Friday, September 14, 2012

What The Mouse Done Ate: Reviving A Gibson BR6

This Gibson BR6, circa 1947 followed me home a few years ago from an auction in Mount Union, Iowa for forty bucks. When I got it home and opened it up I found a lot of gnawed wire and caps and one very dead mouse. No Bromo Seltzer was gonna  save his skinny ass.

I started the task of rewiring it and then for some reason put it aside until this week. All I really had left to do was to finish the power supply and the phase inverter which took a few hours of thinking how I was going to engineer it. The solution amounted to mounting some of the components on terminal strips. I glued a pair back to back and mounted them on the power transformer studs which is where the phase inverter resistors went..

I had decided to build it as a BR6F circuit, which used a pentode preamp tube, mostly because I like pentodes but there's another reason. The volume control in the original BR6 circuit is ahead of the first preamp tube grid which makes for a noisy and feedback prone amp, because it's running wide open all the time. You have to attenuate the signal with your guitar which cuts out a lot of the tone. Maybe lap steels are different but that layout isn't any good for guitar. I had the same problem on an old Flotatone I have, and in that case rewired it as a 5C3 Deluxe. It solved the noise and feedback problems so I knew it was a good fix. The BR6F is a much improved circuit.

I also installed a 250 ohm cathode resistor bypassed by a 22 uf electrolytic, which qualifies as a modification but is an improvement to my way of thinking. It probably would sound a bit more raw without the capacitor but I'm all about clean tone. It's good the way it is.

The wiring is not as pretty as it could be but remember, I started this project a number of years ago. 

Surprise! It fired right up at the first flick of the switch and now purrs like a kitten, which is a first for me It sounds pretty good within its limits and will make a good addition to the fleet or some trade bait.

I spent a couple of happy hours yesterday putting this amp through its paces and it's the equal, if not the better of any Valco of similar age and configuration. Aside from the Ruby 5AR4 blowing its wad while I was watching, it's a good performer.

1 comment:

  1. Very nice job! FWIW I did the same thing (converted from BR-6 to BR-6F) on a very beat up amp I bought several years ago and also greatly prefer the sound now, as a BR-6 it was high hum and low gain...felt OK about doing this since Gibson did the same and the conversion is completely reversible.