Hot Rod Deluxe Bias Ramping up ... all by itself.

[nosaj]

I like the AM radio suggestion. The weird thing is...the HRD sounds fine with a guitar plugged in. No fog. No crazy overtones or distortion. It's bias current escalates. Having said that, I will see if there is an AM radio in the house.

That's the reason I suggested it . It's cheap its possible someone you know has one and will let it go from free. Even a handheld transistor one will be fine. They can also be used to find interference around the house, like if a flourescent light ballast is going bad or some other appliance is causing electronic hash.

nosaj

[christarak]

I am sorry I took so long to get back. I have found the problem. As an industrial instrument engineer one of the questions I ask myself, is "who checks the checker", ie how good is our calibration/measurement tool. In the case of the drifting bias current, I believe with 90% certainty that the problem was in my bias probe for measuring the Cathode current. I changed back to my old Weber BiasRite (which I can only calibrate to read current correctly, not V&I at the same time), and the cathode current was rock solid. I didn't know whether to be furious, or smile and put this one down to learning.

I would like to say a HUGE THANK YOU to everybody who contributed to the fault diagnosis. I am humbled, and have learned so much about diagnosing amp issues by following each of your suggestions. Brilliant. Thank you.