Up to serial #5200 used magnetically operated tremolo control switched from console, although many were field updated to the later style same as used today (later than sn 5200).

31H series c. 1951 to 1958 (may be as late as 1960 on special order, hopefully I will find out before the few people left that know for sure die). Same box a 31A; top louvers are now divided on 3 finished sides, but still upside down. Amplifier now dubbed "32H". Series II starts at sn 5200 and has modern tremolo control circuit, otherwise amp is the same as previous except it does not use an interstage transformer.

Later 31H's (sn 8000 and later) use the 32H series III amp, which is almost the same circuitry as previous but now housed in the now-familiar narrow chassis that also fits into the bottom of the 21H box. Same size chassis metalwork design similar as used today. The reason for this was that the 21H originally used a 21H 20 watt amplifier on the narrow chassis design. The 21H proved to be popular residentially, but artists demanded additional power. Since the 32H series II amp would not fit in the bottom of the 21H box, the amp was redesigned/repackaged into the smaller 21H chassis and dubbed the 32H series III (what else!), and the results were as expected - which essentially signed the 31H's death warrant. The 21H w/32H amp was 95% of the sound with 75% of the weight and size of the 31H.

From that point on (c.1953), the 32H series III amp was optional in the 21H, but was the only amp used in the 31H (after sn 8000). All of the 31H's used the PM V21 and F15LL (as did the 21H).

Sal Azz