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Guitar Amp Cutout

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(@mcdouggy)
Trusted Member
Joined: 20 years ago
Posts: 57
Topic starter  

Hey, I have a Seymour Duncan 84-40 Tube Amp, and it cuts out a lot. I was at band practice earlier today and mid way through a song, the guitar dies away (takes about 3 seconds to go from loud to dead quiet), it mysteriously comes back again about 20 seconds later and then acts as if nothing happens. This dieing away happens many times (say 5 times in 10 mins). I have already had one loose solder connection resoldered, but that hasnt seemed to make much difference. Also I have checked the usual things, leads, fuses, guitar etc. It's definitely the amp. I was wondering if anyone has any ideas on what could be causing this.

Thanks
Doug

I have a schematic if anyone wants one, and the amp runs with 3x12AX7/ECC83s, and 4 EL84s at 40 watt if that's any help.

EDIT: Hmm..maybe this should be in the Guitar Amp forum..oops.

Visit my band's website!


   
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(@forrok_star)
Noble Member
Joined: 21 years ago
Posts: 2337
 

Here's a quick list of things to look into. Intermittent problems are sometime the hardest to figure out. Generally something is at the very edge of failing. Some very common thing maybe Mechanical vibration, Heat, or Voltage stress. A few others would be when a Tube develops a problem when it gets really hot. Power tube screen resistors are a common for this. Resistor's or capacitor's go bad when they get hot. Bad solder joint or broken part or badly drifted resistor value. If the amp is cutting out when its being driven hard usally caused by a temporary bias shift.

How long has it been since the output tubes have been changed and Biased.

Hope this Helps point you in the right direction.

Joe


   
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(@mcdouggy)
Trusted Member
Joined: 20 years ago
Posts: 57
Topic starter  

Hey, I had another problem about a month ago, they replaced the filter caps for cheap as I only got it in May. All the tubes were changed and biased last just before I bought it in May as well. One preamp tube had cracked somehow so I replaced that, that didn't happen in my care though, that was when the tech replaced the caps (maybe he used a hammer? :roll:). I don't think it's anything in the power supply at all as the lights are still on on the front and the tubes are still lit. It does cut out when driven hard though, any idea if that bias shift could last for as long as 15 seconds or so?

Doug

Visit my band's website!


   
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(@ricochet)
Illustrious Member
Joined: 21 years ago
Posts: 7833
 

Doug, this is starting to sound familiar. Are you the guy with the Marshall that's had so many problems, discussed in the "Amps" forum over on http://guitarists.net ?

"A cheerful heart is good medicine."


   
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(@mcdouggy)
Trusted Member
Joined: 20 years ago
Posts: 57
Topic starter  

Hmm nope, have you got a link to that post in case it's any use?

Doug

Visit my band's website!


   
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(@ricochet)
Illustrious Member
Joined: 21 years ago
Posts: 7833
 

Not sure how much help you'll get from it.

Here's one thread: http://www.guitarists.net/forum/view.php?forum=5&thread=51711

The author had posted earlier about problems with his new amp, returned it to Marshall, and still had problems with it. I can't find the earlier thread at the moment.

"A cheerful heart is good medicine."


   
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(@sarton)
Estimable Member
Joined: 18 years ago
Posts: 95
 

I'd recheck the replaced caps. It really sounds like something is overheating (echoing what forrok said).

I've been happy with my 84-50, so I hope you get yours working.

A sucking chest wound is Nature's way of telling you to slow down.

Godin Freeway Classic, PodXT Live, Seymour-Duncan 84-50.
(All this so I could learn 'Twinkle, Twinkle, Little' Star for my youngest.)


   
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