# Uh oh...one tube not glowing as bright as the others...



## GuitaristZ

I looked in the back of my marshall dsl50 today, and it seems one of the power tubes is not glowing as brightly as the others. Is this a sign that it will fail soon? and should I get it replaced?

thanks for you help!!! I want to take good care of my gear 


edit: I also read today that you can pull out any number of power tubes to lower the wattage without damaging the amp in any way...is this true?


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## Stratin2traynor

From my limited experience and based on my similar question a couple of months ago I think it doesn't really matter as long as it doesn't look like it could light up your whole house! But I'll let the experts respond.


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## GuitaristZ

yeah its a bit dimmer than the other 2 power tubes, so I just want to make sure.


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## Wild Bill

GuitaristZ said:


> yeah its a bit dimmer than the other 2 power tubes, so I just want to make sure.


I wouldn't worry, Mr. Z! 

First of all, tubes don't glow brighter as they work harder. They light up and that's about it! They're lit or they're dead.

Some tubes like preamp tubes may not show the glowing heater/filament that well anyway. Tungsol 12AX7's can be almost impossible to see if they're lit but if you try to pull them when they're hot you can sure feel it!

Most important, you don't have 3 power tubes in that Marshall! A 50 watt Marshall DSL has 2 EL34's and a 100 watt unit has 4 of them. Virtually all guitar amps have output tubes working in pairs or combinations of pairs.

So there's a pair of EL34's in the output stage and 4 of the smaller 12AX7/ECC83 preamp tubes. All of the preamp tubes are in the signal path. If one fails, nothing can get past it so the amp sound goes dead.

In the words of Tony Soprano: "Fuggedaboutit!":smile:

:food-smiley-004:


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## GuitaristZ

:rockon: thanks a lot Wild Bill. You are a very helpful fellow. 

Do you recommend getting my power tubes replaced any time soon?

I have used the amp maybe 20 times on stage with the volume pretty high (3 or so o'clock) but not more than that (too heavy to carry around)...and the rest of the time, I play the amp at home about 4 times a week, perhaps 2 hours each time, at low volumes.


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## Wild Bill

GuitaristZ said:


> :rockon: thanks a lot Wild Bill. You are a very helpful fellow.
> 
> Do you recommend getting my power tubes replaced any time soon?
> 
> I have used the amp maybe 20 times on stage with the volume pretty high (3 or so o'clock) but not more than that (too heavy to carry around)...and the rest of the time, I play the amp at home about 4 times a week, perhaps 2 hours each time, at low volumes.


Whenever you replace power tubes you should always have the bias checked and set. This ensures that the amp is always running in the "sweet spot" and that the tubes will have the best life. If you don't do this it will be a total crap shoot as to how the bias setting is affecting your output tubes (the big ones). If it's setting the idle current too low the power will drop and the sound will get glassy and thin. Too high and the tubes will burn out sooner.

How long should they last? If everything was set correctly a good set of tubes in an amp that gets played a couple of gigs a week should last a couple of years or more, in my opinion. The preamp tubes (smaller ones, usually marked 12AX7) do not wear out like power tubes. They handle voltage, not big gobs of current like 6L6's and EL34's. They can develop faults and need replacement but other than that they can last for years. I've pulled tubes over 50 years old from old amps that still work just fine. In your case with such light duty I would't be surprised at 4-5 years, or more for the output tubes!

As for the preamp tubes, run 'em till they die!:smile:

How do you know when they're getting weak? Well, that can be tricky. Usually the power drops off and the tone can get muffled. I tell my customers that after a year or two if they're worried about it they should bring the amp back to me. I can put the tubes in my tube tester and also re-check the bias to see if it has drifted. This is a fairly quick thing that takes about an hour and my shop rate is $50/hr. I also get a chance within that time to get a look inside the amp for a quick inspection. If I have to replace any tubes I can do that within the hour and so the only extra cost is the tubes you'd have to buy anyway.

Some amps bring the bias test points and adjustments to the back or top of the chassis so you don't have to open up the amp when changing the power tubes. Me, I pull the amp out of its case anyway even though I don't have to. I like to check inside! The amp is on my bench anyway and the customer is paying for a full hour so why not do the inspection? If your car is on the hoist for an oil change they're gonna take a quick look to make sure your transmission isn't falling out or something!:smile: Same with an amp.

You had asked about pulling tubes for less power. It's often done but you have to worry about a few things.

First off, only the power tubes are involved. That's the big ones that I told you work in pairs or combos of pairs. A 50 watt amp will have two output tubes and you can't pull one without unbalancing things and asking for trouble. A 100 watt amp will usually run 4 tubes, as a pair on each side of the circuit.

You can pull one pair. That would be the two outer or the two inner tubes, NOT two from one side! That will drop the power in half. You will have changed the load on the tubes through the output transformer.This means that if you had an 8 ohm speaker cab you should set the amp to the 4 ohm setting. If the cab is 16 ohms then set the amp to 8. 

This last bit is not super critical. With tube amps if the speaker setting is not right the tubes are usually tough enough not to care. The tone might change slightly (maybe better!) and you might lose a week or two of tube life but that's about it.

Hope this is useful!

:food-smiley-004:


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## GuitaristZ

wow...thank you Bill! I appreciate the information. Looks like I wont be needing a tube change after all for a while. thanks again


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## stimps29

Wild Bill said:


> You will have changed the load on the tubes through the output transformer.This means that if you had an 8 ohm speaker cab you should set the amp to the 4 ohm setting. If the cab is 16 ohms then set the amp to 8.
> 
> This last bit is not super critical. With tube amps if the speaker setting is not right the tubes are usually tough enough not to care. The tone might change slightly (maybe better!) and you might lose a week or two of tube life but that's about it.
> 
> Hope this is useful!
> 
> :food-smiley-004:


Would you be able to tell me how to do this or link me to a demo of someone doing this please. Thanks.


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## sysexguy

I've seen this before:

Dear Abby;

I think my wife is cheating on me. I am a working musician and, as you would expect, travel a lot. I have been noticing strange things happening when I get home. Her mobile phone rings and she steps outside to answer it or she says, "I'll call you back later". When I ask her who called she gets evasive. Sometimes she goes out with friends but comes home late, getting dropped off around the corner and walking the rest of the way. I once picked up the extension while she was on the phone and she got very angry. A buddy of mine plays guitar in a band. He told me that my wife and some guy have been to his gigs. He wanted to borrow my guitar amp. That's when I got the idea to find out for myself what was really happening. I said "sure, you can use my amp but I want to hide behind it at the gig and see if she comes into the venue and who she comes in with". He agreed. 

Saturday night came and I slipped behind my Marshall JCM800 half stack to get a good view. I could feel the heat coming off the back of the amp. It was at that moment, crouching down behind the amp, that I noticed that one of the tubes was not glowing as bright as the other three. Is this something I can fix myself or do need to take it to a technician?

Thanks,

Very Concerned.

LOL, I had to post that

Andy


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