The first Leslie amp

[Rhodesplyr]

I got into a discussion of the design of the first Leslie amplifier, the Model 30A, on another site, and there are some things about this 1939 design that puzzle me.

Some background: This cabinet used both a field-coil woofer and a field-coil horn driver, and it uses the DCR of the horn's field-coil in series with the ground return of the PT to derive a negative bias voltage that's applied to the 6L6 output tubes through an interstage transformer. It's also got a strange NFB circuit, taking the feedback signal from the high-pass section of the crossover. (The thing that looks like a toggle switch on the schematic is the horn.)

I hate to look at a schematic and not understand what a part does, so I'm wondering what the purpose is of the two 400 Ohm resistors attached to the secondary of the interstage transformer. Any ideas? I've never seen a tube amp circuit quite like this

They used 'M' instead of 'k' for resistor values on the schematic.

[Steve Conner]

Might be to damp resonance between the interstage transformer's self-capacitance and the inductance of the voice coil and crossover. Since the whole circuit is a feedback loop, the phase shift associated with that resonance could make it oscillate.