So until I pick up a second cab, I plan to use only one channel of the 2:90. Is it safe to use the 2:90 with no load connected to the second channel ? I assume it is, but want be sure before I try it and fry something.
If thi si sthe 4-6L6 rack power amp I had one, and yes it's perfectly fine to use one channel, you just need to be sure to turn the volume off. I ran a Quinn SDO preamp into this amp and had no issues.
The manual is confusing. It sounds like you can. But call mesa first
These two pots control the volume level of each side or channel of the 2:90. Since there's
only one Standby Switch on this unit they may also be useful as channel muting controls.
NOTE: If only one speaker is to be used on one channel and no speaker load on the other,
always turn the unused channel’s Level Control to zero. To prevent oscillation (motorboating)
turn the Presence Control of that channel all the way up"
You got lucky. Most manufacturers, including MESA recommend a shorting jack. Some tube power amps have a separate HT power switch, that works too...