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Classic gear from yesteryear; vintage audio standing the test of time.

RE: Brook 12A bass response?

The 1000pF coupling capacitors are original, square mica.

The tapped driver plate choke was missing, and someone appears to have started to modify the circuit to get around its absence, but gave up and left things disconnected. I've sourced a replacement plate choke from Heyboer to put it back to to a more original configuration. The wax-dipped paper caps have to go, obviously, unless someone just wants to put it on an altar and worship it.

That said, I'm not sure how that's relevant to the question. It came to me in non-functional condition. I've tested the power and output transformers for faults, and I'm mapping out the restoration. I can calculate the high-pass filter formed by a capacitor and grid-leak resistor, and it puzzles me that the original circuit appears to have two high-pass points that would seem to me to limit the low frequency response.

I also work on guitar amps, and I've seen guitar amps from the 40s and 50s that have the same sort of bandwidth-limiting high-pass filters. In that application, it's often a good idea.

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