# Defective Tube?



## SCREEM (Feb 2, 2006)

I have a Peavy classic with 4 el84 in the power section, one of them is glowing brighter than the others. When I hit a powercord it glows even stronger. I'm also hearing a scratchy garbly sound even when unplugged from the amp, not loud but anoying.

Bad tube???


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## Jeff Flowerday (Jan 23, 2006)

SCREEM said:


> I have a Peavy classic with 4 el84 in the power section, one of them is glowing brighter than the others. When I hit a powercord it glows even stronger. I'm also hearing a scratchy garbly sound even when unplugged from the amp, not loud but anoying.
> 
> Bad tube???


Definately a bad tube, but it could be preamp tube just as easily.


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## SCREEM (Feb 2, 2006)

pretty sure its a tube as well, I'll oder a set of tubes from eurotubes they have amp specific options/pkgs.


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## SCREEM (Feb 2, 2006)

new tubes = problem solved...sounds tighter and smoother than the original tubes as well....a beast of a little amp evilGuitar: 

I was trying it out at the office after hours and one of those ugly white ceiling tiles fell


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## Wild Bill (May 3, 2006)

*tubes - bless 'em!*

Those EL84's are pushed kinda hard in amps like your Peavey. The extra glow you're seeing is the screen in the tube carrying so much current it's starting to shine!

With steady gigging 2 years is pushing it for life expectancy with EL84's in such amps. The Vox AC30 can kill a new set of modern tubes in less than a year if you play out 3 or 4 nights a week.

You also might notice that when the tubes are getting old they are sensitive to vibration. Banging the case can cause bad noises and in severe cases the tubes go microphonic where they respond to the acoustic vibration from the speaker and actually feedback.

While preamp tubes can cause scratchy noises there's no way they would cause extra glowing in the power tubes. There's circuitry in between that isolates such effects. Preamp tubes are like light bulbs - they can blow at any time but often run for years. I've found 12AX7's in 30 year old Fenders that still work just fine. They handle only voltage and not true current or power - must less stressful on a tube. Output tubes work with large amounts of current and that's why they have a much shorter life in guitar amps, where the specs have been pushed way over the limit. The idea is that a customer will cheerfully trade having to buy new output tubes once in a while if it makes for a killer sounding amp!

I could follow the lead of many music stores and routinely get the player to change all his tubes including the preamp ones but it's not necessary. If the power tubes test ok but are getting kinda old I leave it up to the customer. I never replace preamp tubes unless they've truly gone bad. That way I can sleep at nights! 

---Wild Bill


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## SCREEM (Feb 2, 2006)

Thanks for the info Bill, does adding a fan to cool the tubes help with their durability?


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## SinCron (Mar 2, 2006)

People do it so my guess would be yes.


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## Wild Bill (May 3, 2006)

*Fan dancing...*



SCREEM said:


> Thanks for the info Bill, does adding a fan to cool the tubes help with their durability?


Absolutely! Heat is the enemy with tube amps! A fan can easily triple your output tube life or even more, in some amps!

The tricky part is getting a fan that is quiet, especially if you do a lot of studio work. You can add a toggle switch so that you can turn the thing off in the studio cheap and easy.

What I do is reduce the voltage so that the fan runs slower. The kicker here is that you shouldn't bother doing this with a fan that runs on AC voltage! Peavey did this with some of their amps and it doesn't work very well at all. You see, AC motors determine their speed from the frequency of the AC, which is 60 hz, or cycles per second with North American line voltage. What happens when you put a resistor in the line to the fan like Peavey did is that the fan is starved for power and it doesn't have enough torque to slice through the air as fast. The fan slows down because the air is too "thick" for the amount of torque available - lead weights on your sneakers!

When an AC motor starts it needs a much higher amount of torque to get it off its ass and moving. That limiting resistor hampers that action. So the Peavey keeps coming into the shop with the complaint: "It takes 5-10 minutes before the fan starts up!" Changing the fan makes it worse. Every brand of fan is slightly different and the value of resistor may not be right. If you're lucky you can find a compromise value that will let the fan start and then come up to a slow and quiet speed in a minute or so. Some people win lottery tickets, too.  Usually there's no compromise so you yank the resistor out, let the fan run at normal speed and put in that fan-defeat switch if the player goes in the studio.

The better way is to use a DC fan. DC motors ARE voltage sensitive for speed! Every computer has a DC fan or two and you can find old computers in dumpsters these days. Now you do have to tap some DC voltage inside the amp and that means you're gonna have to know what you're doing. Most amps won't have the voltage you're looking for - 6-8 volts for a 12 volt fan. You have to modify/add a bit of stuff inside the amp.

Once you've got that DC fan running at the lower voltage it will turn nice and slow yet be quiet as a mouse! Don't worry if it doesn't push like a leaf blower. You don't need a big flow of air when adding a fan to an amp that never had one in the first place. All you need is a steady flow that will keep pushing the hot air OUT of the amp and away from the tubes. You just want to eliminate that "pocket" of hot air around the tubes and stuff.

Once you've done this watch out for drummers who fling their coat across the top of your amp and "blanket" all the heat from getting out! I had to charge $80 to one player for the repair bill... 

---Wild Bill


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## SCREEM (Feb 2, 2006)

I have a 100mm Fan that runs quiet on 110v, I salvaged it from an old photocopier, I was thinking of splicing it to my amps power cord terminals so I dont have to use 2 power outlets. I usually use the fan on the power tubes when cranking the amp, I find I get more headroom on my clean channel...less breakup with a smoother sound.


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