@cflayton,
This brings to mind an issue that was discussed in the following thread, in which someone was seeking an explanation as to why changing the input impedance of his amp from 100K to 47K cured a problem in which the amp’s bias would fluctuate wildly:
https://forum.audiogon.com/discussions/why-did-this-fix-my-problem
Some excerpts follow from the posts in that thread by Ralph Karsten of Atma-Sphere. Note the reference to an ARC amp. Also note that the input impedance of your amp is 300K balanced and 150K unbalanced, which is much higher than the input impedance of both the D-150 Ralph mentions and the amp used by the OP in that thread. Everything else being equal the very high input impedance of your amp increases the likelihood that the phenomenon Ralph describes is what was going on when the distortion and power cycling occurred. (In saying that I’m assuming the preamp has coupling capacitors at its outputs; I couldn’t find a schematic for the 12B but schematics I looked at for several other somewhat more recent Bryston preamps do indeed show output coupling capacitors, which in fact have relatively large values such as 47 uF which also increases that likelihood substantially).
If the distortion/power cycling problem should re-appear it would seem to be a good idea to call this possibility (which I think is a very strong possibility) to Bryston’s attention.
Regarding the gain reduction that occurred, I have no thoughts at this point.
Regarding shorting plugs, a search at eBay for "RCA shorting plugs" will turn up a number of inexpensive sources.
Good luck. Regards,
-- Al
This brings to mind an issue that was discussed in the following thread, in which someone was seeking an explanation as to why changing the input impedance of his amp from 100K to 47K cured a problem in which the amp’s bias would fluctuate wildly:
https://forum.audiogon.com/discussions/why-did-this-fix-my-problem
Some excerpts follow from the posts in that thread by Ralph Karsten of Atma-Sphere. Note the reference to an ARC amp. Also note that the input impedance of your amp is 300K balanced and 150K unbalanced, which is much higher than the input impedance of both the D-150 Ralph mentions and the amp used by the OP in that thread. Everything else being equal the very high input impedance of your amp increases the likelihood that the phenomenon Ralph describes is what was going on when the distortion and power cycling occurred. (In saying that I’m assuming the preamp has coupling capacitors at its outputs; I couldn’t find a schematic for the 12B but schematics I looked at for several other somewhat more recent Bryston preamps do indeed show output coupling capacitors, which in fact have relatively large values such as 47 uF which also increases that likelihood substantially).
Atmasphere 8-3-2012
The problem is that there is a power supply instability in the preamp. The output coupling cap, when driving a 100K load, represented a frequency pole that was lower than the frequency pole in the preamp’s power supply.
The result is low frequency instability. With many amps this may not manifest with anything, especially if the amp does not have good LF bandwidth, but I think the interaction occurred due to the fact that you do have enough bandwidth in the amp and the power of the amplifier was able to mess with the AC line voltage, which in turn exacerbated the LF instability of the preamp.
So lowering the input impedance of the amplifier solved the problem by knocking off an octave of LF bandwidth.
IMO/IME there are much better ways to solve this problem. The power supply of the preamp needs to be either repaired, redesigned or perhaps regulated, depending on why it has the instability....
... I first saw something similar occur in the 1970s with an ARC D-150 and an ARC preamp that had a bad regulator in the power supply.
The preamp would make some low frequency noise, the amp would amplify it, the AC line would sag, that caused the preamp to do it again, repeating the cycle.
If the distortion/power cycling problem should re-appear it would seem to be a good idea to call this possibility (which I think is a very strong possibility) to Bryston’s attention.
Regarding the gain reduction that occurred, I have no thoughts at this point.
Regarding shorting plugs, a search at eBay for "RCA shorting plugs" will turn up a number of inexpensive sources.
Good luck. Regards,
-- Al