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Post subject: 5150iii 6l6 combo issues
Posted: Sun Feb 02, 2020 11:41 am
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Recently bought a used 5150 combo 6l6. Have an issue with it all channels work the red channel works completely normal full volume. The blue and green channel however will work at full volume for about 30-45secs then they drop drastically in volume with volume and power scaling at full they are still only bedroom playing level,they will also change in volume after the drop it goes up and down in volume in a very consistent pattern trying to figure out what my issues may be thanks


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Tue Feb 04, 2020 3:33 pm
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
Ouf :(
I never work on this amp. I have the schematic.
IMO amp must see a good amp tech
I'll follow the thread.

Good luck


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Tue Feb 04, 2020 5:11 pm
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I’d like to try to sort it out myself being as I only gave 250 for it. I’ve determined it’s in the preamp. Reviewing the schematic They use jfets to mute the channels wondering if that could be my issue.


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Tue Feb 04, 2020 5:57 pm
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Does the volume change abruptly or gradually?
Is this a later model with the concentric volume controls?


Welcome to the forum, and good luck.


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Wed Feb 05, 2020 11:29 am
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Professional Musician
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Yes, the JFETS do give problems. They are cheap too.


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Wed Feb 05, 2020 12:13 pm
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$0.90 Newark electronics

https://www.newark.com/on-semiconductor ... dp/58K8917


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Wed Feb 05, 2020 2:17 pm
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This is an older model combo, the volume drop is very sudden upon turning it on the sequence of events is like this (full volume , loud pop , drop in volume , loud pop, another drop in volume , clean channel now becomes somewhat distorted and has no real clean tone, volume is very low , volume begins oscillating)


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Wed Feb 05, 2020 3:46 pm
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Sounds like an intermittent short in a power tube.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Thu Feb 06, 2020 6:09 am
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Replaced power tubes it’s not that unless it’s an issue in the circuit but plugging into effects return running from the preamp of another amp it’s bone crushing loud


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Thu Feb 06, 2020 1:22 pm
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Bad solder on a power resistor... It probably has lead free solder from the factory. Reflow every joint in the amp


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Post subject: Re: 5150iii 6l6 combo issues
Posted: Fri Feb 07, 2020 6:41 am
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I’ll check solder joints and report back. Kind of annoying that this may be the course of action to repair.


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