# Amp volume dropped then faded to nothing



## MetalTele79 (Jul 20, 2020)

I was playing my Marshall clone earlier and half way through some Sabbath riffs the volume suddenly dropped by about half then faded to nothing. The amp still powers on but there's no more skundy. How can I tell if it's a tube issue or something internal? The amp uses two El84 power tubes and two 6AX5 rectifiers.


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## jb welder (Sep 14, 2010)

Does it have an FX loop? Anything more you can tell us about the model or build?


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## MetalTele79 (Jul 20, 2020)

No FX loop. 8 and 16 ohm outputs. Built by Steve Moratto. Here are some more pics that might help?


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## dtsaudio (Apr 15, 2009)

First thing to check are the rectifier tubes. Your symptoms seem to point to loss of high voltage.


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## nonreverb (Sep 19, 2006)

Fading to nothing also sounds like a heater that died. Do all the tubes light up?


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## Lincoln (Jun 2, 2008)

very interesting rectifier system using two low power rec. tubes. I haven't seen that used before.

Better sag maybe?? 

I love the sound of an 18W TMB


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## Mike_Blaszk (Sep 16, 2021)

In my experience, this definitely sounds like a tube issue and is your rational starting point. Then go from there.


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## Alan Small (Dec 30, 2019)

cold solder joint ?


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## Latole (Aug 11, 2020)

When is not working : 
You have to read tubes's heater voltage P.T. and at each tube
Read B+ voltage too


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## MetalTele79 (Jul 20, 2020)

Thanks for all the replies. I forgot to mention that I have no idea what I'm doing on the technical side.

Yes, all the tubes are lighting up but the rectifier tubes are pretty bright and I don't remember if they were that way before.

If I replace the rectifier tubes do I need to do any re-biasing?

Is there any danger in checking the voltages?


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## Latole (Aug 11, 2020)

Rectifier tube look ok
No rebiasing if you replace it, 
Danger to checl voltage ? Yes, it can kill you. Don't do it if you don't know how to


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## MetalTele79 (Jul 20, 2020)

Latole said:


> Rectifier tube look ok
> No rebiasing if you replace it,
> Danger to checl voltage ? Yes, it can kill you. Don't do it if you don't know how to


Thanks. 

I've been making lots of trips for gear repair recently. Are there any online courses or books that teach this stuff so I can learn to tinker safely?


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## Paul Running (Apr 12, 2020)

There's a pile of info out there. I would start with basic electricity, learn it's behaviour...how it flows. Like all forces of nature, learn to respect their potential and do not guess...if you don't know, find out.
Norman Crowhurst has published many books on the subject of audio electronics.
There are some good textbooks from the 60s and the military training manuals are excellent...I was trained in the early 70s by the CAF and their training manuals were superb.


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## MetalTele79 (Jul 20, 2020)

Paul Running said:


> There's a pile of info out there. I would start with basic electricity, learn it's behaviour...how it flows. Like all forces of nature, learn to respect their potential and do not guess...if you don't know, find out.
> Norman Crowhurst has published many books on the subject of audio electronics.
> There are some good textbooks from the 60s and the military training manuals are excellent...I was trained in the early 70s by the CAF and their training manuals were superb.


Good point with the military manuals. I am a dental technologist and when I was in school our main resource was a manual produced by the air force.


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## DaddyDog (Apr 21, 2017)

MetalTele79 said:


> Thanks.
> 
> I've been making lots of trips for gear repair recently. Are there any online courses or books that teach this stuff so I can learn to tinker safely?


While there is info online, imagine their liability if you happen to kill yourself while tinkering. Compare it to knowing how your car works: there are some basic things you should/can know that are easy to maintain or troubleshoot. But at a point, you need to take it to a pro.

There's a podcast The Truth About Vintage Amps. Fantastic source of info from a guy that's been servicing amps for 40 years. Everything from simple troubleshooting to circuit design mods.


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## MetalTele79 (Jul 20, 2020)

DaddyDog said:


> While there is info online, imagine their liability if you happen to kill yourself while tinkering. Compare it to knowing how your car works: there are some basic things you should/can know that are easy to maintain or troubleshoot. But at a point, you need to take it to a pro.
> 
> There's a podcast The Truth About Vintage Amps. Fantastic source of info from a guy that's been servicing amps for 40 years. Everything from simple troubleshooting to a circuit design mods.


Thanks I've added the podcast to my follow list. I'll be sure to give it a listen.


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## jb welder (Sep 14, 2010)

MetalTele79 said:


> Are there any online courses or books that teach this stuff so I can learn to tinker safely?


The rest of the website is good, but start with this page: Safety


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