Gentlemen, in the last week several of you have emailed me about failure conditions if one tube goes out in the hybrid. Most of you were concerned about the PS.
In this situation, there isn't any risk to the PS because the current demand from the tubeless channel is not much lower than a working channel and the additional heat in the PS will not be severe nor damage it.
However, these questions forced me to look at the hybrid amp again just to explore this failure condition more.
Most of you would probably power down quickly if one side of the amp went out, but if the amp is left on with one (or both) tubes not lit then resistors R31 to R34 will exceed their power dissipation. If you want to be safe against losing these resistors, if you don't turn the amp off right away or it is left unattended, the easy fix is to use 1/2W resistors instead of 1/8W.
You can use 71-CCF55-75K or 273-75K-RC. They are longer than the RN55D but they should still fit if you tilt them at an angle.
There are quite a few hybrids out there in service and no one has reported this problem yet, but it will surface when tubes start to die during operation.
I'd like to have caught this a while ago gents, but just didn't see this corner case.
Thanks to everyone who asked the question.