Dave,
Am loving the amp "as is" so far but caught a 7189A glowing slightly red today. FWIW, I haven't dismissed your EFB mod, I just want to get to know this amp in it's stock form before I go and change things.
Inspired by your work, I separated the two (Channel A and B) test points into four so that I may check each individual tube's bias as opposed to checking pairs (something you thought Fisher should have done in the first place.) As with the original design, jumpers are placed from each test point to ground during operation thus bypassing and removing the resistors from circuit. (I state the obvious and created a new thread for those who may have experienced a red plate and would like to protect from damage).
I checked the individual tubes and of course found that the tube in question was drawing more current (close to 70ma), while sucking current away from the others. Can't really find a reason why this occurred as the problem seemed to have been corrected as I readjusted the bias and DC balance controls of each channel. Maybe the components are still breaking in, maybe the potentiometers need a good cleaning. For the time being, I am running the amp without jumpers and checking the current and DC balance of each channel often.
I noticed that in your version of separating the test points, no jumper is used to bypass the 10 ohm resistors during operation. I am currently using 1 watt resistors and am wondering if reducing their power rating to 1/2 or 1/4 watt and leaving the jumpers off would afford any protection should a tube begin to draw too much current? Would one resistor "pop" and if so, would there be any consequence to the other tubes? As both channels share the same bias supply, I've noticed that decreasing the bias of one channel increases that of the other. If one channel were to shut down, would your EFB design maintain regulation for the other?
I don't think I have a bad tube as the problem hasn't recurred. The overall bias seems stable enough, drifting only very slightly. I have less than 25 hours on the amp, so I think it's still settling in.
What do you think? I am particularly interested in modes of failure and their consequences.
Rich
Am loving the amp "as is" so far but caught a 7189A glowing slightly red today. FWIW, I haven't dismissed your EFB mod, I just want to get to know this amp in it's stock form before I go and change things.
Inspired by your work, I separated the two (Channel A and B) test points into four so that I may check each individual tube's bias as opposed to checking pairs (something you thought Fisher should have done in the first place.) As with the original design, jumpers are placed from each test point to ground during operation thus bypassing and removing the resistors from circuit. (I state the obvious and created a new thread for those who may have experienced a red plate and would like to protect from damage).
I checked the individual tubes and of course found that the tube in question was drawing more current (close to 70ma), while sucking current away from the others. Can't really find a reason why this occurred as the problem seemed to have been corrected as I readjusted the bias and DC balance controls of each channel. Maybe the components are still breaking in, maybe the potentiometers need a good cleaning. For the time being, I am running the amp without jumpers and checking the current and DC balance of each channel often.
I noticed that in your version of separating the test points, no jumper is used to bypass the 10 ohm resistors during operation. I am currently using 1 watt resistors and am wondering if reducing their power rating to 1/2 or 1/4 watt and leaving the jumpers off would afford any protection should a tube begin to draw too much current? Would one resistor "pop" and if so, would there be any consequence to the other tubes? As both channels share the same bias supply, I've noticed that decreasing the bias of one channel increases that of the other. If one channel were to shut down, would your EFB design maintain regulation for the other?
I don't think I have a bad tube as the problem hasn't recurred. The overall bias seems stable enough, drifting only very slightly. I have less than 25 hours on the amp, so I think it's still settling in.
What do you think? I am particularly interested in modes of failure and their consequences.
Rich