Timely thread as I recently experienced similar problems with mine. I am not a tube amp expert and the extent of my capabilities is changing tubes and biasing.
I purchased the amp new last year online from the Chicago Music Exchange. The problems for me starting with some ringing noises while playing clean or gain channel a few months ago. Others didn't notice as much, but I did. I figured it was a microphonic preamp tube or it just needed to be re-tubed. Since it was a year old, I figured it was past due anyway and the "factory tubes." I have been messing around with modelers lately and put off retubing. Two weeks ago, I played it and the gain channel was sounding as you described above.
Again, I am no amp tech and not the smartest dude. So I tend to ask a lot of questions and follow directions to a fault since I don't know what to troubleshoot if there is a problem.
I contacted Eurotubes (I have purchased tubes from them for several different amps). Since the documentation for this amp is limited and what is available sucks, I contacted PRS for anything they could provide (which is limited to which tube type). No diagrams, no bias info, nothing. They did provide through the support email the bias range.
I ordered the following:
QTY PRODUCT
================================
1 5881 Matched Pair (5881-2)
5 ECC83S Standard (ECC83S)
1 ECC83S Balanced (ECC83S-B)
Yesterday I swapped out all the tubes. Before rebiasing the amp, I left it on standby for about 30 mins. Plugged in my guitar just to check it out- ringing noise had gone away and but now both channels had the noise. I went back through my check list and realized I had not cleaned the tube sockets as they go on about here:
http://www.eurotubes.com/store/pc/eurotubes faq.htm#x::351:32Tube:32Changing
This next tip is the MOST important of all. Whether your amp is old or new you should always clean the tube sockets before installing new tubes and we HIGHLY recommend doing this! Failure to do this can and often will cause you to burn out tubes and here is why. The pins on tubes are all different just like a fingerprint so when you remove an old tube and replace it with another tube the pins will contact the socket collars in a slightly different location. Cleaning the sockets can easily be done by spraying electrical contact cleaner on the pins of one of your new tubes, and working the tube in and out of each socket a few times. Do this for both preamp and power tubes. If you do not have any contact cleaner, then at the very least you should work each new tube in and out of it's socket a few times to help scrape in a decent connection but if you do this then watch the power tubes closely! Just running them in and out without using contact cleaner will NOT insure good contacts!
Anyway, I pulled all the tubes, cleaned the tube sockets and put the new tubes back in. Turned it back on and no strange noises. I let it sit for about 30 min on standby and set the bias. Played it about 2 hours last night and no problems at all.
Also, forgot to mention. When I pulled the "factory" tubes, there was a mix of ECC83s and had 6l6's in it (one larger than the other). Not sure what to make of that. Having a hard time believing it was shipped that way from the factory, but who knows. I can think of a number of scenarios in which CME changed out tubes (shipping damage, etc). I don't want to think it was a floor model or a return....
Hopefully this is helpful. I have no idea what the problem was/is or why it is fixed and now sounds great. Sounds even better than before. My only other option would have been to ship it to PRS for repair since no one that I have local is authorized to work on them. My only rant is the documentation for the Archon series sucks and pathetically non-existant for the Archon 25.
