newbie bf champ blown fuse, cracked resistor

[sheppadizzle]

built aa764 circuit according to tino zattola's guitar amp building book. everything looked up to spec and then...smoke, blown fuse, cracked 1k resistor on the cap can, and pin 6 on the 5y3 looks a bit burnt. please help!!

[Enzo]

HArd to say, but something failed. COUld be a part, could be a solder joint you made, could be a stray piece of wire that touched something, etc.

If the 5Y3 shorted, that would put AC on the filter cap, which would not like it. Any resistor that burns has excess current runnign through it. That means that something on the downhill side of it is shorted to ground or extremely leaky.

Look at the 5Y3 socket closely. ANy burn marks? It could be arcing. Pull the 5Y3 out and turn on the amp without it. Does it sit there and the fuse not blow? Is the proper high votage AC present on pins 4 and 6 of the tube socket? If you see any tiny blue sparks on the socket, that is bad - replace the socket. And be very careful there are not blobs of solder close to other pins, and no hanging wire strands that might touch something.

With power off, and no 5Y3, measure resistance across each filter cap section - or across each filter cap if they are separate ones. Are any now shorted? That is a low resistance? The resistance reading should slowly climb upwards as your meter charges the cap.

Go along the whole project and look carefully at each socket, looking for stray wire and solder blobs that might touch two pins together.

[Bruce / Mission Amps]

built aa764 circuit according to tino zattola's guitar amp building book. everything looked up to spec and then...smoke, blown fuse, cracked 1k resistor on the cap can, and pin 6 on the 5y3 looks a bit burnt. please help!!

Assuming the 1K is the screen node resistor, replace it with a 2 watter and try 2.2K to 3.9K instead.
It sounds like the power tube failed and took the screen with it.
5Y3, Pins 4 and 6 are the high voltage secondary lugs.
You could of had a flash over there so check it out carefully.

[bob p]

one approach that i find helpful is to remove the preamp and power tubes when you're troubleshooting a problem like this one. that helps to eliminate some of the variables when you're trying to track down the problems.

[Enzo]

I must be tired Bruce, as often as I have suggested that failure mode, it never entered my mind. yes, of course that is most likely what happened

[sheppadizzle]

So I managed to replace the screen node resistor per Bruce with a 3k and now my amp at least turns on but has a hideous screeching oscillation with all the knobs at 0. Any suggestions?