Recently having some issues with both my vox ac4's 1. my ac4tv is having an issue that after about 10-15 minutes it gets this quiet ugly distortion sound no matter what the volume is or if it is in 1/4 watt mode or 4 watt mode. If I turn it off for 10 seconds and then back on the sound goes away for another 10-15 minutes more or less. I replaced the pre-amp tube so far but the shop didn't have an el84 and I am 14 hrs away from anyone who will repair amps. So I have ordered an el84 to give that a shot. 2. the second ac4 I have is the recent handwired edition. This one has some sort of volume issue after a bit of playing it sort of fades to a thinner and quiter sound without distorting and then back in again. I have swapped the preamp tube on this one as well. I am hoping that this a tube issue but maybe that is wishful thinking for an easy solution. Any suggestions?