No Millen station would be complete without the one-and-only high power amplifier
ever produced by James Millen. The 90881 was designed to be a 500 watt amplifier
using a pair of 812A triodes in push-pull configuration. Both input and output circuits
are designed for balanced lines which is interesting considering the 90800 and 90801
transmitters had 50 ohm unbalanced outputs! Band switching is achieved via plugin
coils for both the input and output tuning. Metering is supplied for both grid and
Notice the small hole to the right of the plate tuning knob (left knob). This is used to allow a shaft drive to connect to the output tuning coils link coupling. Although this type of external link adjustment isn't shown in any of the Millen ads for the 90881, it does appear on many units physically seen. Thus it appears Millen did indeed make this change to the amplifier along the way.
An external DC supply is needed and the user supplied the filament transformer as well on earlier units. This is because the 90881 could be used with other power tubes besides the 812A's. However, on some models, like the one pictured above, the filament transformer was probably supplied as the front panel has a pilot lamp and switch to turn on the filament. Earlier 90881's did not have the lamp and switch assembly. Perhaps the link adjustment came about at the same time as the aforementioned assembly.
This 90881 belongs to Gary Carter, WA4IAM. He purchased it directly from James Millen Electronics, so it is likely to be one of the last in the series of amplifiers.