JTM45 Build - Mains Fuse Blowing

[Janglin_Jack]

I finished my JTM45 build and I started the power up and get 120v from the wall. I hit the power on switch and the first time, I was able to measure 360v at the standby switch and 5v at the recto and 3.15v for the heaters. Then the fuse blew. I thought it was due to the fast action fuses, but even with the variac, it blows at around 60-70v. I have checked and rechecked my wiring and from what I can tell, I have it all correct. I can't get past the power on with no tubes and the mains fuse blows 3A slow blow. Where should I look at this point? After the first fuse blow, I noticed a bit of a smell, like melted plastic. It was faint and I can't tell where it is coming from, but fuses blow now when I get the variac up near 90v or so. I have never had so many issues between this amp and my 5E3 build!!! Help!

Mike

[Janglin_Jack]

I clipped the 50v bias tap wires and took a bias feed from the primary. I change the resstor to 150K at the bias pot/network. Turn the amp on, the fuse still blows, (no tubes). I am stumped. Any ides of what to check? I have been over and over the wiring and can't find anything.

Stupid question, but for the power cable, I have a convienience jack and removeable power chord. I matched up another chord I have with bare wire ends to figure out what was the "white" wire and "black" wire. I am sure I have this right, but can someone tell me if these are reversed, would that cause a problem?

Mike

[chubs]

First, the HT fuse should be 500mA ... and the Mains (120v) is the 2-3A fuse. If you have them reversed ... the .5A fuse will blow every time

You won't blow the mains fuse if you invert the Neutral / Hot wires from the power cord. Blowing the 3A fuse with No tubes would indicate a short between hot and ground..... or my first suggestion above.

I would check that first.

[Janglin_Jack]

I said HT fuse, but I what I should have said is the mains fuse 3A fuse. The 500mA HT fuse is fine and now blowing.

Mike

[Janglin_Jack]

I think I found it. The bias pot goes from the 56K resistor the ground connection at the other side. I have the wiper going back to the resistor side of the pot, (input) instead of to ground Will modify tonight and see if that is it.

Mike

Made the change and still pops the fuse!!! OMG - going to cry now.

[Janglin_Jack]

OK, should my dedicated 50v bias supply be interrupted by the standby switch or should one lead go from the PT to the bias network and the other to ground?

Mike

[Janglin_Jack]

Well, thanks to Steve, ("SDM") and some extra $$, we got the amp working. Steve noticed a couple resistor values that needed to be changed, (one for the bias network 27K down to 470 ohm) and the 100K resistor on V2, (mine was 100 ohm). As Steve pointed out, those would probably not cause PT failure. I am not sure if I had an earlier wiring error, or if my Solid State Recto plug did it, but Steve helped me test to determine my PT was fried!!!

Anyhow, got a new PT and wired it all in. Tested everything and I am happy to say, the amp is alive, (it just occurred to me I should have said that like Dr. Frankenstein since it was fixed on Halloween......it iiisssss aaaaAAAALLLLIIIIIIVVVVVEEEEEEE).

It may be just my luck with this amp, but my daughter was moving my 2x12 cabinet with the head on top and SLAM, the head dropped to the ground. I hope all is well with it, haven't tested it yet.

Anyhow, thanks to everyone for their help with this build, I think this was the most frustrating yet!! But it sounds monstrous, will post a clip soon.

Mike