Bias Circuit Question

[eschertron]

When either there is no input connection ... then the bias voltages go to MAXIMUM NEGATIVE Volts (set by the negative supply) effectively turning the output tube idle currents to MINIMUM.

Does forcing the output tubes into cutoff when the input jack is unplugged have a significant affect on tube life? Does it have any positive affect on the lifetime of the power supply? I'd think allowing B+ to drift high would be a bad thing, but I'm no engineer

I imagine the output would be mighty quiet with the output tubes biased that cold (when the guitar cord is unplugged). Is that part of the design possibly driven by a desire to get cred as a quiet amp?

[Gingertube]

eschertron,
It probably does not cut off the output tubes completely, just reduces their idle currents to a couple of mA each. Combine that with the front end/phase splitter current draw and you probably have a minimum of say 10 to 15mA draw. So B+ will drift a bit higher but most likely not dangerously unless filter caps etc are dangerously close to max voltage spec in normal operation.
As far as output tube life goes it should extend their life.
Cheers,
Ian