Blowing fuses

david eaton

Member
Messages
754
So here's the situation...

About two weeks ago, the fuse fell out of my Mesa Studio .22+ and was promptly run over by a vehicle. Since it is impossible to find a new fuse cap (the spring loaded cap that screws into the amp), Mesa sent me a new fuse assembly.

Last night, an electronics guy and myself pulled the amp out of the cab and replaced the fuse holder. Today I turned the amp on and after about 10 seconds the fuse blew. Replaced the fuse, same thing.

I'll check to see that the tubes are seated properly (though it sounded fine for the few seconds it played before it blew). Any other ideas of the cause for this?
 

John Phillips

Member
Messages
13,038
Are you using the right type of fuse - anti-surge/slo-blo/T ?

Although it doesn't sound like that since a fast-blow would go immediately when you turn on, if you needed an anti-surge... which isn't the problem.

Are you absolutely sure you didn't disturb some other wiring or something while you were in there?

It actually sounds like a blown power tube to me... odd coincidence, but it could happen. The easiest way to find out is to pull the power tubes and power up again.
 

david eaton

Member
Messages
754
Thanks, I'll give that a try. The fuse assembly and the fuses came from Mesa so I think all is ok there. I have some new power tubes, seems like that would be an odd coincidence though...
 

fullerplast

Senior Member
Messages
6,781
Make sure it is the correct value slo-blo fuse, first of all. If it is, next thing to try is removing all the tubes and then see if it blows. If it still blows, you are probably looking at a filter cap or rectifier diode. If it doesn't blow, put in just the preamp tubes. If it still doesn't blow, you've at least isolated it to the power tube section. From there, you'll need to get a schematic to go further or take it to a tech.
 

WailinGuy

Member
Messages
1,286
If you have been putting the correct type and value of fuse in there, then I would assume the problem is not directly related to installation of the new fuse holder (unless something else got disturbed in the process). With only two wires to unsolder and then solder, it would require some serious carelessness to screw that up!

This would be a helpful hint: does the fuse blow after turning just the power switch on (leaving standby off) or only after you've turned on the standby switch? (In general, the standby should be turned on only after all the tube filaments are fully lit up. If you are turning both switches on at the same time, then I would wonder if the 10-15 second delay is related to the warmup time of the tubes.)

The more I think about it, the more likely it seems that you are using a slo-blo fuse, but of a lower value than what is required by the amp. During the short period of operation, the power tubes continue to "settle in" and warm up just a little bit more (i.e., they draw more idle current). causing the total amount of current the amp draws from the wall outlet to excceed the ratings of the fuse. At least, this is my best guess, given what you've told us so far.
 

tater

Member
Messages
5
I had the same sort of fuse problem with an amp of mine. I spent hours swapping power tubes in and out to no avail.

I finally replaced the rectifier tube - presto! Fixed.

Could be worth a try?
 

WailinGuy

Member
Messages
1,286
I finally replaced the rectifier tube - presto! Fixed. Could be worth a try?
Th amp were talking about (Mesa .22+) doesn't have a rectifier tube; the power supply uses silicon diodes.
 

tater

Member
Messages
5
Originally posted by Jim Salman
Th amp were talking about (Mesa .22+) doesn't have a rectifier tube; the power supply uses silicon diodes.
Sorry, I didn't mean to interrupt you experts. Couldn't possibly be the rectifier could it?
:p
 

fullerplast

Senior Member
Messages
6,781
Originally posted by tater
Sorry, I didn't mean to interrupt you experts. Couldn't possibly be the rectifier could it?
:p
from an earlier post:

If it still blows, you are probably looking at a filter cap or rectifier diode.
Unfortunately, we've gotten no more feedback from the original poster, so we may never know what the problem was.
 

david eaton

Member
Messages
754
Originally posted by Jim Salman
If you have been putting the correct type and value of fuse in there, then I would assume the problem is not directly related to installation of the new fuse holder (unless something else got disturbed in the process). With only two wires to unsolder and then solder, it would require some serious carelessness to screw that up!

This would be a helpful hint: does the fuse blow after turning just the power switch on (leaving standby off) or only after you've turned on the standby switch? (In general, the standby should be turned on only after all the tube filaments are fully lit up. If you are turning both switches on at the same time, then I would wonder if the 10-15 second delay is related to the warmup time of the tubes.)

The more I think about it, the more likely it seems that you are using a slo-blo fuse, but of a lower value than what is required by the amp. During the short period of operation, the power tubes continue to "settle in" and warm up just a little bit more (i.e., they draw more idle current). causing the total amount of current the amp draws from the wall outlet to excceed the ratings of the fuse. At least, this is my best guess, given what you've told us so far.
I trust the fuse is the right value (it was shipped from Mesa with the new fuse holder). They recommend 1.25 amp. I have some 1.5 amp fuses that I could try.

The standby is off on power up.

I'll likely try to send the amp off to a tech. I would think if we messed up installing the fuse holder it would be that we didn't completely connect one of the wires, but then we'd have an incomplete circuit and the amp wouldn't turn on at all.

I bought another amp over the weekend to i) tide me over until I get this fixed and ii) serve as a good backup. There are no good techs around here and it'll be a couple of weeks before I can get the amp to Nashville to have someone look at it.
 

WailinGuy

Member
Messages
1,286
I trust the fuse is the right value (it was shipped from Mesa with the new fuse holder). They recommend 1.25 amp. I have some 1.5 amp fuses that I could try.
From experience, I never trust that a fuse holder contains the correct value fuse!! (I always take a look at the fuse because you never know when some moron has stuck a 20 amp automotive fuse in there.) So what value fuses have you been trying since the one that Mesa sent with the new holder blew? Additional 1.25 amp fuses or some other value(s)? It's probably OK to try 1.5 amp fuses, but I wouldn't go any higher if the amp calls for 1.25. You might even want to go a bit lower (1 amp) since we know there's a problem and we want to be sure no further damage occurs. (The recommended fuse value is always somewhat more than what would be required to operate the amp at low to moderate volume.)

The standby is off on power up.
Yes, but you didn't tell us when the fuse blows. Does it happen after initial power up (but before taking the amp off standby), or after flipping on the standby switch?

I'll likely try to send the amp off to a tech.
You may wind up needing to do this, but I'm still wondering about the 10-15 second time delay before the fuse blows. Even a slo-blo fuse will blow much faster than that once the current draw through it has exceeded its rating. I'm still suspecting it's related to tube warmup. Something like a bad cap or diode would most likely blow the fuse almost immediately. You said you replaced the power tubes and the fuse still blew, but have you tried removing all the preamp tubes and then turning the amp on? It's worth a try. One more thing - while you're in there removing tubes, check the tube sockets (especially the power tube sockets) for any carbon arc traces.
 

david eaton

Member
Messages
754
Originally posted by Jim Salman

Yes, but you didn't tell us when the fuse blows. Does it happen after initial power up (but before taking the amp off standby), or after flipping on the standby switch?

Sorry, it blows before I take the amp off of standby.
 




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