Mesa Subway Blues Hum

[AtomicMassUnit]

This am develops a severe hum after 30-45 minutes of play time. It is an early model with the separate inputs. I have a schematic, attached below, and a service mod that is supposed to fix the problem. There is a .01 cap connected to a "potted module" inside the amp. This cap is supposed to be changed to .68uf. I haven't been able to find one locally, and will be ordering one, but in the mean time, what is the reasoning behind this fix? Is this cap the one coming out of the preamp after the reverb? Could I try a .15 (largest value I have on hand), or even two together (.3uf) and see if that would correct the problem? That seems like a huge increase in value for the part.

[booj]

My first guss is that our output tubes are running hot. Check in a dark room when the amp starts to hum and see if there is an orange glow coming from the pates. I'm working on a Subway Rocket that had that issue. I modified the bias circuit.

[AtomicMassUnit]

My first guss is that our output tubes are running hot. Check in a dark room when the amp starts to hum and see if there is an orange glow coming from the pates. I'm working on a Subway Rocket that had that issue. I modified the bias circuit.

Thank you! It ended up being that .1 cap sub that fixed it. edit* .6uf.