Blowing mains fuses in a scratch-built Ceriatone JCM800 2204

[dfricano]

Hi there: first time posting, first time trouble-shooting my amp. Newb alert!

A friend of mine scratch-built a JCM800 2204 for me from the Ceriatone kit, and after a week or two of playing wonderfully I am now blowing the mains fuse 1-1.5 seconds after powering the amp on. I'm taking it upon myself to finally learn something about maintaining a tube amp, but am pretty well in the deep end at the moment. The story so far:

1) I've replaced the mains fuse to no avail.
2) After removing both power tubes, the mains still blows out.
3) HT fuse is fine.

I've been told that the problem may be in the rectifier stage, possibly the rectifier diodes. That last sentence makes about 35% sense to me in that I know what the diodes look like. Otherwise, I am lost.

I've got a multimeter that I know little of how to use. I've got the amp unplugged, standby and pwr switches set to "on," and have discharged the filter caps (though I haven't checked for voltage -- not sure how) and still have the jumpers set up.

Kind people of the Metro Amp forum, I humbly lay myself to your tender mercies and voluminous knowledge base when I ask "HELP!" Please tell me if there's other info/pics you need and I will get it to you!

[neikeel]

You need to look at the schematic to see what is in circuit with HT off and mains on, usually the PT with diodes and first (mains) filter caps.
First unhook the filter can and retry - if blows it is the can
Second lift both the HT ac feeds between PT and diodes ?
If it still blows it is your PT.
diodes are cheap and easy to sub if in doubt.
Other point to mention is correct rated SLO BLO fuses must be used for these mains and ht