Quick tube question

[Tony Allgood]

I have a Crate Blue Voodoo-60 guitar amp. The owner of it has asked me
to swap the two 5881s inside for two EL34s. The manual confirms I
can do this ... the auto-bias circuit can deal with the change. I was
suspicious. But I tried it.. and it oscillated madly on positive output
cycles as soon as you cranked the power up. Any ideas? Sort of makes
sense to me if the tubes have different gains.

Regards,

Tony Allgood  Penrith, Cumbria, UK

Modular synth circuits: www.techrepairs.freeserve.co.uk
My music: www.mp3.com/taklamakan