[Mondial]

There are two separate heater windings used on the 37-116 chassis. Some tubes are fed from one winding while others are powered from a second winding. Perhaps there is a higher load on one winding which results in a greater voltage drop and reduced heater voltage?

The main reason for the two separate heater windings is for hum balancing of the directly heated 6B4G push-pull output tubes. Each filament is fed from a separate winding with a balance pot in each. Because the filaments are directly heated with AC, there is a AC component generated in the output. By balancing the filament windings voltage to ground with the balance pots, the AC hum can be cancelled resulting in minimum hum. In the latest production of the 37-116, the 6B4G tubes were replaced with 6A5G's which are similar but have heater-cathode construction. Without AC on the cathodes the need for the balance pots were eliminated.