1 post / 0 new

HD130 biasing question


Tom C
Posted on Friday, February 18, 2005 - 11:44 am:   

According to the service bulletin for this amp, the 6CA7's should be biased to 50 mA (.5v/10 Ohms). Isn't this very excessive for a plate voltage of 725V? Seems like it should be half of that. What gives?
michael kaus
Posted on Saturday, February 19, 2005 - 10:43 am:   

Remember, that's for two tubes, not one. That equates to 25mA per tube. I would say however that even that is a little suspect. I suggest you bias individually. Each tube should be checked on it's own. The MM's were bad about eating tubes anyway at that voltage. You could have one tube reacting at 30mA and the other at 20 and get 50. You need to check each tube so that the hottest tube is around 23mA and that way you are safer. Mike.