What is really going on in 10W mode?

[MusicManJP6]

90W uses all 4 tubes, and 45W uses the inner tubes. I can see this clearly in a dark room when switching modes. 10W mode does something strange. I see it switch to the left three tubes (third tube from left is just barely lit up blue). I assume it is using the Rectifier tube as well in this mode. Does anyone know exactly what is going on here? I'm curious.

 

[Scary]

I read somewhere (possibly the manual) that it uses one tube for power, and another to offset the transformer somehow. I guess in theory it's wrong but somehow it works.

 

[MusicManJP6]

Then why is a third tube lighting up slightly?

 

[litesnsirens]

It's using two tubes in single end class A and the reictifier tube... explains it in the owner's manual page 31. And it's the two tubes next to the recitifier tube that the amp uses for the single ended class A. So that would be the three tubes that you are seeing lit up all next to each other.

 

[Boogiebabies]

From the schematic, it shows the left pair or V7 and V9 is switched to triode. Each tube also gets a 1.6K 25 Watt resistor place between the plates
and the OT's primaries to lower the output and the ground reference is raised with two 370 Ohm 7W resistors. V8 and V10 are technically on, but there is
a relay on the phase inverter that turns off the signal that rides on the bias to these two tubes. Mesa uses V7 and V9 wired in parallel to look single ended.
They did this so they did not have to make it truly single ended using one output tube and having to fool the output transformer to think it's a gapped core.

 

[rabies]

so the bottom line is it's using two power tubes in 10 watt class A. I was wondering about this myself as supposedly a true class A is single-ended (one power tube operational) and that would mean one of the tubes in the power section pair is being worn out more than the other. But this is apparently not the case as two are running in 10 watts class A.

 

[MusicManJP6]

From the schematic, it shows the left pair or V7 and V9 is switched to triode. Each tube also gets a 1.6K 25 Watt resistor place between the plates
and the OT's primaries to lower the output and the ground reference is raised with two 370 Ohm 7W resistors. V8 and V10 are technically on, but there is
a relay on the phase inverter that turns off the signal that rides on the bias to these two tubes. Mesa uses V7 and V9 wired in parallel to look single ended.
They did this so they did not have to make it truly single ended using one output tube and having to fool the output transformer to think it's a gapped core.

Holy tech talk, batman. Thanks for the detailed info!