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Amp fuse blowing repeatedly.


Huw
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Just used my new (second hand) mesa (bass 400+) bass head for the first time tonight in practice and it didn't go so good.

The issue:

left it for a good 7-8 mins to warm up then took it off stand by. messed around with the settings a little then we started playing a tune. 3 notes in it cuts out. checked the fuse and it was blown... replaced this fuse.

Second time, pretty much exactly the same. Waited less time before taking it off standby though coz the tubes were already warmed up. Replaced the fuse. switched to the other output jack.

Third time struck the first note before even playing a song and blew another fuse.

Was using it with an 8 ohm trace elliot cab out of one of the 8 ohm jacks? is this correct?

Can anybody help me with trouble shooting this problem as i've got a gig tomorrow and i'd really like to play it. My thoughts are that i could be loose tubes or the wrong output jack? any ideas please?

Huw

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Just used my new (second hand) mesa (bass 400+) bass head for the first time tonight in practice and it didn't go so good.

The issue:

left it for a good 7-8 mins to warm up then took it off stand by. messed around with the settings a little then we started playing a tune. 3 notes in it cuts out. checked the fuse and it was blown... replaced this fuse.

Second time, pretty much exactly the same. Waited less time before taking it off standby though coz the tubes were already warmed up. Replaced the fuse. switched to the other output jack.

Third time struck the first note before even playing a song and blew another fuse.

Was using it with an 8 ohm trace elliot cab out of one of the 8 ohm jacks? is this correct?

Can anybody help me with trouble shooting this problem as i've got a gig tomorrow and i'd really like to play it. My thoughts are that i could be loose tubes or the wrong output jack? any ideas please?

Huw

i'm guessing the ht fuse is the one that is blowing (usually it's a small value - a few hundred mA, but could be bigger on a bass amp)

if so, replace power valves (nay cheap - but could be worse)

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Just used my new (second hand) mesa (bass 400+) bass head for the first time tonight in practice and it didn't go so good.

The issue:

left it for a good 7-8 mins to warm up then took it off stand by. messed around with the settings a little then we started playing a tune. 3 notes in it cuts out. checked the fuse and it was blown... replaced this fuse.

Second time, pretty much exactly the same. Waited less time before taking it off standby though coz the tubes were already warmed up. Replaced the fuse. switched to the other output jack.

Third time struck the first note before even playing a song and blew another fuse.

Was using it with an 8 ohm trace elliot cab out of one of the 8 ohm jacks? is this correct?

Can anybody help me with trouble shooting this problem as i've got a gig tomorrow and i'd really like to play it. My thoughts are that i could be loose tubes or the wrong output jack? any ideas please?

Huw

When my flatmate and I went halfs on a Dual Recto many years ago we had a similar problem, turns out we were so excited about plugging it in and pretending we were in Metallica we didn't read the note that came with it instructing to take off the plastic protectors on the tubes before turning it on. They melted to the tubes and when it wasn't popping fuses it sounded like gash, leaving us with an expensive replacement job.

Not to say you'd make a rookie mistake like this, but the point I'm trying to make is ensure everything's well ventilated, overheating tubes will make fuses pop too :)

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Hi Huw,

I've always found this site to be incredibly helpful when trying to troubleshoot problems with valve amps.

Tube Amplifier Debugging Page

From there:

Fuses blow quickly for massive overloads, like AC shorts to a grounded chassis. They blow with a some time lag - maybe only a second or two, up to minutes, as the overcurrent gets closer to the fuse's actual rating. Anything that uses enough power to cause the AC line current to exceed the rating of the fuse will eventually cause it to blow.

It is important to remember that a fuse NEVER blows without something else being wrong. It could be that:

* fuse is the wrong rating - replace it with the correct rating

* power tube shorted

* rectifier tube shorted

* power supply filter cap failing

* Carbon trails on the output tube sockets between the plate lug and the other electrodes, especially the heater electrodes.

* power tubes have lost bias or biased incorrectly

* power section of the amp is oscillating at too high a frequency to hear

* there is an ac wiring short or high leakage

* power transformer is faulty

* choke (if present) is shorted/leaking to chassis

* output transformer is faulty

* AC power wiring or B+ power wiring is faulty/shorted

Given that it could be due to any number of (complex) problems, you'd be best to take it to a tech to be honest. Take it to Dennis Marshall; he's the boy.

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Hi Huw,

I've always found this site to be incredibly helpful when trying to troubleshoot problems with valve amps.

Tube Amplifier Debugging Page

From there:

Given that it could be due to any number of (complex) problems, you'd be best to take it to a tech to be honest. Take it to Dennis Marshall; he's the boy.

Stop replacing the fuse!!!

The mesa 400+ takes a "slo blo" fuse and replacing the fuse might lead to more damage (if there is any).

Unscrew the top cover, a gently push all the tubes down a little to make sure that they are in.

It seems that you are overloading the amp somehow, since you are getting sound before the fuse goes. Don't drive the inputs too hard, thats the mistake that I made when my amp decided to die on me the first time.

How high did you have the input gain?

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Thanks to Scotty B this is fixed as he mentioned. Heroic.

Will get new valves though. Do all the valves in an amp have to match ior can some be one type and one be another, like KT88s, 6L6s (I can't rememeber if they are the same type or not - but you get my drift.)

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