQ are the same except for construction of internal shield.