Marty (Viridian), you may find it helpful to take a look at the biasing procedure described in the manual for this amp. I would think that the wildly fluctuating meter reading is most likely a true indication that the bias is wildly fluctuating.
Rluthy, it's hard for me to envision how a speaker defect could be causing this problem. Bias current is DC, which flows on the primary side of the output transformers, while the speakers are of course connected to the secondary side. And transformers don't pass DC, aside possibly from extremely miniscule amounts of leakage. And in any event it sounds like the speakers don't have any defects, based on their performance when the amp is in triode mode.
My speculative guess is that some sort of oscillation is occurring, involving an interaction between the amp and either the speakers or the preamp. An oscillation involves a path that is a closed loop, and I'm guessing that part of that loop may include the effects of the AC current draw of either the Zeus amp or the speaker's bass amp on the AC line voltage being seen by either the other of those two components or the preamp. Some thoughts and questions:
1)Do the woofers appear to be moving when the problem occurs?
2)Are you following the manual's instruction to turn off the preamp when doing the adjustments?
3)Even though the manual says it's ok to change from triode mode to UL mode while the amp is powered up, have you tried changing into UL mode while the amp is turned off, and then turning the amp on and observing the bias meters?
4)Try plugging the speakers into a different AC branch than the one the amp and the rest of the system are plugged into, using a long extension cord if necessary.
5)Beyond that, all that occurs to me to suggest at this point is to go to Radio Shack and pick up either a couple of cheap speaker drivers, or alternatively a pair of 8 ohm resistors rated to handle substantial power levels (I think they sell one rated for 20 watts), and seeing what happens with those connected in place of the Summits. Although if the bias readings do not fluctuate with those dummy loads in place, per my comments above it would not necessarily be indicative of a speaker defect. My suspicion in that case would be that some kind of interaction between the amp and the speakers is resulting in an oscillation.
Good luck. Regards,
-- Al
Rluthy, it's hard for me to envision how a speaker defect could be causing this problem. Bias current is DC, which flows on the primary side of the output transformers, while the speakers are of course connected to the secondary side. And transformers don't pass DC, aside possibly from extremely miniscule amounts of leakage. And in any event it sounds like the speakers don't have any defects, based on their performance when the amp is in triode mode.
My speculative guess is that some sort of oscillation is occurring, involving an interaction between the amp and either the speakers or the preamp. An oscillation involves a path that is a closed loop, and I'm guessing that part of that loop may include the effects of the AC current draw of either the Zeus amp or the speaker's bass amp on the AC line voltage being seen by either the other of those two components or the preamp. Some thoughts and questions:
1)Do the woofers appear to be moving when the problem occurs?
2)Are you following the manual's instruction to turn off the preamp when doing the adjustments?
3)Even though the manual says it's ok to change from triode mode to UL mode while the amp is powered up, have you tried changing into UL mode while the amp is turned off, and then turning the amp on and observing the bias meters?
4)Try plugging the speakers into a different AC branch than the one the amp and the rest of the system are plugged into, using a long extension cord if necessary.
5)Beyond that, all that occurs to me to suggest at this point is to go to Radio Shack and pick up either a couple of cheap speaker drivers, or alternatively a pair of 8 ohm resistors rated to handle substantial power levels (I think they sell one rated for 20 watts), and seeing what happens with those connected in place of the Summits. Although if the bias readings do not fluctuate with those dummy loads in place, per my comments above it would not necessarily be indicative of a speaker defect. My suspicion in that case would be that some kind of interaction between the amp and the speakers is resulting in an oscillation.
Good luck. Regards,
-- Al