I've got a 5150 III 50w and a 100w Stealth. I run a Fortin Zuul off the key input connected to the guitar signal with the gate in the effects loop.
It's really, really quiet when gated, even if I have a mountain of gain on the red channel or an HM2 clone before the preamp dimed. When the gate is working you should only hear a tiny bit of noise from the power amp. My understanding is that the power amp is essentially on full for these amps from when you power up the amp and all volume control comes from the strength of the signal from the prramp. It shouldn't matter which channel/how much gain/volume you are running if your gate is working properly off the guitar signal.
The power amp doesn't care which channel you are using, so a loud red channel while gated would be indicative of the gate not being able to kill the signal from the preamp (like others have said above), or possibly an issue with the amp. If you can borrow another gate to rule out the possibility of a gate issue I'd give that a try. I'm assuming you have played with the threshold and other settings on the gate, and that you can hear an audible reduction in noise when the gate is engaged on the blue channel. If the blue channel is about the same whether the Sentry is on or not, you might not have the proper settings on the Sentry to get the gate to do its thing.
It's really, really quiet when gated, even if I have a mountain of gain on the red channel or an HM2 clone before the preamp dimed. When the gate is working you should only hear a tiny bit of noise from the power amp. My understanding is that the power amp is essentially on full for these amps from when you power up the amp and all volume control comes from the strength of the signal from the prramp. It shouldn't matter which channel/how much gain/volume you are running if your gate is working properly off the guitar signal.
The power amp doesn't care which channel you are using, so a loud red channel while gated would be indicative of the gate not being able to kill the signal from the preamp (like others have said above), or possibly an issue with the amp. If you can borrow another gate to rule out the possibility of a gate issue I'd give that a try. I'm assuming you have played with the threshold and other settings on the gate, and that you can hear an audible reduction in noise when the gate is engaged on the blue channel. If the blue channel is about the same whether the Sentry is on or not, you might not have the proper settings on the Sentry to get the gate to do its thing.