I purchased my JTM45 about four years ago and last summer it started blowing its fuse. We were playing an outdoor gig in Minnesota on one of those hot and humid nights. After the fourth song it blew its fuse (the fuse that can be replaced from the back of the head). I have replaced the 2A fuse but the head will blow a new fuse as soon as you turn it on. I have tested all the tubes in a tube tester and they all look good. There was no smoke or burning smell. The inside of the head looks OK--there are no exploded caps, no burnt components, no weird smells. Continuity checks all appear good. All resistors measure OK. I cannot check any voltages because the fuse blows the instant I turn the head on.

Anyone have any good next steps in troubleshooting?

+1.

Turn it on with just the fuse and see if it blows.

Then turn off.

Turn the amp on with the fuse and rectifier and see if it blows.

Then turn off.

Turn the amp on with the fuse, the rectifier and the power tubes and see if it blows.

I'm guessing by then you'll have an answer.

If it blows with the rectifier and power tubes removed, chances are you have a short.

If it blows after a particular tube was installed, try a replacement tube(s).  If that doesn't help, there is an issue at the socket.

I might be going too far back....
But, I would lift the entire secondary of the Power Transformer, all you would have is mains power coming into the PT and that is it.
Does it still blow the fuse.? I  would bet not.
I would then connect the secondaries one at a time.
What do you have on that side.?
High Voltage
Heater
Rectifier Tap
Is that right.?
I would connect those (2 wires at a time) one at a time and see what snaps the fuse. Kind of thinking heater or rectifier taps.
I usually let Wyatt handle this stuff. He is WAY better/knowledgeable than I am. If he suggests different, I would surely defer to his recommendations.
Best

OK, it's been a while since I started this topic. I have finally got back to troubleshooting my JTM45.

I have removed all tubes, upon applying power the fuse blows. So I have unsoldered all the output connctions from my power transformer and it still blows a fuse.

I suspect I have a faulty power transformer and I must acquire a replacement. Any advice on where to purchase a replacement?

Thanks,
Rocko

I have purchased a Magnetic Components OT (Magnetic Components calls it a "50W Marshall Power, Laydown Mounting"). Before I rip and replace I can't quite line up all the connections. I live in Minnesota (North America). Here's a chart listing my connections:

Ceriatone            Magnetic Components
Labels                 Labels

E                         ?
O                         ?
120V                   120V
0                         5V
5V                       5V

350V                    345V
0                         ?
350V                    345V
3.15V                   315V
0                          ?
3.15V                   315V

1. Any idea what connection I use for the Ceriatone "E" connection? This connects to ground, but the Magnetic Component (MC) OT has no connection labeled either E or ground.
2. The Ceriatone connection labeled O (adjacent to the 120V) connects back to the AC input. Would I use the MC connection labeled COM?
3. The Ceriatone connection between the two 350V labeled O; would I use the MC 345V center tap connection (labeled 690V CT @ 150mA)?
4. The Ceriatone connection between the two 3.15V labeled O; would I use the MC 3.15V center tap labeled 6.3V CT @ 5A?

Did I purchase the wrong output transformer--or am I just challenged by conflicting labels?

Any advice would be greatly appreciated.
Regards, Paul.

Have good knowledge with material of interest. Benefit from your web very much.