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Peavey XXX burned components, gut shots inside..


Iron Lotus
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Okay, long story short...

I bought matched kt77s, & a few different preamp tubes.

I've tested them again after this incident, all tested GOOD.

I noticed after I bought the amp & installed tubes one power tube

stopped being under load. (No slight purple glow) A few weeks later I noticed another tube beside it did the same. Now I've been

in alot of amps, I do it safely, test with meter before starting.

Handy with soldering iron etc etc, for years. No worries.

 

So I took the amp apart to upgrade the 4 screen grids.

(even tho they looked fine) Changed them all to 5w-1k 5%

 

Noticed while inspecting that the 2 resistors were burned

on the R86 & R82. See pics. So I don't know why but I slapped

2 more of the 5w-1k in their place, not sure why I did that. But I did. Also went ahead & did the c3 mod to a .0022 nf

 

Put it all back together, double checked all connections, fuses, plugs. Powered it up. Tubes light up, seems good. No channel lights though, flipped the standby, nothing. Tubes glowing like they are under load, but still no channel lights, no sound.

 

What did I fluck up exactly? Feel like it was the 2 burned resistors that I replaced with the ones I put in there... if thats the case what needs to go there? Stock values? And why were they burned? ....not sure so thought I would ask the folks here. All fuses still good after this btw. Have the amp apart again awaiting responses.

 

Thank you!

Ps-randall satan will be here in 2 days. I'm stoked.

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This is just a wild guess - I'm not familiar with that particular amplifier.

 

Some modern tube amps use logic circuits to facilitate channel switching. These logic circuits work on low voltage and a cheap and dirty regulated low voltage power supply uses a zener diode and a power resistor. Sometimes these resistors get quite hot and can cause weird things to happen in the logic circuit. The heat can often leave burn marks on the circuit board or on the resistors themselves.

 

Since R86 and R82 are surrounded by filter capacitors I am going to assume they are part of the power supply and are quite likely used in the low voltage supply. If replacing R86 or R82 affected the logic power supply then that could be causing your problem.

 

 

In the picture above, the original R86 and R82 are 50 ohm resistors. If you replace them with 1K ohm resistors then they will not function properly. I believe that is the cause of your problem.

 

 

 

Edited by onelife
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This is just a wild guess - I'm not familiar with that particular amplifier.

 

Some modern tube amps use logic circuits to facilitate channel switching. These logic circuits work on low voltage and a cheap and dirty regulated low voltage power supply uses a zener diode and a power resistor. Sometimes these resistors get quite hot and can cause weird things to happen in the logic circuit. The heat can often leave burn marks on the circuit board or on the resistors themselves.

 

Since R86 and R82 are surrounded by filter capacitors I am going to assume they are part of the power supply and are quite likely used in the low voltage supply. If replacing R86 or R82 affected the logic power supply then that could be causing your problem.

 

 

In the picture above, the original R86 and R82 are 50 ohm resistors. If you replace them with 1K ohm resistors then they will not function properly. I believe that is the cause of your problem.

 

 

 

Indeed, great information. I figured somehow those resistors might have been the cause but didn't understand how. So I have to say thank you for your informed analysis!

 

So to the issue of the stock values being burned from the heat, & yes the underside of the circuit board beneath those two burned resistors was slightly browned as well when I removed them.

 

Is there anything I can do to prevent that from happening again? A different value perhaps, that will still allow it to function properly, or just put the stock values back there again & hope it doesn't happen again?

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why? why? why do you think you can replace some resistors with ones 20 times different the value??? *facepalm*

 

at least you put again 5w resistors in there...

the initial resistors do not look burnt to me. can you measure with an ohm meter if they still have between 45-55ohm (10% tolerance), best before you solder them in again? if so they are fine.

 

those are 5w resistors cause they have to handle a lot of current and voltage and therefore get very hot, they can put on some color and some smell when doing so. when burnt, you see a lot of black some even worse smell and that i got melted the one or the other way.

 

i can only advice, do not touch the stuff, when you don't know what you are doing. and if you must replace something, replace it with a part with the exact same specification (size, tolerance, wattage) what it is already there.

 

you can only change values of one or the other part, if you exactly know what this part is doing and you know what outcome the desired change will have.

 

this is electronics, its science and its math, so the changes are calculateable and predictable.....

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why? why? why do you think you can replace some resistors with ones 20 times different the value??? *facepalm*

 

at least you put again 5w resistors in there...

the initial resistors do not look burnt to me. can you measure with an ohm meter if they still have between 45-55ohm (10% tolerance), best before you solder them in again? if so they are fine.

 

those are 5w resistors cause they have to handle a lot of current and voltage and therefore get very hot, they can put on some color and some smell when doing so. when burnt, you see a lot of black some even worse smell and that i got melted the one or the other way.

 

i can only advice, do not touch the stuff, when you don't know what you are doing. and if you must replace something, replace it with a part with the exact same specification (size, tolerance, wattage) what it is already there.

 

you can only change values of one or the other part, if you exactly know what this part is doing and you know what outcome the desired change will have.

 

this is electronics, its science and its math, so the changes are calculateable and predictable.....

 

Thanks for your opinions.

And yeah, science is great.

Experimentz. Yayyyyy.

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I have to go with Tele on this one. It seems you've missed many steps in your analyses and are way down a bunny trail leading to disaster.

 

First off What Peavey amp are we talking about here. Different amps recommend different tubes especially on these high gain Peaveys.

 

Second what tubes exactly. Telling us its a matched pair doesn't mean jack. What brand of tubes?

 

Third, the most important step, you didn't mention a thing about biasing the tubes. You need a bias probe to check and see if the tubes are getting the proper bias.

 

Fourth, The purple glow you think is good and the absence of it as being bad is actually bass ackwards. Purple glow is a sign the tubes aren't biased properly. The lack of purple glow may mean its properly biased if there are no other mitigating problems. Cherry red glow of the elements is always bad news and a sign the tubes are cooked.

 

Fifth. Installing high wattage ceramic resistors is an unwise move. This is a tube amp with a PCB board. The standard carbon resistors will open saving the rest of the circuitry is there are output problems. Ceramic resistors wont blow, they will simply heat up and eventually bake the board turning it black. Once it does, the board is carbonized and becomes conductive instead of being an insulator.

 

I've done several amp repairs where people added these ceramic resistors and cooked the boards. I actually had to cut away the baked board and glue in a new piece of PCB to prevent the high voltage arching across it and cooking the board even more.

 

Seventh. Replacing resistors with the wrong values is about a poor a choice as you can make. I have to ask are you wanting the amp to fail? I never, ever, use substitute components unless I have the reasoning and logic for making those changes to the original design. Most of these circuits today are designed on cad programs where exact values and tolerances supplied form the manufacturer are plugged in. The completed circuits are stress tested and run in real world situations, failures are tracked and modifications are made to get the most reliable products. If the circuit could have benefited by using ceramic resistors instead of carbon, the manufacturer would have chosen them. Its a competitive market and the best designs last.

 

Amplifiers are all about properly "balancing" voltages and currents using specific components to get optimum results. You cant just go in and say this is close enough and stick a component in there. you have to know the theory in back of what you're doing and measure what the consequences will be, otherwise you wind up exactly where you are, on the rocks.

 

First rule in electronics, you run into a problem, reverse gears and undo the dame you've done. Techs are often their own worst enemy and often create the worst damage by carelessness of lack of knowledge. You get no place plowing forward like you have been compounding problem after problem. You must undo everything you've done to get back were you began.

 

Increasing the grid resistor decreases the grids ability to control electron flow. Lowering the value shuts the tube off completely. There's a balance between on an off where the power tubes produce a maximum controlled signal without clipping or shutting off. Grids need to be a lower potential then the plate to properly gate the electron flow.

 

The whole theory is a small voltage change on the grid from the preamp (small turn on a water valve) Makes a big change in the electron flow through the power tube. (Water flowing through a hose) Like many water spigots, you have the most control on water flow in the first 1/4 turn or so. Once your above the 1/2 way point you may not notice much change in the water flow.

 

Raising the resistor value, is like setting the valve so its open most of the time. You no longer have the ability block electrons flowing from the anode to cathode. Raise it too high and the tubes are running wide open, like a spigot with a broken valve that's always on.

 

This brings up a point. Excuse me for preaching here a bit, but its pet peeve of mine since got in the business. I a necessary point. Its not directed at you. Its your amp and you can do what you want with it but you may gain some useful insight for future endeavors. I do encourage musicians to learn as much as they can about repairing their own gear, but as Clint Eastwood once said. "A Man has Got to know his limitations". .

 

I work with techs all the time and because I'm their service manager I have the responsibility or overseeing and fixing their screw ups on a regular basis. There is no point where you say something cannot be fixed, you simply run out of sound economic methods of tackling the problem.

 

Along with that its my job to educate techs so they do repairs quickly, efficiently and cost effectively.

The techs you can educate have a positive attitude to learn. The ones that eventually wind up with a pink slip all have something in common. They have a lazy troubleshooting method, mostly because they aren't forced to use a systematic approach.

 

Even veterans like myself has to go back to the most basic step by step approach of finding the problems. Of course you learn where the short cuts are and begin with the most common approaches. If you have a power indicator lit, your problem is unlikely to be the power outlet, but even then it may have been the cause of the failure so you have to be open to looking at things that appear to be working and verify them using test tools.

 

I being this up because you never mention using a meter before you started swapping parts. No tech does that. he measures components before replacing them unless he has the wisdom to know its a weak area likely to fail and does it as part of a preventive maintenance.

 

Even then he knows he has a good chance of creating more problems the he solves, especially in the area you are messing with. If this is a self biasing amp you just screwed it up because the tolerance and drift of new components can be way off throwing otherwise properly biased tubes way off. A tech often has to match bias resistors just like he matches tubes to get a good balance.

 

 

Troubleshooting requires self discipline and a step by step systematic approach in ruling out what parts are good and in the process finding which are bad. If you skip a step you can skip either the entire cause to the failure or possibly a small step in a cumulative failure which spans across several components.

 

In order to troubleshoot you must be trained and skilled using test tools and know how to evaluate the results you get making those tests. This is pure science. A lab tech must measure his results to know if his formula is actually resulting in a test tube. His theory may be good but his real world results may have unforeseen consequences.

 

It may also be his real world results may be a defective formula. How can you know unless you test as you go. A blind man cannot navigate an unfamiliar room unless he's learned how to use the a blind mans tools to navigate. Electricity is invisible. You must be competently trained to navigate your way through circuits and properly use the test tools designed to test and measure this invisible force.

 

Troubleshooting is not swapping parts without testing them first. That's what's commonly called throwing spaghetti against the wall and hope it sticks to know if its properly cooked. just use a friggin timer and then you'll know if its cooked, or in the case of a circuit, get a meter out, remove the component and test it. Troubleshooting is a systematic step by step process based on deductive reasoning to both confirm what's working and what is not. You cant reliably troubleshoot based on symptoms because there are too many causes for the same symptoms. Symptoms can help point you to the causer but a tech does not put his faith into trusting them. he always looks beyond them as a safety measure.

 

You must know all the theory and math in back of the science and then develop the working hands on knowledge to build up enough experience to have a good mental image of what is actually going on in order to use test tools in an efficient manor. Even then its no guarantee you will be good at troubleshooting. I work with dozens of techs every day and have trained hundreds of others. Even after a tech has received a 2 year degree in electronics he is still not much better then an amateur. He knows some theory but has no real world hands on skill using that theory.

 

If a tech is not mentored by other skilled techs on the job he wont pick up the good habits needed to make him a good tech nor reach his full potential in the field.

 

Breaking poor habits is the hardest of all. Its why I'm ranting on here. Poor work habits are both dangerous and costly. I'd rather see someone take a little extra time and confirm he is making a wise choice them do a bunch of things that yield no results, mainly because you can introduce more faults trying the fix the original problem which is often tomes so simple you'd stumble over it if you were able to recognize it.

 

Even with excellent mentoring few are gifted enough to be a really good troubleshooter. One in twenty is pretty common. Truly gifted is maybe one in a hundred. The rest usually wind up doing grunt work being told what to do and never knowing why they are doing it. There's a Big difference between knowing what will fix a problem and guessing what might.

 

In summary, Troubleshooting is pure detective work based on finding bad components instead of bad people. Few become a Sherlock Holmes. The other half is more like being a doctor or surgeon. You diagnose the circuits illness then cut out the bad organs and replace them with good ones. Then you correct the behavior that lead to the illness, the use or abuse of the amplifier in this case of having it fail (if it even did) in the first place.

 

There are limits to where an amateur can venture without doing serious damage. You seemed to have crossed that line when you ventured beyond replacing the tubes and having the bias set (or learning how to set bias). Even if the amp is self biasing, If the amp developed an issue after changing the tubes, its an obvious sign specific tubes will work in peavey amps and others will not.

 

My advice is, Reverse everything you've done so far and keep your fingers crossed you haven't damaged anything permanently. You may get lucky and your amp may come back to life. If not then take the amp to a tech who knows what he's doing and keep your fingers out of circuits till you know what you're doing.

 

I realize allot of what I posted here isn't of much use in actually fixing your problem, but you never did mention what your original problem was. Techs must have details to give you good advice. I don't need to know where you ended up, I need to know why you even cracked the amp open because that's going to tell me more about the problem and the correct solution to fixing it. We can then backtrack out of the place you're stuck and take the proper path to resolving the problem.

Edited by WRGKMC
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I think a slight purple haze under load is normal, since the 5150, Mesa dual rectifier, peavey ultra plus, and this peavey xxx have all done it.

 

I paid 400 for the amp.

I also have or "had" an egnator armageddon that arrived last week.

And a randall satan on the way this week.

Didn't like something about the egnater voicing.

Literally returned it after 2 days & ordered the satan.

 

I highly doubt those 2 resistors messed the amp up permanently, but if they did, I'd sell the amp before wasting money taking this **************** to a "tech". Lol. I'm happy with my electronic work & I've fixed many amps for others as well. Not saying Im a trained pcb tech but im perfectly capable of doing repairs, thanks for the tech reccomendations, but no thank you for trying to be ****************************s about it. Ive never needed one in 20 years of playing & repairing, probably dont need one now. I gambled a bit with the 2 resistors simply because that's what I had at my disposal, I just don't see them ruining the amp. But I'll surely let you techsnob mind frame people know how it turns out so you can say how you were right, or I can say I did fine on my own... either way it's all in fun & games. I love working on my own stuff, always will. I picked up the stock values today & will be putting those in the amp after I get done welding what I need to at my job tonight.

 

Thanks for the input, and no thanks for the overly asinine responses. Although I appreciate both. Thanks again everyone.

Edited by Iron Lotus
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I highly doubt those 2 resistors messed the amp up permanently...

 

As do I.

 

You probably just temporarily disabled the power supply for the solid state components that are used for channel/fx switching. This could easily (and quite likely) cause the interruption of signal flow through the circuit.

 

 

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As do I.

 

You probably just temporarily disabled the power supply for the solid state components that are used for channel/fx switching. This could easily (and quite likely) cause the interruption of signal flow through the circuit.

 

 

Makes sense man. Thank you for the helpful & insightful responses, this place benefits greatly from guys like you.

 

The stock values were 50 ohm, 5 watt 10%

They only carry 56 & 47 ohm versions of 5w 10%

So, I went ahead & had them order the 56 ohms.

Should be in tomorrow, I don't see why they shouldn't work well.

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I would go with the 47 ohms because the value is closer to 50 ohms and within the 10% tolerance.

 

56 ohms is just outside the tolerance - the reason I think this might be important is that 47 ohms is a very common value while 50 ohms is not - there must be a reason why the designers chose a less common part.

 

That being said, if it works with 56 ohm resistors then that is great.

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Makes sense man. Thank you for the helpful & insightful responses, this place benefits greatly from guys like you.

 

The stock values were 50 ohm, 5 watt 10%

They only carry 56 & 47 ohm versions of 5w 10%

So, I went ahead & had them order the 56 ohms.

Should be in tomorrow, I don't see why they shouldn't work well.

 

you should be fine, have you measured the ones you removed with an ohm/multi meter?

 

if you don't have one i suggest to order one together with the resistors

 

i don't want to bother you, but what you are doing, can be very dangerous, it can burn down your amp, which can light up your house to a big born fire and cause the end of your or someone elses life, and therefore its better to be save than sorry

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I have to say the same thing. R86 & 82 didn't look bad to me from the pics. Did you test them. The leads had some heat corrosion but that's normal. When resistors go bad they usually get hot enough first to bake the lettering off before they open up.

 

Again, it seems you're trying to visually troubleshoot what's going on using symptoms, not through logic and making tests to verify that logic.

 

I'd also ask these questions. Was the amp working before you changes the tubes. second, what brand of tubes did you buy. Third were the tubes new. 4th how did you test the tubes? An inexpensive tube tester can tell you a few basic things like the heater being bad and whether its shorted but they wont tell you the complete story.

 

Back when I was a kid and knew anything I kept blowing the screen resistors in my Bassman head because I was running a high impedance cab and the head was rated for 4 ohms. The tubes would go cherry red and cook the screen resistors. I'd take the tubes down to radio shack to test them on the tube tester and they read good yet I knew the tubes were shot. I thought it was the tubes going bad on their own. I was able to get free replacements a couple of times due to my insisting they were bad but the fact was I was burning them up by running a 16 ohm load on a 4 ohm head.

 

Once I figured out what was going on, the problem never occurred again and has been running like new for the past 40 years.

 

If you're still trying to get those tubes to run, my first advice would have been to try another set. I bought a set of Ruby Tubes from MF a few months ago. I figured it was time to do a complete re-tube on the Bassman because it had been about 20 years. I put them in and within 20 seconds I heard loud popping and noise. I checked and cleaned the sockets to make sure all was kosher and tried them again. Got the bias set and tried a few guitar chords, and again, there was all kinds of popping noise.

 

These were bran new tubes. I figured the trip from china to Ruby where they test and match them, then the trip to the MF warehouse then the mail run to my house wound up shaking the elements enough where one of the screens got bent or had a bad weld so I returned them for credit.

Hopefully they will send them back for credit and not send them back out to some customer.

 

I bought a set of JJ's after that because they are noted as being very rugged and so far no problems.

 

This is just my most recent experience. Back when I sat at a bench for a living, we bought tubes in bulk. When we did a re-tube we'd pull a pair and match them ourselves. put them on a scope and make sure we were getting a symmetrical wave, burn them in and test them again. We'd get maybe 1 out of 10 tubes that just didn't work right. Sometimes they were dead, sometimes it made noise, sometimes it was so far out of specs it wouldn't bias right or it would drift all over the place.

 

Hopefully you bought the tubes new through a reputable dealer. You cant trust anything you buy on EBay or any other auction site. The tubes can look brand new and even test good but they may be completely beat. There's no way to tell you how many hours they have on them and NOS of popular tubes is completely dried up. Back when tubes were made in the US the ink they'd use to stamp them would fade as the tube was heated. This aided a technician when he visually inspected the amp in determining weather the tubes were new or old. The ink they use in China and Russia doesn't fade. I pulled the groove tubes out of my amp and it still looks like it did 20 years ago.

 

So this would have been my first advice. Try another set of tubes, preferably from another manufacturer that's noted to work well in peaveys.

Second I would have tested the voltage across the screen resistors to see if the tubes were biased in the safe zone which should be 45 volts which gives you a 37ua reading.

 

Note, Peavey has a defect in some of their bias literature. I found this warning while liking for their ratings.

 

https://www.eurotubes.com/store/pc/peavey%20amp%20page.htm

 

An update 10/09/07 We just measured the bias on a XXX in the EL34 mode and with the bias set at 42 volts using the factory test points (Peavey recommends 42.5) the bias was a whopping 57mA which is over 100% of dissipation! You will burn up your tubes with all but the coldest grades of EL34's or KT77's. With the bias set to 45 volts at the factory test points the bias went way down to a very acceptable 37mA. So in this particular amp a variation of only 3 volts at the factory test points made an incredible 20mA difference in bias!!

 

Do NOT bias your XXX ,JSX or 3120 for any EL34 type tube including the JJ KT77's using the test points! The 55 volts that Peavey recommends at the test points for 6L6GC's is on the other hand, on the cool side but using the test points is so completely inaccurate we simply cannot warranty tubes to players who use the test points to bias these amps! It was brought to our attention that there was a post on the Peavey forum and it stated that the bias on the XXX did not have enough range to get the power tubes too hot. This is not true!! There is plenty of adjustment especially in the EL34 mode to fry any tube! Moral of the story? PLEASE do not rely on the test points to bias your amp! You will be lucky to get in the same neighborhood as the ballpark, let alone in the ballpark. These are not cheap amps and the bias is SO easy to set accurately that you owe it to yourself, your amp and your tone to do it right! There are short bias probes available on Ebay and from several other sources including our own Eurotubes bias probe for 25.00 so it's a VERY cheap tool, very accurate and very easy to use

 

 

I come across this kind of stuff all the time. I had to dig really deep to find the correct procedure and voltage for my Music Man head. It had EL34 tubes in it and I couldn't figure out why the bias read 10X the amount. I later discovered the head was designed for 6CA7 tubes and the readings for the EL34 were correct. I later found reissued 6CA7's and was able to bias them to factory specs. It was only one line in an ancient blog that lead me to this discovery but it all made perfect sense after that.

 

The XXX head will run 6L6 or EL34 type tubes. It may be the person who owned the head before you had the bias set improperly based on the Peavey test points documentation that found on line.

 

It perfectly describes what happened after installing the new tubes you bought. The bias was set too high and just cooked them away to nothing in a short time. Again, I urge you to backtrack and undo what you've done to the amp. If it had carbon resistors put them back in. Those resistors act as fuses when something goes wrong. They are cheap and inexpensive to replace and may save you from blowing a power or output transformer some day. Putting in high wattage resistors where they aren't needed is like putting in over rated fuses. you may think you are beefing the amp up to make it better and it may work fine but its simply a roll of the dice. When a tube shorts you'll simply blow something else that's much more expensive to replace.

 

Not trying to be snarky or snobbish here. I truly have your best interests in mind and want the head repair to be safe on a long term basis.

 

Good luck

 

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