Thursday, March 15, 2012
Biasing A Kustom TRT100
I had one of these land on my doorstep recently. It's a tube head, made in China, allegedly 100w and it came to me completely dead. A bit of inspection revealed that two of the 4 1k grid resistors were smoked and the associated diodes were iffy, and three of the four power tubes were more or less defunct.
After repairing the power tube board and replacing the grid resistors, sense resistors, B+ fuse and diodes, I came up with a set of matched tubes. The plate voltage registered an energetic 490v more or less. But how to set the bias? My usual bias adapter disagreed with the circuit and caused one of the grid resistors to burp and backfire, which caused all sorts of problems with me not seeing it when it died.
Once the offending resistor was replaced it became obvious that R218 and R219, being 1 ohm resistors must be in the cathode string, one for each pair of tubes. Measuring across the resistors in turn gave us 72 mv, which makes it 36 ma per tube. It's easily adjustable with the handy pot provided.