Help with a Seymour Duncan KTG2100 Power amp

[fusionbear]

I have a strange problem with this Seymour Duncan stereo KT88 power amp. If I check each channel individually, they work great. The moment I test them together, I get a loud oscillation that sounds horrible. I thought it might be the preamp tubes, but I changed them all out as well as the power tubes. Any ideas?

Does anybody have a schematic they could send me?

Thanks...