Gibson BR-6F amp

[dkevin]

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!

[Leo_Gnardo]

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!

Yes bumping up the Rk to 250 or 270 ohms is often a good move. Remember our power companies are sending us commonly upwards of 120V these days, back then 110 to 117 was the norm, so voltages in your old amps often run a bit high. One other thing to be alert for, caps carrying signal to output tube control grids are often leaky in old amps, and that will run your bias current to excess as well. Get a good set of Mallory 150 or Panasonic metallized polypro caps, or orange drops of one sort or another if you prefer, and you may find the bias current settling down to something approaching normal.

[dkevin]

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.

[Leo_Gnardo]

That sounds a whole lot better, glad to hear it's all under control. You might check bias current on each output tube separately, if they're out of balance with each other you'll have a hum you can't beat any other way.

[dkevin]

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.