Express Fuse Problem

[RockNRollDr]

This is my third TW build (my second Express style build) and it is the first one that didn't fire up perfectly the first time, and I'm not sure what is happening here. Although I think it could be a very simple fix, here is what happened on the first flip of the switch:

I cut the amp on and let it warm up for about 5 minutes and everything was fine, then I plugged my guitar in and played for about 30-60 seconds and I blew a fuse, so I changed it out and turned it on again for about 10 minutes and all was well until I plugged my guitar in and again I played for about 30-60 seconds and blew another fuse. So I swapped it out and disconnected the wire going to my power switch from the fuse holder, then I hooked an amp meter between the fuse holder and the wire I disconnected, and when the amp was powered on it read 0.5 amps which was where it stayed until I plugged in my guitar, then it rose straight to 1.5 amps and steadily increased for about 30 seconds until it got to 3.0 amps and I powered the amp off.

Any ideas will be greatly appreciated.

[selloutrr]

http://www.geofex.com/ampdbug/fuseblow.htm

this should help let us know what you find.

[billyz]

I assume you did try a different set of Power tubes. Sounds like some kind of oscillation feedback , maybe too high frequency to hear. Check your layout against known good ones.

[jumpbluesdude]

Id bet its a parasitic oscillation from your description, but its out of your hearing range. Thats why it doesnt happen until you plug in. A lot of times these will completely shut an amp off in the audio range and it will apper your amp is dead, but its just pushing everything into a frequency you cant hear. I think taking a close look at your lead dress would be a good place to start. I think Billyz nailed it.

[Kregg]

I blew two fuses the same way once when I was setting up the amp in my listening room. The bias was out of whack after the amps shipment from LA to Atlanta. I guess that's why current limiters and variacs are handy devices.