An unexpected and unwanted road trip has come up and preparing for it has been consuming my thoughts and my time. I must leave in 48 hours and will be gone for a week. Consequently I cannot pursue things with this amplifier untill I return.
At present, all 8 output transistors are removed - 4 from the left channel and 4 from the right. With absolutely no output transistors in place, the DBT glows steadily when the variac reaches about 50 volts.
Jumper between C and E of Q404 is still in place.
If it will be of any benefit, I am prepared to reinstall whatever output transistors I am advised to and resume voltage checks with variac at full power and DBT in place. I suspect this will cause the DBT to glow at it's maximum brightness. And perhaps blow the fuses.
Regarding normal background current draw.
Before the bias pot let go and caused whatever damage has occurred, the DBT went dark immediately after amplifier power was switched on. At idle, there did not seem to be enough current draw to make the DBT produce any visible glow. At least not under normal room lighting. In a dark room, there may have been a perceptible, feeble glow but I did not observe it under that condition.
If there are any further suggestions, I will gratefully accept them but cannot act on any until I get back home. I appreciate the patience.
At present, all 8 output transistors are removed - 4 from the left channel and 4 from the right. With absolutely no output transistors in place, the DBT glows steadily when the variac reaches about 50 volts.
Jumper between C and E of Q404 is still in place.
If it will be of any benefit, I am prepared to reinstall whatever output transistors I am advised to and resume voltage checks with variac at full power and DBT in place. I suspect this will cause the DBT to glow at it's maximum brightness. And perhaps blow the fuses.
Regarding normal background current draw.
Before the bias pot let go and caused whatever damage has occurred, the DBT went dark immediately after amplifier power was switched on. At idle, there did not seem to be enough current draw to make the DBT produce any visible glow. At least not under normal room lighting. In a dark room, there may have been a perceptible, feeble glow but I did not observe it under that condition.
If there are any further suggestions, I will gratefully accept them but cannot act on any until I get back home. I appreciate the patience.
It sounds like something else is amiss to me.
I would look at the voltage on each main rail, the positive and negative rails which would go to the collector of the output transistors. Each rail should be equal and opposite (polarity wise). See what voltage each comes up to when the bulb is bright.
Do not fit the outputs, no point at this stage and it might confuse things.
Perhaps the original faults have taken a diode out in a bridge rectifier
That suggests something has happened beside a normal output stage issue. Check the rail voltages first with the bulb lit and report back what you measure.
I returned from my road trip yesterday. I got some very sad news at my destination so it was not a pleasant one.
I am hesitant to take that action for fear of causing gross damage from sustained current draw. As mentioned, the DBT (40 watt) is fairly bright when the variac is at approximately 50V. But it is not at it's brightest.
How bright should the DBT be? Or, at what voltage should the variac be at to measure voltages at those output transistor collector connection points?
Should I increase the voltage past 50 volts before making the measurements?
I am hesitant to take that action for fear of causing gross damage from sustained current draw. As mentioned, the DBT (40 watt) is fairly bright when the variac is at approximately 50V. But it is not at it's brightest.
How bright should the DBT be? Or, at what voltage should the variac be at to measure voltages at those output transistor collector connection points?
Should I increase the voltage past 50 volts before making the measurements?
I'm sorry to hear that news 
The DBT is there to protect. If a diode in the bridge is failed for example then nothing bad will happen, if the output stage were conducting (if the transistors were fitted) then again nothing bad happens because the current is easily within the capabilities of the transistors.
Take it to the point the bulb stops increasing in brightness and/or the point the rails stop increasing. A 40w bulb is pretty low in the scheme of things, a 60 or 100 would be more the thing to use for a power amp... just don't overthink it at this point, just try and get a handle on what is going on and where the current flow is occurring.
The DBT is there to protect. If a diode in the bridge is failed for example then nothing bad will happen, if the output stage were conducting (if the transistors were fitted) then again nothing bad happens because the current is easily within the capabilities of the transistors.
Take it to the point the bulb stops increasing in brightness and/or the point the rails stop increasing. A 40w bulb is pretty low in the scheme of things, a 60 or 100 would be more the thing to use for a power amp... just don't overthink it at this point, just try and get a handle on what is going on and where the current flow is occurring.
I now understood what was shorting, causing DBT to glow.
It was collectors of the output transistor traces shorting to ground. My fault for not seeing it before but it hit me when I was about to try voltage checks again with a 100W DBT. I eliminated the shorts (there were several) and now DBT behaves properly - goes dim immeduiately after amp is turned on.
Output transistors in this amp are TO220 so a metal tab as collector. There is a screw that anchors the transistor and it is those screws that attaches the PC board to the heat sink. When I removed the output transistors, I replaced the screws so that the heatsink and PC board remained coupled. Without an insulator behind the TO220's, collector traces and metal heat sink were in direct contact.
So no screws in place and the DBT is happy.
Since I now know why the DBT was staying bright and since it no longer does, should I begin replacing the output transistors? I would install them one at a time and check the DBT after each one was put into place.
It was collectors of the output transistor traces shorting to ground. My fault for not seeing it before but it hit me when I was about to try voltage checks again with a 100W DBT. I eliminated the shorts (there were several) and now DBT behaves properly - goes dim immeduiately after amp is turned on.
Output transistors in this amp are TO220 so a metal tab as collector. There is a screw that anchors the transistor and it is those screws that attaches the PC board to the heat sink. When I removed the output transistors, I replaced the screws so that the heatsink and PC board remained coupled. Without an insulator behind the TO220's, collector traces and metal heat sink were in direct contact.
So no screws in place and the DBT is happy.
Since I now know why the DBT was staying bright and since it no longer does, should I begin replacing the output transistors? I would install them one at a time and check the DBT after each one was put into place.