I just tested my DC30.  Checked everything and all appears fine except for a high pitched squeal at around 12-13kHz that kicks in at around 30% volume on both the 12AX7 and EF86 channels when the master volume is >50% or bypassed.

Although it's hard to tell at the frequency, it sounds like a sine wave and the frequency shifts slightly when the volume level is changed.

Given the nature of this squeal appearing within the preamp stage and the fact that it comes in when increasing the volume on either preamp channel when the master volume is up suggests to me that this is some HF feedback.

Could anyone provide some suggestions on how to track this down and eliminate it?

I'd appreciate any suggestions.

Cheers,

Crusty

Okay, checked tubes by swapping out.  No change.

Did some more trouble shooting and found that the squeal disappears when the cut control is maxed to knock out the higher frequencies.

Explored the wiring from the cut control and tone stack to the EL84s and noticed that moving the wires altered the frequency of the squeal.

Further investigation revealed that the squeal is definitely related to the current going IN to the output transformer.  Moving the wires for the output tranny makes all sorts of horrible squealing noises.

Is this a defective output tranny?

Cheers,

Crusty

Well Done!
If not tubes, it sounded like a lead dress/proximity type problem. Nik is a very knowledgeable and helpful guy. I would be interested in seeing some pictures of your solution........
Good Luck