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Brook 12A bass response?

I'm working on a Brook 12A 2A3 amp for a customer. This one is in rough condition, not a museum-piece, so there is not a lot of concern about absolute originality. It's been previously modified and monkeyed with.

In strategizing the project, I'm noticing some high-pass filters, one at the input (0.01uF into 470k) and a 1000pF mica capacitor into the bootstrapped 6SN7 cathodyne phase inverter. Now, I know that a bootstrapped cathodyne PI has a very high input impedance, but even calculating it at 10Meg, you get a 16Hz high-pass node.

Not having heard a Brook 12A before, was this amp purposefully bandwidth-limited on the low end for the speakers and musical signals of the 1940s?

On another site, someone working on a 12A clone omitted the input capacitor and bumped the 1000pF coupling cap (C9) to the PI to 0.1uF. And, in fact, on this unit, someone changed the input wiring to bypass the 0.01uF capacitor to the first stage, making it DC coupled at the input.

I was thinking of omitting the input capacitor and perhaps increasing the PI input capacitor to 0.01uF or 0.022uF. Or is it the case that this amp doesn't need the bandwidth extended for late 20th/21st century recordings?


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Topic - Brook 12A bass response? - Thermionic27609 14:35:46 03/12/17 (16)

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