amp problem

i have had the problem for a while and just dealt with it. Whenever i play with my fender princeton 65 amp with my band It starts to ummmmm....fart out on me. It doesnt cut out, it just gets sounding all wierd when i play low notes. I have a zoom 505 and at first i thought it might be that, but i still cant figure it out. Thanks for the help.

Have a tech check it out. I suspect the power tubes need re-biasing.

Thanks