This is the same '79 Champ I started a NAD thread on last week. I played it just about wide open at the shop I bought it from and at around 1 or 2 at the condo while on vacation (to keep the peace mainly). There was no weird noise at any of those times.
I got back home today from vacation and immediately went to jam on my new toy. The 6v6 power tube had somehow fallen from the socket in transit; I'd noticed that the tube sockets were a little loose before. It looked fine, so I put it back in and played the amp. Now I am getting what sounds like a secondary fuzz tone that is about 1/8th or so the volume of the main amp signal (no pedals, straight into amp); that is, it sounds like a separate signal that is generating definite harmonic content like a fuzz would. Other than this, the amp sounds like I imagine it should (no other glaring problems).
Has anyone had experience with this and knows what it might be? Could the power tube be causing this somehow due to damage? I'm not sure if it matters, but the tube is a Raytheon 6v6, labeled: 280 548.
I can get in there and take some pics if needed.
I have repaired three SF Champs of late that ALL had this issue.
In all three cases, it was the speaker.
One good jolt and those old 8" give it up especially if they have been dormant awhile.