High pitched noise 15 watt tube amp Monoprice

[sausetron@yahoo.com]

Hi, does anyone know what this sound is, and how to fix it?  High pitched noise coming from Monoprice 15 watt tube amp.  Things I've tried:  switching guitars, changing rooms, disconnecting reverb tank, changing cables, turning down all the knobs, going directly from guitar to amp.    Thanks!

[Ronquest]

Here's the basic schematic for that amp.  Does the sound continue if you plug your guitar into the FX return?  Also try pulling preamp tubes V1, then V2, then V3. and let us know if it stops and at which tube.  Don't touch the power tubes, they're hot!

[sausetron@yahoo.com]

Hi, my friend replaced one of the tubes after pulling the preamp tubes as you suggested, and it fixed the issue, thank you for your help and suggestions!