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In Reply to: RE: Brook 12A bass response? posted by briney on March 12, 2017 at 18:55:50
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.
For some reason I thought you had a working 12a. Good luck with your rebuild.
Thanks. It should work when I'm done. I tested the OPT in a few ways including running a signal into the secondary and checking the primary for any distorted or unbalanced waveforms, plus a HV leakage test using a condenser checker as the supply. With amps this old, you never want to get to the end of a repair only to turn it on and find that the iron is bad.
With point-to-point circuits, I find that it's best to think through any possible mods or updates before you start soldering components to terminal strips.
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