Marshall JCM800 (2205) 50w problem

[malbi]

Friday night, my band was playing at a local club for a Halloween party. The club is notorious for problems with electric and sub par sound...

My amp is now screwed up and was wondering if anyone can advise what could have caused the problem and best course of action for repair. Here is a series of events:

1) we get on stage and we have to share guitar cabinets. no big deal. the one I was using was a mesa boogie.
2) I plug up and I believe my amp was set on 16 ohms. everything was working fine.
3) the owner of the cabinet tells me it should be 4 ohms so I switch my head over to 4 ohms
4) our other guitarist is on the other side of the stage and can’t find an outlet to plug in to.
5) when he does find one and turns on his amp, my volume cuts down. maybe coincidence, maybe not
6) I go over to my amp and I smell something not right. like something was burning.
7) I shut the standby and power off right away.
8) then I go to turn my amp back on and nothing…no power at all
9) long story short, I had to use the guy’s triple rectifier too. (that was really cool of him to do that)

After our set, I take my amp upstairs to investigate. My 3 amp fuse was blown. In the 20 years of the amp’s existence, I have never blown a fuse.

My questions is, what would cause that? Did me changing my ohm setting from 16 to 4 do it? Or do you think it was something with their power situation?

Also, it calls for a 3 amp 250 volt fuse. Should i use slow blow fuse or fast acting?

[drewl]

Is that the mains fuse?
If so it should be a slo blo.
If the voltage sagged when the other rig was turned on it may have caused your bias voltage to drop faster than the high voltage, causing the tube to draw too much current blowing the fuse instead of killing the output transformer.
You won't know if there's any premanent damage until you try it again with a stable supply voltage.

[malbi]

Yes, I do believe it is the Mains fuse. I'll be able to try it out tonight. I'll buy a 3amp slo-blow fuse and see what happens...

i'll report tomorrow.
thanks,

[jrfrond]

If you haven't blown a fuse in 20 years, congratulations. Fuses don't age like fine wine or cheese, more like moldy bread. Their rating drops with age, so that 3A fuse after 20 years might only hold at 2A. Moral of THIS story: ALWAYS carry spare fuses.

Second issue: turn the amp on, you have low volume then you smell something cooking and the fuse blows. My guess: a power tube shorted, reducing volume, toasting a screen grid resistor at the same time, and then the fuse gave out as a result.

Like I said, it's just a guess, but I have seen it countless times go down just like this, and I do NOT think it had to do with anything but coincidence. However, I could be wrong.

[malbi]

I replaced the fuse with the correct 3amp slow burn fuse. The amp powers up fine. Both tubes glowed.

But when I plugged in my guitar, and flipped the standby, there was a low hum but I could play fine; both channels. But after about 5 minutes at the most, I heard a little crackling noise and the volume dropped.

I stopped playing and looked at the back to check out the tubes. The tube on the right was glowing extremely bright. Brighter than I have ever seen any tube glow. I thought it was going to blow up… The sound by this time dropped almost completely out.

In an effort not to damage it anymore, I flipped the standby and let it sit for about 2 minutes. Then I cut the power switch. I waited about 5 minutes and turned the power back on but did not switch on the standby.

Obviously, it is not stable and needs some attention.

What do you think? Bad power tube, cap, resistor? I don’t know.

I know i'll be taking it to a tech soon but wanted to get other opinions beforehand

[R.G.]

Obviously, it is not stable and needs some attention.
What do you think? Bad power tube, cap, resistor? I don’t know.
I know i'll be taking it to a tech soon but wanted to get other opinions beforehand

I'd guess that it was the elephant hiding under the standby switch...

The real bottom line here is that hearing a dozen guesses about what it might be has no bearing whatsoever on what your tech actually finds. Your first sentence is correct; everything else is handwringing.

If you're going to play tube amps, get yourself a complete set of replacement tubes, fuses, cords, anything that you can plug and unplug without having to reach inside and court electrocution. That is both the minimum you should be able to do and the maximum you can do unless you have the skills to work inside the amp.

The lit-up tube is highly suggestive, but it could be shorted, or could have lost its bias voltage. Your part in this is to get the amp in to the tech and say a few prayers that the extra current drain didn't burn out the primary of your output transformer or the high voltage secondary of your power transformer, those being the two most costly single components in the amp.

The purpose of the AC power line fuse is not, as is widely believed, to protect the amp. It's to keep the amp from starting house fires when it fails. It's both possible and desirable to fuse other positions inside an amp to protect the power and output transformers, but only one production amp I know of does a good job of this.

Finally, fuses do not gradually drift down in current carrying capability. The wear out mechanism is metal fatigue from being flexed by heat expansion on power up surges.

Each power surge at turn on heats the element, and it expands. If it does not crack, the cross sectional area and current carrying capability are unchanged. When the metal finally fatigues and a crack starts, that part heats most because the cross sectional area is reduced by the crack. Once a crack starts, life is measured in a few and maybe only one power on surge.

[jrfrond]

Educated guesses by experienced techs are not totally invalid. This guy smelt something burning, and it seems pretty obvious that he has problems beyond his troubleshooting and repair abilities.

This is one that should see a tech before tubes or anything else is replaced.

[drewl]

Well he saw a tube glowing bright red, it's either bad or it lost it's bias voltage.
The screen resistor is in the process of frying from the current draw and that's what he's smelling.

[malbi]

It is going to my tech tomorrow...Whatever it is is beyond my capabilities and i'll be the first one to admit that. I'll reply with how the story ends once i hear back from him. Thanks for all the input.
Mike

[malbi]

Well, i just got my amp back from my tech. One of the power tube's lost it's bias. I had the two power tubes replaced, biased and all is in order. no damage to transformers either, luckily!

[rabies]

consider buying an EL34 dual monobloc with JMP-1. you'll get very similar sounds to your 2205 and you won't have to worry about this problem again essentially.

also, consider a monster powercenter.