Problem with my Marshall JMP 2203 (blown mains)

[Rampage]

Hey guys,

I was just playing my Marshall and the mains fuse blew. Here's a time line:

- Powered the amp on and let it warm up for about five minutes
- Switched off standby
- Played for around twenty minutes
- Notice a significant hum at the 20-25 minute mark which progressively got louder for about ten seconds followed by a pop (Mains fuse blew)
- The sound cut out (no hum coming out of the speakers in or out of standby), but the power indicator does stay on (red switch on Marshall)
- I immediately powered down the amp and let it cool, but I also tried to smell around the amp to see if I could smell anything burning, which I couldn't.

Now, does this sound like a blown tube, or something worse? I do not have an extra set of power tubes around so I cannot test that. I just had the amp in at a local tech last year who replaced the filter caps, so I doubt that the problem would be related to an old cap. I haven't checked yet, but I will pull the chassis tomorrow and look for any burned out components. Could this be an ominous sign of a worse problem such as a transformer issue?

Thanks in advance!

 

[glip22]

Hey guys,

I was just playing my Marshall and the mains fuse blew. Here's a time line:

- Powered the amp on and let it warm up for about five minutes
- Switched off standby
- Played for around twenty minutes
- Notice a significant hum at the 20-25 minute mark which progressively got louder for about ten seconds followed by a pop (Mains fuse blew)
- The sound cut out (no hum coming out of the speakers in or out of standby), but the power indicator does stay on (red switch on Marshall)
- I immediately powered down the amp and let it cool, but I also tried to smell around the amp to see if I could smell anything burning, which I couldn't.

Now, does this sound like a blown tube, or something worse? I do not have an extra set of power tubes around so I cannot test that. I just had the amp in at a local tech last year who replaced the filter caps, so I doubt that the problem would be related to an old cap. I haven't checked yet, but I will pull the chassis tomorrow and look for any burned out components. Could this be an ominous sign of a worse problem such as a transformer issue?

Thanks in advance!

More than likely just a tube going south.The hum you were hearing was probably a tube redplating . When one tube plates the second one in the pair usually will redplate also. You may have opened a screen resistor. I would check those with all tubes out. If they check out and you do not see anything out of the ordinary, I would replace the fuse and throw the entire quad out and replace, rebias, and cross your fingers you got lucky. As soon as I hear that certain sound I know what happened and power down immediately. I had to learn the hard way quite a long while back. I hope you didn't keep the amp powered too long. I took out an OT this way. But it's happened to me several times with zero issues. It really depends on if the fuse did it's job.

 

[Rampage]

More than likely just a tube going south.The hum you were hearing was probably a tube redplating . When one tube plates the second one in the pair usually will redplate also. You may have opened a screen resistor. I would check those with all tubes out. If they check out and you do not see anything out of the ordinary, I would replace the fuse and throw the entire quad out and replace, rebias, and cross your fingers you got lucky. As soon as I hear that certain sound I know what happened and power down immediately. I had to learn the hard way quite a long while back. I hope you didn't keep the amp powered too long. I took out an OT this way. But it's happened to me several times with zero issues. It really depends on if the fuse did it's job.

Hopefully it is indeed just a power tube problem. Tomorrow morning I will pull the chassis and look closely at the tube sockets for arcing and on the boards for any burnt components. I don't have any fuses on hand of the right value so I'll swing by a local strip mall where there's a Checker and Radioshack, I'm sure they'll have some.

I was playing at a very low volume (under 1 on the master) and I turned the amp off maybe a minute (at most) after the pop. Hopefully it wasn't a transformer because these Drakes are part of the magic in these old 2203s and 2204s.

 

[Rampage]

I just pulled the tubes and chassis and everything actually looks pretty good. I did not notice any carbon tracing on the power tube sockets nor did I notice any burned out components inside the chassis, screen grid resistors included. I am going to dig around and see if I can find my little homebrew filter cap drainer. If I can, I'll drain the caps then I'll pull out the multimeter to test each screen grid resistor to make sure it is still good. Heck, I may also try the 6550 to EL-34 conversion while I'm doing all of this as well.

 

[jerrydyer]

power tube

 

[killertone]

If the mains fuse blew, not sure that it is a power tube. If it were the HT fuse it would most likely be a power tube.

Am I on crack?

 

[Rampage]

If the mains fuse blew, not sure that it is a power tube. If it were the HT fuse it would most likely be a power tube.

Am I on crack?

That's why I asked. I've always thought that the HT fuse blowing was a sign of power tube failure, not the Mains Fuse blowing.

From my reading, it looks like this could be a simple power tube failure; however, it could also be a transformer issue. I would be shocked to see an issue with the transformer, though. They amp has played fine and has been used at relatively low volume levels with the right ohm load, but crazy things happen, I suppose. It would be a shame if the OT/PT went out though because I can't afford to dump that much more repair money into this amp.

I may just take it to a local tech to make sure it is a tube issue.

 

[MrDan666]

If the mains fuse blew, not sure that it is a power tube. If it were the HT fuse it would most likely be a power tube.

Am I on crack?

Sometimes a bad power tube will short across pins 2 and 3, which is the plate and heater connections. In that case, the Mains fuse will blow

 

[Rampage]

If the mains fuse blew, not sure that it is a power tube. If it were the HT fuse it would most likely be a power tube.

Am I on crack?

Sometimes a bad power tube will short across pins 2 and 3, which is the plate and heater connections. In that case, the Mains fuse will blow

Would that make sense with the symptoms I experienced? Normal playing, then a hum that gradually got pretty loud, then the popped mains fuse?

 

[MrDan666]

Would that make sense with the symptoms I experienced? Normal playing, then a hum that gradually got pretty loud, then the popped mains fuse?

I think it was just a bad power tube. If all looks ok and nothing is burnt or blown, then i would try a different quad of power tubes to see if they fix the problem. If you have any spare sets you can use, then do so

 

[jerrydyer]

If the mains fuse blew, not sure that it is a power tube. If it were the HT fuse it would most likely be a power tube.

Am I on crack?

if the main blows immediately its usually PT, he said after it wamrs up it goes so, its an assumption. Impossible to tell from the computer.

 

[Rampage]

Would that make sense with the symptoms I experienced? Normal playing, then a hum that gradually got pretty loud, then the popped mains fuse?

I think it was just a bad power tube. If all looks ok and nothing is burnt or blown, then i would try a different quad of power tubes to see if they fix the problem. If you have any spare sets you can use, then do so

Unfortunately, I do not have a spare quad on hand. Perhaps I'll just buy another quad of 6550s (man they've gotten pricey!) from Doug's Tubes and see if they alleviate the issue.

 

[Rampage]

If the mains fuse blew, not sure that it is a power tube. If it were the HT fuse it would most likely be a power tube.

Am I on crack?

if the main blows immediately its usually PT, he said after it wamrs up it goes so, its an assumption. Impossible to tell from the computer.

Yeah, the main didn't blow right away. I had been playing for a while (~20 minutes) and then I heard a hum which gradually became louder (I stopped playing when I heard it come on) and then the loud pop from the fuse. After this happened, the indicator light in the power switch remained on.

 

[jerrydyer]

check valve wueen or cedist.com
sometimes $38 a quad.

 

[Rampage]

check valve wueen or cedist.com
sometimes $38 a quad.

I may pull the trigger on a quad of Svetlana 6550s from Valve Queen. I liked the EL-34s in my Shiva and $116 isn't too bad for a new set.