After a break, I am back on this. I have some new information, which some might be able to help with?
The primary of the output transformer (lifted from C4, not in circuit), has an impedance of 36KΩ @ 1kHz, when the secondary is loaded by a 1k resistor. This was measured with multiple techniques including the first method described on
this page. Incidentally, the DC resistance of the primary is about 650Ω.
Reiterating, The secondary impedance is still 1690Ω at 1kHz, 171Ωdcr with the primary connected to amplifier circuit.
The transformer has a label beneath the amber insulation. On the label -
"Neumann
Nr.
900 - 0,07
200 - 0,18"
Does this look like any of the standard transformers. I don't see any White windings.
FYI
All passive components are newer, in good shape, and measure accurately, with 3 differences to the schematic. C4 was changed to a 1uF cap, and R2, R6 are 180MΩ. On my barely legible schematic, it appears like R2 and R6 should be 150MΩ? These changes were present when this mic sounded great, and I can't see how they would be involved in lowering the plate voltage (C4 is not leaking any current, measured between 30 and 100vdc).
Again,
Thanks for any insight.
Here is a bad photo of the transformer/mic. I will try to get a better one soon.