Monday, October 26, 2015

Biasing the Albion TGT-50

An Albion TGT50 head came my way for some routine service and it occurred to me that it would be worth checking the output tube bias to make sure it was reasonably close to what it should be.

Of course I couldn't find any service information readily to hand. So I opened the small hatch on the rear of the chassis and saw what looked pretty close to something I've seen on Marshalls and on a Randall head I used to own-a pair of potentiometers and three pins. I then removed the chassis for a general lookover and it seemed like a pretty conventional 50w channel switching head. Nothing was remarkable except that the tube sockets are chassis mounted and wired to the board.

A nice touch, and one that makes service people happy, if you followed my adventures with hard wiring a Blackstar 100w head recently because the board had burned up.

After using my bias probes and measuring the plate voltage at about 470v DC I adjusted the pots and got the results I needed. I then went back to the original setup, set my multimeter to millivolts and measured the left and right pins with the ground probe to the center pin.

The results were quite similar which led me to believe that the designer of this amp, Steve Grindrod,  used the same setup that Marshall used-not surprisingly when you figure that he worked for Marshall for a while. I figure I was reading across a 1 ohm resistor in the cathode string for each power tube.

Or, of course, you can take the better path and check bias with a pair of probes and a couple of dedicated multimeters. It'll give you a chance to look around inside while you're visiting.

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