jmd 1 head fuse keeps blowing

[Rroxon]

Not sure how to deal with this now... I've changed out the EL34's from JJ... The JJ preamp tube did not work.. So I reinstalled the original ... The F1 1 amp fuse still keeps blowing.. Bias on both sides are set at 78mv.. Any thoughts on this issue ??? I'm stumped..

[surfnorthwest]

When this happens it is usually a bad power valve, but if you are sure they are good then you have a short in the circuit somewhere most likely created by a bad power tube. It is time for service.

[Anitoli]

Surf, you have a JMD1 correct? What fuse is he referring to? He states this is a 1 amp fuse. JMD1 has a power draw of what 375 watts? That fuse would have to be bigger than a 1 amp. A 6100 is a 375 watt load and it requires a 4a fuse.

[Anitoli]

The correct mains fuse should be a T3.15A 250V. To the OP if it is a different fuse please advise.

[surfnorthwest]

The correct fuse is a T3.15 20mm Fuse (3.15AMP) 250V, indeed 375 watts. The fuse is inside the head. Could also be a issue with the bias resistors

[Rroxon]

The fuse is the F1... If viewed from the bottom of the chassis.. It's the one to far right.. That fuse had been in place for the last 4 years before it blew the first time... On the spec sheet it says.... F1AE-100W T400mAE-50W...

[Anitoli]

Can you post a picture of it's location?

[Rroxon]

I'll have to research how to do that.. Been a long time since I've posted any photos in a forum thread... I do know there is a photo from a few years ago in the jmd thread... if viewed from the bottom of the head... The fuse would be the one under all the power feed wires from the on-off switch and the standby switch... it says F1 and is on the very far right

[Rroxon]

If you view the second photo in this link.. You will see the fuse on the left side of the pic... It's under the yellow wires that are connected to the power and standby switch's...


viewtopic.php?f=48&t=10568

[Anitoli]

Unfortunately i can't find any schematics showing the circuit for that part of the amp so i don't know what its connected to.

[Rroxon]

I found 4 pages of them... I'll see if I can find the link

[Rroxon]

Here's the one that gives the most info.. Link .. On the second page in the upper right hand corner... I believe that is the fuse that keeps blowing
download/file.php?id=3079

[Anitoli]

Ok, there are two in the upper right corner an F3 and an F4. Which one is it?

[Rroxon]

Your looking at the first page... The fuse is in the second page, Scroll down you'll see F1 T1AE-100W-T400 mAE- 50 W.......... Thanks

[Anitoli]

Got it. Try this just to rule out the power tubes. Remove all 4 tubes and turn on the amp, take it off of stand by, put it on standby and take it off again, and then back on. Does this kill the fuse?