Trouble with Peavey 6505 (blowing fuses)!

[makinrose]

I'm helping a friend fix his Peavey 6505. It blows the fuse immediately when turned on. I've hooked it up to my trusty light bulb limiter. I removed the power tubes and the fuse does not blow. However, after replacing them with a matched quad the amp still wants to blow a fuse. It is biased for -55V at the bais test point. All the plate voltages seem correct (465v~) without the tubes and all the grids are getting negative voltage. Any ideas as to what is wrong? Thanks!!

[Enzo]

Which fuse is blowing? MAins? High voltage? heaters? Low voltage? And are you replacing it with what the schematic calls for?

And when you say immediate when turned on, do you mean when the main power switch is turned on, or not until the standby switch is turned on too? This is important.

If it holds a fuse OK without the powr tubes, that generally means all the other stuff is OK. Such things as shorted flyback diodes or OTs would not care if tubes were present or not.

SO I am left to wonder if one of your new tubes was also bad.

Tube sockets can arc, but usually they will do it tube or no tube, once they are inclined to arc. But I suppose it is conceivable that a tube in the socket makes it arc. Movement in the pins or some such.

Try this. Of the amp won;t even power up, well we need to rethink, but try just One tube in the end socket. Wil lit fire up and go off standby that way? If so, then standby off, move that tube to the next socket over, and fire back up. Move through all four sockets. Does that single tube work in all four? We want to know by this if the amp can basically function, and if there is something about maybe one of the sockets causing grief. If that all works, go back to socket one. Set aside our test tube - no pun intended - and try one of the other three tubes there. We already know the socket works, if the previous test went well. Then the third tube, and then the fourth. At the end of this, we have tested each socket, and each of the four tubes.

[makinrose]

Thanks for the help! I got it figured out. Stupidly I did not realize the limit bulb would shine brighter with a 100 watt amp. Duh! Power up and now and sound just fine.