Please pardon my ignorance when it comes to obvious questions but I figured it's better to ask than remain a dummy! I had some friends over on Saturday and found out that two of my trusty amps were behaving badly. My Silvertone 1484 head would make a loud blaring noise that would go away every so often. I turned it off, much to my one friend's dismay .. he loves that amp. Then my MiM Hot Rod Deville 2x12 started making what sounds like a tuning fork tone while at rest. From all the research I've been able to do over the weekend it seems to me that both amps may have microphonic preamp tubes? The Silvertone takes four 12AX7's (it also has two 6GC7's?) and the HR Deville takes three 12AX7's. My questions are, do I have to replace all the preamp tubes at once or can I replace just the bad ones (providing I can find them) and do I have to have the amps rebiased (I think the MiM can be since it has an internal variable bias control?) or is that only necessary when you replace power tubes? Will any 12AX7's do or should I be looking for specific ones for those two amps? Thanks in advance for helping a tube dummy out! -R.