Escalating (in Frequency) Oscillation in Blackstar 100 Head

[thud33]

So I was doing some work on a Blackstar 100 Series One 6L6 head. It is a crazy complicated thing with no schematic available. 
After doing nothing more than cleaning up the contacts on the input jack and setting the bias on the power tubes, I tested the amp, and it sounded great. Then later in the day, I powered it up again, and it began to make a high pitched squealing noise that sounds like an oscillation. Changing out all of the preamp tubes made no difference.
Here are more details on what happens:
1. It happens upon start-up after the amp has been powered off for quite awhile.
2. After the initial tube warm-up on standby, the squealing starts with full power.
3. The high pitched squeal gets higher and higher in pitch until it is inaudible.
4. It then never is audible again during that playing session.
5. The volume of the squeal follows the volume control.
6. The relative volume of the squeal seems to be related to cascading gain in that on the clean channel, it is pretty low relative to the guitar signal. On the highest gain channel, it sounds like a loud teapot whistling.
This amp has five preamp tubes. The PI is solid state.
Any ideas or suggestions? Thanks so much.

[shooter]

sounds like a parasitic oscillation before the volume pot;
if you don't have a scope;
monitor the cathode VDC of each tube before the vol pot, when the squeal happens, you should see instability in the voltage.

[thud33]

shooter, thank you for the recommendation — I will do that.

[pdf64]

Maybe a HT decoupling ecap has developed excessive ESR?
Has the amp been left unused for at least a few months?

[thud33]

pdf64, probably — though the amp’s owner just told me that it has done this in the past.