- 2022
- Apr
- 14
1959 Blonder-Tongue Audio Baton (B9) Graphic EQ Rebuild
(See my later post for the Sams folder / schematics for the BT B9.)
A long time ago when eBay was still fairly new, a set of heavily modified Blonder-Tongue Audio Batons showed up for a cheap price. These are the first commercially available graphic equalizer, featuring 9 bands. Tube complement for this device is 5 12AX7 used as amplifiers in the EQ stages, and 1 6X4 rectifier to provide power for the unit. 3 lamps to backlight the adjustments ran off the AC filament transformer.
I’m not sure what was going on with these units, but two of the potentiometer holes were punched out on one of the chassis, and two extra tubes had been installed. Multiple modifications had been made to the electronics, which were in somewhat of a state of decay. Old carbon comp resistors had drifted, capacitors had become leaky, and all of the mods just made a mess of the unit. The other unit didn’t have mods, but the electronics were in the same state of decay.
The first thing I did was remove all the components except for sockets, transformer, and terminal strips. New components were purchased, including orange drops and mylar film capacitors, carbon film resistors, and of course, new wire. Holed punched in the chassis were re-filled with washers of the correct size. Potentiometers were saved where possible and cleaned. The chassis were re-wired and all of the new components were placed. I decided to do my own layout and wound up with a more compact layout than the OEM layout.
The original case, of which I had one, used weird bi-pin lamps to backlight plastic tubes with a candy-cane stripe on it. The stripe appeared in a slot cut in the face, and as you turned the pot the stripe “moved” up the slot. This was re-assembled and, unfortunately since the face had been damaged, I used stick-on labels to identify the bands. The odd number sticker was left on the face. I assume this is from a previous auction.
The second chassis was more of a challenge. Since I didn’t have a case, I settled on a rack chassis from SES-COM, a company that used to manufacture such items but has since transitioned to audio products. More space allowed for some added some amenities such as VU meters using a precision rectifier circuit and a push-on/off control circuit for both units. I replaced tubes as needed, using NOS RCA and other brands as available in my personal stock. A final check, and they worked - probably just as good or better than new.
I was never able to determine what the mods were, save that I’ve seen others reference strange mods where the chassis was punched out and extra tubes added.
In the end, there was more fun had rebuilding these units as opposed to using them. They’re simply graphic EQs that consume a lot of power and have the added noise of a tube circuit. A modern unit works just as well, and has more bands. I ended up relisting them on eBay around 1998 (I think?) and selling them to a studio. That’s something I kind of regret, but as I said - in the end it’s just an old version of a new circuit.
Unfortunately, since this build pre-dated digital cameras, all I have left are these photographs taken with a friend’s Samsung point-n-shoot. They’re not the best, but here they are.
