I had problems finding info on this, so I'm sharing it with anyone who needs it... After a couple months playing my pf-500 through my 4ohm 210 with no problems, I took it to my band's practice and plugged it into an 810, and it peed it's pants. Turned it on, and the fault light came on, line normal, but didn't turn off. I didn't notice for a couple minutes or so, we were all chatting and hadn't started playing. When I noticed, I shut it off, and switched it back on. Lights came on for a couple seconds, then a small pop sound, and all the lights, except the power switch, went off. No pop through the speakers, it came from the head. No burnt smell either. Sounded like a fuse popping. It was plugged into the same power conditioner as 2 guitar tube heads. Only problem I saw was the 1/4 in speaker cable wasn't fully inserted... So I took it home, popped the top, and pulled the fuse. I don't have a multimeter to test the fuse, so I went on the hunt for a new one. That wasn't easy.... E-mailed ampeg, they said send it in. Its not warrantied, so I wanted to try to fix it before shelling out money to send it in. A few e-mails later, they gave me the part number, but I couldn't just order one from them... I e-mailed the local service department, which is 2 hours away, and they sent me a radio shack page, and the item was sold out. But I got the info I needed. Its a ceramic fuse, 5mm X 20mm 10A 250V - or the part number - FUSE SB 10A/250V 5X20MM UL CSA Service guy said any slow break fuse that size would work, but I thought it would be best to find a ceramic. Well, 2 radio shacks, 3 music stores, 3 electronic stores and I couldn't find one. Found one on eBay, where I should have looked to begin with. So the fuse is headed my way. I wanted to post this in case anyone else has this same problem, because I couldn't find any info on the fuse anywhere. Hopefully this will help someone out there down the line. I'll update when the new fuses get here, which can't be soon enough!