Rectoverb Solo Button Problem

[vincidious]

I have a Rectoverb Series II. I was playing last night and after a while the solo button on my footswitch stopped working. The only thing that's changed recently is that I put in a set of EL34s.

Everything else works fine. I'll check the cable and replace the power tubes, but if this has happened to others, I'd like to hear your ideas.

Thanks.