UL power tubes question

[apndan]

Hello,
anybody knows anything about these tubes and what is the difference between KT 88?

I found this in my Pitbull Ultra Lead with old VHT logo and without graphic EQ. I have it almost two years. I think this is the only UL in my country.

[Van Noord]

Your UL sounds a lot like mine. A 1993 G100 model.
Info I found on the KT90 I found is a follows;

The KT90, or in full, "Kinkless Tetrode 90", is a beam power tetrode and features the same octal socket as its smaller variant, the KT88. It may therefore be used as a substitute, given appropriate re-biasing when used in push-pull configuration.

The KT90 is currently manufactured by Electro-Harmonix, who claim that, despite its different construction, it possesses similar sound characteristics to the EL34 valve.