Ht fuse blown on 1974x - solved, ez81

[Gouna15]

Hi

I bought a used Marshall 1974x three weeks ago. According to the label, it dates back to 2009. The EL84 tubes are 2011 Sovteks, and the EZ81 is a JJ.

I’ve turned the amp on about 15 times over the last 15 days (playing for 15 to 30 minutes each time). No issues. I haven't gone past volume level 1, except for one time outdoors when I tested it at 10—it sounds loud and great.

Last night, I turned the amp on again—volume at 1 for a clean tone—and played very dynamically with a heavy pick attack. After 3 minutes, the sound cut out completely; total silence. The HT fuse blew; the glass isn't blackened, but the wire inside is broken.
One of the two EL84s has slight brownish marks inside, while the other looks perfectly clean.
So, the HT fuse wasn't the original one; it was a 500mA fuse instead of a T315mA... That concerns me!
I’ve ordered some T315mA fuses; I'll see if the problem comes back...
I hope the power transformer isn't the culprit.

Thanks for your thoughts and advice

[Daviedawg]

Hi. While waiting for the fuse replacement I would fit new or known good EL84s. Those have done their stint. It is possible that a failing one has caused your fuse to blow. Don't risk more serious damage.

Dd

[Iain T]

It certainly seems plausible that a failed output tube could be the cause of the blown HT fuse.

This could be simply from ageing of the tube or some other factor. If it's just valve age then replacement is an easy fix. If the cause is somewhere else (e.g. bias, leaky coupling cap, etc.)

At this age it's very wise to have an amp serviced - any sus electrolytics replacing, resistor values checked, etc. As a minimum you'd want to make sure that the various voltages are roughly where then need to be.

[Gouna15]

Thanks for your replies.

I received some T315mA fuses.

So...

Test 1: without the EL84s, with the EZ81= the fuse blows (no noise, no smoke).

Test 2: without the EL84s, without the EZ81= the fuse doesn't blow.

I'm getting a new EZ81 soon...

[Bieworm]

when you buy a new EZ81 MAKE SURE it's NOT a JJ!!! those are terrible and fail very very often

[Gouna15]

Oops...

[Bieworm]

Oops...

Fingers crossed then

[Gouna15]

I tested the new JJ EZ81, and the amp works!

However, I still have that little bit of unwanted noise (crackling); you can hear it clearly at the end of a chord:
https://youtu.be/H4SFgB19ym4

I've had this noise ever since I got the amp.

[JMPGuitars]

Crackling is often caused by a bad (probably cold) solder joint.

Thanks,
Josh

[Gouna15]

Okay, problem solved: two new Sovtek EL84s—no more unwanted noise!