I recently bought another "fixer" and am seeking some helpful information. I am seeking operating voltages for this amp. It has a field-coil speaker and the voltages are quite high. The schematic does not show the voltages. This amp uses a 6SJ7 preamp tube, 6SN7 PI, 2 ea. 6V6's in push/pull configuration and a 5Y3 rectifier. I took my time tracing a layout of the components and man!, what a mess! I chose to place the electrolytic caps on terminal strips I with dropping resistors. Likewise, I moved the PI components to a terminal strip (from the tangled tube sockets). The amp sounds good but I am concerned about the voltages. The Weber bias calculator says my 6V6's are running at 18 watts per tube with 55 ma of plate current. I can upsize the shared cathode resistor (200 ohms/23 vdc) to drop the cathode voltage but I really would like some advice about the voltages that others may be encountering from this beast. Thanks for your help!
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Gibson BR-6F amp
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Originally posted by dkevin View PostI can upsize the shared cathode resistor (200 ohms/23 vdc) to drop the cathode voltage but I really would like some advice about the voltages that others may be encountering from this beast. Thanks for your help!This isn't the future I signed up for.
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I rechecked my bias numbers and found that I had made an error in the size of the cathode resistor...it was 294 ohms...not 200 ohms as the schematic suggested. As I re-ran the numbers, it turns out the watts per output tube is 12W (not 18W) and the plate dissipation per tube is 37Ma (not 55Ma). I am working to minimize a low-grade hum by tightening up the grounding scheme. The interstage caps feeding the 6v6 grids are all new (not-leaking). The amp sounds good and I am ready to reinstall the chassis and enjoy it.
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Thanks for your help! When I received this amp all of the transformer wires were disconnected. The PI and dropping resistors were all mounted on the (unused)tube pins. It made for quite a puzzle trying to imagine the schematic according to the helter skelter array of parts. I chose to re-imagine the circuit with most of the components mounted on adjacent terminal strips. It should be easier to test and repair/replace drifted resistors/caps when the need arises. I will keep your advice about the mismatch hum issues as I continue to experience the amp. As designed, this amp would be (as several of my other Gibson amps were) very difficult to troubleshoot and repair. The tubes are all (5) in a straight line squeezed into a space about the width of an octal socket.
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