Amp volume fades in and out

LiveeviL2000

LiveeviL2000

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My 5150 has had the volume fade out and come back a couple of time now.
I tried researching probable causes and the internet is full of possibilities. Most point to power tubes over heating, but there are a dozen other things that are suggested.

The volume fades out after a good 20 - 30 minutes of the amp being turned on. The after a minute or two it comes back.

Any help will be welcome
 
LiveeviL2000 said:
Should I replace both?
Is a "matched pair" a real thing or is that a myth?
Click to expand...
One thing is also to have a look at how they are lighting up, but not always telling. The matched pair thing supposedly is a good thing, but really isn't necessary. You should re bias after replacing power tubes though. I am pretty sure 5150's are generally biased on the cool side.
 
LiveeviL2000 said:
Should I replace both?
Is a "matched pair" a real thing or is that a myth?
Click to expand...
"Matched pairs" make biasing easier if the amp just has one higer/lower trim pot.
If you have individual tube bias, there's more flexibility.
One old tube and one new tube is usually not a great long term plan, as you can guess which one will probably fail first.
Also, depending on your tone preferences, a slight imbalance may create a desirable result.
 
LiveeviL2000 said:
My 5150 has had the volume fade out and come back a couple of time now.
I tried researching probable causes and the internet is full of possibilities. Most point to power tubes over heating, but there are a dozen other things that are suggested.

The volume fades out after a good 20 - 30 minutes of the amp being turned on. The after a minute or two it comes back.

Any help will be welcome
Click to expand...
Is this a Fender EVH 5150 or a PV 5150?
 
LiveeviL2000 said:
Would tapping the tubes tell me if the are going bad or do I have to swap them one by one and see if the problem persists.

This is an EVH 5150iii 50w 6L6
Click to expand...
I did see a problem in the last one I worked on, that causes the output tubes to overheat...
there is something inside which needs to be addressed.
Do you have a volt meter to take readings?
In any case don't replace anything until you know what's going on.
 
LiveeviL2000 said:
To be clear, the exact model I have is the EVH 5150iii 50w 6l6 Stealth. I don't think the Stealth means much, although they do charge more for it.
Click to expand...
tubes out
turn the power on, leave standby in warm-up position.
Do not turn standby to operate position.
Set meter for DC volts.
Black probe attach to chassis.
Red probe connect to pin 5 of output tube socket.
What is the DC voltage on pin 5 of each output tube socket?

The pin 5 voltage is the most important voltage in the amp.
This voltage controls the output tube current.
When this voltage malfunctions, the output tubes can burn up....or worse.
This is why pin 5 voltage is critical.
 
Last edited:
Good luck, hope you get it sorted easily. Remember - safety first!

If you end up trying new power tubes:
As @syscokid reminded me, the Sovtek 5881/6L6WGC is not a 30w 6L6, but 23-26w depending on where you look.
Bias accordingly.

I mistakenly biased mine based on 30w max and ran it for quite a while with no ill effects before re-biasing based on 23w.
 
Amp Mad Scientist said:
tubes out
turn the power on, leave standby in warm-up position.
Do not turn standby to operate position.
Set meter for DC volts.
Black probe attach to chassis.
Red probe connect to pin 5 of output tube socket.
What is the DC voltage on pin 5 of each output tube socket?

The pin 5 voltage is the most important voltage in the amp.
This voltage controls the output tube current.
When this voltage malfunctions, the output tubes can burn up....or worse.
This is why pin 5 voltage is critical.
Click to expand...
I will get on that after work.
 
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