Often I am asked which amplifiers will interchange. Below is a list of direct interchange by amp model, but there may be a slight difference in sound quality.

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The C, D, and E amps will interchange with no alterations.
 The F through I will interchange also.

The Continental through K mono amps are all the same as well as the Stereo amps from the same machines.

MRA-1

MRA-2

MRA-3

MRA-4

MRA-5

HFMA-1

HFMA-2

SHFA-1 & SHFA-2 will interchange

SHFA-3, SHFA-4, SHFA-5 will interchange, but 5 is the only amp with a connector for the side speakers found on the DS.

You are really limited here to a few interchanges:

Any 501 and 503 will swap.  In most cases, they will also work in a 700.
The 1800 through 2200 will interchange.

I'll have to do some research on Rockola amps as far as interchange data is concerned, this info will appear in a later column. The 1422 and 1426 amps are the same, the 1428 is unique.