9 pin miniature preamp tube (Amplification Factor = 100). This is the ubiquitous Chinese 12AX7 with added selection. The gain is high and the American "Groove Tubes" must be filtering out the noisiest ones.
For more information about our 12AX7 tubes, see our tech corner article on 12AX7s.
This tube is closest in specification to a 12AX7 and can be exchanged with a slight drop in gain. Most companies changed over to the 12AX7 as it became more common.
This tube can be tried in later Marshall amplifiers in the first gain position, where the owner of these amps would like to drop the background noise and lower the gain level to plexi or early 70s vintage gain levels with its returned wider frequency response.