Heater fuse breaks when I start the amp.

[blueserik]

Hi.
Dont know if I am alowed to post here because it isnt a D-style amp.
But this is the only place I know with so many talented people.

Well here we go.
I have a Marshall 4140 Club and Country amp. I just love it. But all of a sudden it didnt work. Saw that the tubes didnt get warm so i checked the fuses inside. There is 4 fuses for the heater. Guess it is 1 for every tube. (100W,amp)

It is always the same fuse that goes off.
Does anyone here have a clue?
I have swapped the tubes but with the same result.

Thanks, Erik

[jjman]

I can't find a schematic that shows the specific routing of each heater fuse. Could be a short from a wiring or socket problem.

[blueserik]

so I pulled the 4 output tubes and the fuse dosent break. Do I only have to get some new tubes or can it maybe be something else?
Thanks, Erik

[Firestorm]

The schematic for the 4150 Bass Club & Country shows 2 fuses in parallel for each leg of the heater circuit, so a heater fault would have to blow at least two fuses to interupt the circuit. Since only one fuse blows (and always the same one) I would suspect that there is a wiring problem at the fuse holder so that only one fuse on that side is actually connected. Since Marshall needed two fuses on each side to handle the current, if only one is in the circuit, it would blow.

By taking out the power tubes, you reduced the current demand to the point where the one fuse can handle it. It doesn't necessarily mean there isn't a bad tube, but the tubes could be just fine. Take the fuses out and check for continuity between the connectors where the fuses' metal caps sit. There should be continuity between the two at each end. Also make sure the fuse holder isn't corroded.

[blueserik]

Thanks for the answer firestorm.
Your right about that theres 2 fuses connected together. I didnt think about that all all. I will check the wires first thing in the morning. Thank you so much for the answers.

Thanks, Erik

[blueserik]

ok. The amp starts now but as soon as i turn up the volume and hit one note it starts with this loud cooking noice.

Dont know if I have the words to describe it properly.
Hmm its like a speaker is blown. That kind of sound. But if i flip the chassis almost vertikal the noice stops.
What the hell can this be?

Thanks, Erik

[M Fowler]

Bad solder joint loose wire perhaps.

[Firestorm]

That might be a tube. Take two tubes out (one from each side) and see how that sounds. If okay, start swapping the remaining tubes one at a time until you find the bad one. If you usually use the 8-ohm setting, switch it to 4-ohms when doing this test to keep the impedance correct for only two tubes.

If a tube is bad, you might think about replacing the whole set. If it isn't a tube, there's more serious diagnosis to be done. Yoiu don't want to screw this amp up. It's cool and pretty rare.

[blueserik]

Thanks once again.
I swapped the tubes around a little bit and found the tube that made the sound. I will change all the tubes. Today I run it with EL34. I will try with some JJ KT77 instead. Supposed to break up sooner.

Thanks you very much for the help. Cant belive how many nice people there are out there.