JTM60 combo blowing filament fuse

[fredcapo]

I repaired a JTM60 last year that was taken in on a trade, and then sold. The new owner used it pretty heavily for over a year, until the 6.3 A filament fuse blew a few weeks ago. I replaced the output tubes, and measured the filament current at idle at around 5 amps. I even increased the fuse to 7 A, but it blew again after the guy took the amp back and the amp was on for 4 hours.

Could this be a heat-rise problem?

I am going to install a old style back panel fuse holder so the owner can change fuses eaily and possibly a small rotron fan to try to eliminate a possible cause.

I would appreciate your thoughts on this. Thanks.

[g1]

It blew while running, or did he power off and turn it back on again?
I've seen this problem from turn on surge, but if it happened while it was running, it could be a bad tube or an arcing issue.
You are using slow-blow, yes?

[Enzo]

I think doing things like installing a fan just in case it might be heat related is really just guessing. I calculate 4.2A, making 5A maybe a bit high.

Also, what we don't know is if there is a steady current overdraw, or if there are current events, like an intermittent short in a power tube. Have we tried a different pair of EL34?

[fredcapo]

The fan idea addresses the general rap on this model amp. I have read posts suggesting to up the fuse to 10A...
But no, I did not think the original fuse was a slow blow but will try one.

Thanks for the input.

[52 Bill]

The schematic calls for a 6.3 amp slow blow.

[g1]

Anything other than slow-blow there will be a problem. I have in the past had to increase the value of that fuse due to nuisance blowing at turn on, though I don't think I went as high as 10A.
But if it is not a turn on surge issue (blows while running), I would not recommend increasing the fuse value.
I can't see how heat could affect heater current draw.

Edit: I suppose if the PT heater winding were faulty, then it could become a heat related issue.

[fredcapo]

Thanks guys. I confirmed with the owner that it is a turn on issue...occurred when turned on again hot after run for several hours. I did not install a slow blow at the time of followup repair. Today I put in a fuse holder that he can access in the tube compartment, and an 8 amp slow blow. Also suggested he try to kep the amp ventilated. Should nail it...

[Audiotexan]

Today I put in... an 8 amp slow blow.

Just my .02, but you're asking for problems using *anything* higher than what it's specced for (as Bill noted, 6.3a Slo-Blo).