Was recording a choir festival. Equipment was as follows: pair of RODE NT5s (XY) to ART MPA ii digital to a Tascam 750 CD recorder and Tascam SS-R1 CF card recorder. Both running through the analog outputs of the ART.
Issue was intermittent losses of power on the left channel. At any given moment, the left channel would lose about 50% power for a few seconds to a few minutes. As the day wore on, the audio drop would last longer and longer until the last few recordings had the loss in power throughout. I replaced the tubes about a year ago. I've had a similar issue before where the internal connection at the "stereo/dual" had worked itself loose. That doesn't seem to be the issue this time.
For those interested in the settings used: both gains set to about 2 o'clock, master set to +2, set to stereo, impedence at about 600, high plate voltage, +20 gain, phantom power on, low cut filter on both channels set to about 60hz audio level selector on the back set to +4.
Am I looking at a tube going out? Any other ideas?