by dcgillespie » Mon Oct 01, 2012 6:41 am
Corn -- Something is very wrong at this point, and it does not sound like new output tubes will resolve the problem, either.
You are indicating that there is a 21 volt drop (310 - 289) across your new R12 (50 ohm) resistor. That equates to a current flow of 420 ma -- or 4X to normal amount. This is supported by your (basically) 1 volt reading across the bias test points in each channel. Such a reading equates to 200 ma per channel, or again, 400 ma of total current draw. I'm sure R12 did go out spectacularly! With the EFB modification, this resistor is normally dissipating just under a watt, so a 5 watt device should be more than ample for the job.
This problem is all consistent with no actual bias being provided to the output stage. Therefore, I would determine the cause before operating the amplifier any further. With EFB, this could be because:
1. The EFB circuit is no longer sensing the B+: i.e., the 360K resistor has opened, the bias pot is defective, etc.
2. Either or both caps in the EFB circuit has/have shorted,
3. The regulator itself is shorted, or
4. The old standby, wiring errors.
I have no doubt that the issue lies somewhere in these areas. The only components within the EFB circuit that have any significant dissipation associated with them at all are the 360K resistor, and the regulator. Therefore, I would assume that the issue lies with one of these two components (assuming that there are no wiring errors).
Assuming a 2 watt resistor was used (it normally dissipates .36 watt), as well as adequate heat sinking to properly dissipate the (typically) 1.5 watts that the regulator is dissipating under normal use, then no component failures have been noted with EFB (that I am aware of) when these cautions are observed. In my original EFB installation, the EFB regulator only becomes mildly warm after several hours of use when bolted to the rear of the SCA-35 chassis, and shown in my article. With your rather unique installation, if your regulator is regularly operating so hot that you cannot touch it for long at all, that is a sign that heat stress may have taken its toll on the regulator, and it finally gave up the ghost. If that is the case, you might want to consider an alternate method of heat sinking the regulator better.
I'm sorry you are having the problems you are, but I have no doubt that you will find the issue soon enough.
I hope this helps!
Dave