I've been driven crazy with bad done over the past week or so. There has been a bout of wet weather so I blamed the humidity. I ran hot air against the speakers in the 4x12 cab, and put a bunch of silica gel dessicant inside the thing. I cleaned jacks. I cleaned tube sockets in the ADA MP-1. I repositioned the cab in the room. But I did the obvious last: try switching to the other channel on the power amp. Aha!
What is the perceived problem? Something harsh and objectionable in the tone in distorted programs, and a stiff lack of touch sensitivity on cleans, which are much more fluid and silky through the other channel.
This morning I cracked open the power amp, and checked the quiescent current in that channel based on the voltage on one of the 0R22 emitter resistors (with the amp completely unloaded: no speaker or resistor). It was around 27 mA. What? I had set it to 5 mA only weeks ago. How can it drift so far, and in such a short time? This is a sign of some trouble. I also noticed some quite marked instability. On power up, the voltage was 6.1 mV on the resistor, soon dropping to around 5.5 mV in less than a minute. Change with temperature is expected, but I don't remember it jumping by such a percentage in a short time.
I haven't had time to do any more investigating than that, so far.
Maybe a driver transistor is going south?
There is no glaring problem with the amp, like pronounced distortion. It reproduces sound and plays loud.
I'm not going to have time in the next few days, but I will run some signal through it into a load resistor and scope it.
It's nice to have a second channel for backup and comparison.
What is the perceived problem? Something harsh and objectionable in the tone in distorted programs, and a stiff lack of touch sensitivity on cleans, which are much more fluid and silky through the other channel.
This morning I cracked open the power amp, and checked the quiescent current in that channel based on the voltage on one of the 0R22 emitter resistors (with the amp completely unloaded: no speaker or resistor). It was around 27 mA. What? I had set it to 5 mA only weeks ago. How can it drift so far, and in such a short time? This is a sign of some trouble. I also noticed some quite marked instability. On power up, the voltage was 6.1 mV on the resistor, soon dropping to around 5.5 mV in less than a minute. Change with temperature is expected, but I don't remember it jumping by such a percentage in a short time.
I haven't had time to do any more investigating than that, so far.
Maybe a driver transistor is going south?
There is no glaring problem with the amp, like pronounced distortion. It reproduces sound and plays loud.
I'm not going to have time in the next few days, but I will run some signal through it into a load resistor and scope it.
It's nice to have a second channel for backup and comparison.