Kingtone SOLOIST squeals except when BOSS pedal is in front of it

[pete_g]

I built a SOLOIST pedal, but when I turn the tone up full and the volume is about 12 o'clock, it's fine. If I turn the volume to full and turn the tone up to full it squeaks with a high pitch. When I place a boss pedal in front of the SOLOIST and both the volume and tone are on full, there's no squeal. What could be causing this? Even if there's no lead plugged in the input of the soloist on its own, I can still hear the high squeal.

[anotherjim]

How is it built? Tagboard? PCB? Is it the Aion FX one?
Basically, it's an unbuffered Tube Screamer and there's a chance of squealing at high gain via positive feedback stray coupling between parts & wires.
A Boss pedal on the input is a buffered low-impedance source which will tend to dampen out the stray feedback at the input.

[pete_g]

It's the Kingtone Soloist. It's Perfboard, but I had to cross some wires over each other cause I built it on a small board. The wires are shielded though. If I build a simple buffer should that fix it? It won't change the tone at all will it? It doesn't sound different to my ears when the BOSS is in front of it

[anotherjim]

Adding the buffer inside might fix it - but it's still in the same box.
How the footswitch is wired can matter. With a 3PDT, put the LED switching in the middle row. That's often enough to break the stray coupling path between in and out.
This Aion schematic may help...
https://aionfx.com/app/files/schematics/king-tone-soloist-trace-schematic.pdf

If the circuit can be heard squealing in the background when in bypass, the switch wiring that also grounds the circuit input can fix that.

[pete_g]

A simple jet buffer fixed the problem. Learnt something new and hopefully if others read it they can too. Thanks