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Help with a tube amp problem


soundcreation

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So I've been having a strange issue with my amp and a thorough google search hasn't turned up a similar issue to compare to.

 

So what's happening is suddenly my amp will get louder, and the distortion gets...how can I put it....fizzy and more intense and there is a total drop off of bottom end. So it's louder but it's all treble and messy distortion. It will often "correct" itself back to normal and then happen again.

 

Any ideas where the problem may lie?

 

Tubes seem to be OK.

 

Thanks!

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On my stuff it's usually a bad connection causing a partial ground and not component failure. Bad joint, oxidized plug/jack, pot trace etc. Finding those is a matter of twiddling and tapping knobs and plugs and failing (or passing) that, looking inside and tapping and poking around with a chopstick looking for the offending joint. I suppose that part would be easier with a multimeter but I'm clueless there.

 

Since your tone gets louder it could be the tone stack shorting somehow and allowing the input to go directly to the rest of the circuitry

 

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It could be any number of things.

 

The valves tend to be the most likely culprit in most valve amps issues. You said you didn't think it was them, how can you be sure? Beside a swap for known working valves, it's difficult to ascertain that.

 

Next, the fact that you describe that issue as cyclical, I'd think at a possible electrolytic capacitor fault. The cyclical nature makes me think it would be possible that a capacitor is slowly charging and discharging, possibly around the tone stack as it seems to increase treble and rob the low end. Certainly a sudden discharge could increase the volume depending on where it is in the circuit.

 

But personally, I'd start with the valves, even if they are newish.

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How bout the amp model? How bout the year made? There's a millions of different amps and not all are the same. An amp built in the 60's is going to have different failures then something built a year ago.

 

Second components rarely go bad and magically fix themselves. Issues that come and go are usually connection/cable/solder joint related. Rarely do components act intermittently and when they do its usually heat related. Tube amps are designed for high temps so the likelihood of a bad cap or resistor is low, especially if the amp is less then 10 years old..

 

You always start with the easiest and work to the hardest.

 

The items that can be abused by the user come first.

 

Swap all cords. Be sure to use only a speaker cord too. Guitar cords for speakers (if it uses one) is only designed to carry a few volts. . A speaker connection can fry them from the inside. The insulation separates, oxygen gets into the cable and then the copper tarnishes making for intermittent connectivity. Always use a zip cord dual wire speaker connector.

 

Next you may have a bad guitar cord, bad foot pedal switch, connectors etc. Plug straight into the amp with a well know cord.

 

Next clean your tube sockets with zero residue contact cleaner. -- Do not use pot cleaner----. Pot cleaner contains silicones or mineral oil lubricants and should only be used on pots. Non residue cleaner wont attract dust and cause shorts. You don't want the pins to slip from the lubricant and vibrate loose either.

 

Next you'd move to your input and output jacks. If they are plastic jacks, don't try and clean them. The contact cleaner will cause them to dry out and fracture requiring replacement. If its a tube amp they should be using metal jacks but since I don't know the amp type I can only guess.

 

Plastic jacks are usually PC mounted. One good tug on a cord and the solder joints can crack. Check the PC traces that connect the jack and re-solder as necessary. If they are metal jacks make sure the nuts are tight. you can clean metal connectors.

 

Effects loop jacks usually have one that's switched. Try a jumper cord between the effects in and out. You can also plug a guitar into the return and bypass the preamp. This will rule out half of your amp. If the problem still occurs the issue is in your power amp.

 

If you have a second amp, plug the effects send/preamp out to another amp. If the noise is still there the problem is in the preamp.

 

Beyond these basic items I suggest the amp may have a cracked solder joint. Common places are connections to the bottom of the tube sockets. Tubes heat up and the solder weakens. I've probably fixed more amps as a tech just re-soldering connections all other faults combined.

 

Tubes going bad would be the other thing to try. Don't immediately go to tubes first. Check the other items because they don't cost a dime and you have a good chance of success there.

 

If the amp uses 12AX7 tubes, number them with a felt marker then try swapping them around and see if the problem changes. You can also tap lightly with a pencil and see if you hear any crackle, static or microphonic ping. If you do the elements may be cracked inside the tube or just so loose they're failing.

 

Power tubes would be the last item on a list - but again I don't know what amp you're using. If its something like an AC30 its tubes could definitely be a problem because of their short life. Others like 6L6 or EL34's are very durable. They often give signs when they fail and they don't magically get better.

 

I've had many amps where the screen resistor connections crack and the tone can go from good to bed just like you explained. You may have a dirty switch. My music man had a dirty Deep switch and the bass kept cutting out. Dirty pots will do it. Just working the pots full turn a number of times may make it go away for awhile.

 

A common issue moving gear - a pot may have taken a physical blow and the shaft was pushed in. If you can push and pull on the shaft and it gives more then the other the wiper inside may not be making a good contact because the pot can is loose..

 

There's many other simple possibilities. What you need to do is rule all of the simple items I listed. Then and only then might you need to open up your wallet.

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Thanks very much for the information guys.

 

The amp is a Crate V1512. Probably...10 - 12 years old. 2 EL84's and 3 12ax7's. It's a pretty straight forward amp. No effects loop. Cathode biased so the power tubes are running full on all the time.

 

I'm thinking that it it may be some kind of connection issue. I have to admit my chord is pretty old (but not abused at all) and it does have it's own connection issues in terms of sometimes it needs a wiggle to work properly. Even this I think might be the input jack on the amp and not the chord, which is something I need to deal with.

 

But this seems to be an issue on top of that. When it happens, wiggling the chord doesn't have an effect.

 

I did the "chopstick" test on the tubes and I've also done some tube swaps just for tone purposes a while back, and they seem solid. I could investigate that further.

 

But I think what you guys are saying might make the most sense. Bad or dirty connection that might need a cleaning. I live in a very humid place and the input jacks on all my guitars often corrode to the point they need a bit of a scrubbing to make a good connection.

 

I'll start simple with chords and contact cleaner and take it from there.

 

Thanks again guys! It's much appreciated.

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This looks to be a PCB tube amp. The sockets mount to the board and not to the chassis which is always a weakness.

 

Since you have issues with your cord - I suggest you begin there. A cord cutting in an out may begin with that issue but it can get progressively worse and change its symptoms. I don't even try to repair them any more. For the cost of a quality connector you can buy the whole darn cord.

 

Monoprice sells these cloth covered cords with quality plugs shrink wrapped for $7.59. http://www.monoprice.com/product?p_id=601415 The connectors alone cost $6 these days. I've had one for about a year now. Other then the fact the cloth coating makes ita little stiff, its been a good durable cord.

 

 

The amp seems to have allot of plug in connections, including the wires going to the jacks. This is a weakness when it comes to intermittent issues like you're having but at least you may have a simple fix there. I suggest you get some non lubricating contact cleaner pull the connectors off one at a time, Give them a tiny shot of then reseat the connections. Do them one at a time so you know exactly where they go. make sure the cleaner is completely dry before powering the amp. Be careful pulling them off too. Some have clips that hold them in place that need to be unclamped using a small flathead screwdriver or the likes.

 

P1050882_zpsuuu1bkw1.jpg

 

Do not use zero residue cleaner on Pots. It will strip away all the lubricants and make the pots super scratchy and hard to turn. If you don't have one cleaner or the other don't substitute. You can use the poh boys method of cleaning the connections using rubbing or denatured alcohol. Use an eye dropper and work the connector a few times. You're simply trying to get rid of oxidation and the smoke and dirt the heat from the tubes cause. As heat rises, dust and smoke are drawn in through convection air currents and it coats the connections. If you got an air can give the entire amp a good blow job to everything's clean in there. A dry paint brush can help loosen any flinging dust.

 

P1050878_zpsgbqvyf45.jpg

 

The EL 84 tubes are the same power tubes used in Vox amps and they tend to have a high failure rate. If you use the amp regularly I think the lifespan is about a year on them. Sound quality drops and becomes flaky when they get old so again, they may be the cause but check everything else out first. The 12AX7 tubes tend to be super durable in comparison. I got 15 years of regular usage out of my last set in a bassman before one tube started flaking out. Of course a combo has more vibration which can cause them to go microphonic more quickly.

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This looks to be a PCB tube amp. The sockets mount to the board and not to the chassis which is always a weakness.

 

Since you have issues with your cord - I suggest you begin there. A cord cutting in an out may begin with that issue but it can get progressively worse and change its symptoms. I don't even try to repair them any more. For the cost of a quality connector you can buy the whole darn cord.

 

Monoprice sells these cloth covered cords with quality plugs shrink wrapped for $7.59. http://www.monoprice.com/product?p_id=601415 The connectors alone cost $6 these days. I've had one for about a year now. Other then the fact the cloth coating makes ita little stiff, its been a good durable cord.

 

 

The amp seems to have allot of plug in connections, including the wires going to the jacks. This is a weakness when it comes to intermittent issues like you're having but at least you may have a simple fix there. I suggest you get some non lubricating contact cleaner pull the connectors off one at a time, Give them a tiny shot of then reseat the connections. Do them one at a time so you know exactly where they go. make sure the cleaner is completely dry before powering the amp. Be careful pulling them off too. Some have clips that hold them in place that need to be unclamped using a small flathead screwdriver or the likes.

 

 

 

Do not use zero residue cleaner on Pots. It will strip away all the lubricants and make the pots super scratchy and hard to turn. If you don't have one cleaner or the other don't substitute. You can use the poh boys method of cleaning the connections using rubbing or denatured alcohol. Use an eye dropper and work the connector a few times. You're simply trying to get rid of oxidation and the smoke and dirt the heat from the tubes cause. As heat rises, dust and smoke are drawn in through convection air currents and it coats the connections. If you got an air can give the entire amp a good blow job to everything's clean in there. A dry paint brush can help loosen any flinging dust.

 

 

 

The EL 84 tubes are the same power tubes used in Vox amps and they tend to have a high failure rate. If you use the amp regularly I think the lifespan is about a year on them. Sound quality drops and becomes flaky when they get old so again, they may be the cause but check everything else out first. The 12AX7 tubes tend to be super durable in comparison. I got 15 years of regular usage out of my last set in a bassman before one tube started flaking out. Of course a combo has more vibration which can cause them to go microphonic more quickly.

 

 

Thanks again!

 

What's a good cleaner for pots? I've got some WD40 contact cleaner (not standard WD40) but it says it 'leaves no resedue'

 

I'd need to know something generic as this stuff is a little hard to come by where I live.

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Don't use WD40 - order a cleaner designed for pots & sliders from someone like Radioshack or Tandy (are they still going in the US?).

 

 

I'm using the WD40 contact cleaner for the connections but I'm not sure what to use for pots. I don't live in the US so it's a bit of a challenge.

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You can use deoxit 5 for the pots. http://www.musiciansfriend.com/pro-audio/caig-deoxit-d5s-6-spray-contact-cleaner--rejuvenator-5-oz.?cntry=us&source=3WWRWXGP&gclid=CL-Brunl8M8CFQ5EfgodakYDFQ&kwid=productads-adid^92666429307-device^c-plaid^157459348602-sku^429167000000000@ADL4MF-adType^PLA

 

Note: You can only clean pots a few times before they need replacement. Wait till the symptoms show up first, otherwise its only a matter of a short time before you're cleaning them again then replacing them.

 

If you have no signs or them getting scratchy when turned you should not clean them. Factory pots in most amps are good for 10~20 years and cleaning them strips some of the carbon away to get the contacts working again. Cleaning is a short term fix for worn pots but it also accelerates failure on pots that may have gone many years without a problem.

 

Amp pots are not like guitar pots that turned constantly and wear the carbon pads out. They are usually set to a setting and left alone. Practically no wear at all. Most of the scratchiness comes from dust and ozone oxidation not from wear. The lubricant in the cleaner has a temporary benefit of removing this dirt but it also accelerates the attraction of the same dirt you're trying to get rid of.

 

Old amps not used in awhile may only need to pots to be worked a bit to free up the dirt and cut through the oxidation. Just turning a pot vigorously several times usually brings them back to full operation.

 

Also many pots have a conductive grease on the shaft which gives them a plush feel when turning. This is stripped away when cleaning and they will first feel super loose to turn. Shortly after they may in fact become sticky to turn, especially the pots that have nylon shafts.

 

This is why you should never clean them as a preventative measure. Only do it when you know the symptoms are definitely there permanently. Otherwise you condemn them to an early death - usually within a year or so.

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You can use deoxit 5 for the pots. http://www.musiciansfriend.com/pro-audio/caig-deoxit-d5s-6-spray-contact-cleaner--rejuvenator-5-oz.?cntry=us&source=3WWRWXGP&gclid=CL-Brunl8M8CFQ5EfgodakYDFQ&kwid=productads-adid^92666429307-device^c-plaid^157459348602-sku^429167000000000@ADL4MF-adType^PLA

 

Note: You can only clean pots a few times before they need replacement. Wait till the symptoms show up first, otherwise its only a matter of a short time before you're cleaning them again then replacing them.

 

If you have no signs or them getting scratchy when turned you should not clean them. Factory pots in most amps are good for 10~20 years and cleaning them strips some of the carbon away to get the contacts working again. Cleaning is a short term fix for worn pots but it also accelerates failure on pots that may have gone many years without a problem.

 

Amp pots are not like guitar pots that turned constantly and wear the carbon pads out. They are usually set to a setting and left alone. Practically no wear at all. Most of the scratchiness comes from dust and ozone oxidation not from wear. The lubricant in the cleaner has a temporary benefit of removing this dirt but it also accelerates the attraction of the same dirt you're trying to get rid of.

 

Old amps not used in awhile may only need to pots to be worked a bit to free up the dirt and cut through the oxidation. Just turning a pot vigorously several times usually brings them back to full operation.

 

Also many pots have a conductive grease on the shaft which gives them a plush feel when turning. This is stripped away when cleaning and they will first feel super loose to turn. Shortly after they may in fact become sticky to turn, especially the pots that have nylon shafts.

 

This is why you should never clean them as a preventative measure. Only do it when you know the symptoms are definitely there permanently. Otherwise you condemn them to an early death - usually within a year or so.

 

 

Cool..thanks...

 

I was messing around with it a bit today and all the knobs turn without and crackles etc....so I think I'll just leave the pots alone.

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Slowly dying capacitor. At least, it sounds like very similar symptoms as my Music Man amp had, and that was the diagnosis. Totally different amp design, granted, so I suppose it could be something totally different. Doesn't sound like a connection or oxidation problem to me as those tend crackle and/or cut out.

 

FWIW, I have a couple of amps with EL84's. One has original tubes from 2003. The other's on its second set, going on 5 years or so.

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Piggybacking on what many have already said this problem (IMHE) usually is a bad tube or bad connection.

 

Tap on every tube with a sharpie or eraser end of a pencil and see if one of them is making a noise. If you're able to isolate the problem then try swapping a tube and repeat. If you're getting the same issue try opening the chassis (BE CAREFUL!!!) and use a sharpie to poke the wires around the socket. Many times just re-soldering a connection at the socket will fix this issue.

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Same as BG76, piggybacking off other replies, using WD40 anything is usually not a great idea for sensitive electronics. I only use CRC contact cleaner. It does tend to have a little over spray, so you may want to use some masking tape.

Most likely you have, IMHO, either a bad socket/connection, or a dying tube, in the Pre-amp. That opinion is based solely on the assumption you are running guitar - amp with no effects in between. Keeping a spare Pre-amp tube around is a great way to troubleshoot.

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I've found lots of middle aged amps with settings that don't change much just need their pots to be run through their range of motion several times (amp off) to take care of this sort of thing. I don't tend to use any spray unless I find a pot that I know is actually causing the weirdness and no amount of moving it works.

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Exactly. Don't clean them unless you're sure its an issue.

 

Someone Mentioned WD-40. You should know WD does make contact cleaner now. https://www.amazon.com/WD-40-Specialist-Electrical-Contact-Cleaner/dp/B00CMT9TFY/ref=sr_1_1?ie=UTF8&qid=1477686310&sr=8-1&keywords=wd+40+contact+cleaner

 

It is NOT a Pot cleaner, its a zero residue cleaner designed to clean charred/oily carbonized switch contacts. It could be used to clean connectors and tube sockets only. Do not use it on pots or it will strip the carbon off the resistive pad and make the pots all crackle badly.

 

Regular WD 40 is a moisture inhibitor, not a contact cleaner. If you use it on pots you'll get a temporary improvement until it dries. Then you'll have a worse problem then you began with on pots switches or connections. WD40 was developed for the military to apply on weapons. It was applied to steel to prevent moisture which could freeze, or make the metal rust and corrode.

 

The only uses I've had for it in electronics is reversing water damage. Solder circuit boards after water has gotten on them can cause the oxidation in lead to accelerate and leach onto a board causing shorts. I've used it successfully as a protectant. You apply it then remove all the excess you can get off then let it dry and it will stop the corrosion. I've used it to arrest rust on transformers and chassis too. You basically wipe and it displaces moisture and the rust stops growing.

 

Because it dries to a thick petroleum paste its the worst thing you can use on electrical contacts, especially in pots where you want good contacts.

 

Contact cleaner uses silicone and/or mineral oil which remains liquid and too thin to prevent conductance. It actually prevent sparks which is what makes the pots crackly to begin with.

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Well I think I've figured out the problem.

 

It appears it is the master volume pot after all. It's not that the pot is "scratchy" as is typically found when turning up or down. This seems to be a problem with it when the pot is "wiggled".

 

Basically the post on the pot feels "loose". So when it's at a given volume...but then moved around at that volume, all kinds of stuff happens from sound drop outs to what I described originally. A sudden burst of fizzy volume. I guess the vibrations from playing send it into it's various states on it's own.

 

So it looks like I'm going to have to try to replace this pot.

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Well I think I've figured out the problem.

 

It appears it is the master volume pot after all. It's not that the pot is "scratchy" as is typically found when turning up or down. This seems to be a problem with it when the pot is "wiggled".

 

Basically the post on the pot feels "loose". So when it's at a given volume...but then moved around at that volume, all kinds of stuff happens from sound drop outs to what I described originally. A sudden burst of fizzy volume. I guess the vibrations from playing send it into it's various states on it's own.

 

So it looks like I'm going to have to try to replace this pot.

 

Before you replace the pot, resolder the wires that connect to it and make sure it is being held tight by the nut that holds it in the chassis.

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Before you replace the pot, resolder the wires that connect to it and make sure it is being held tight by the nut that holds it in the chassis.

 

 

Yeah I'll give that a try. It's on a PC board though so I'll have to take that out to do the work. I don't really have experience with doing work like this but I'll give it a go, considering there aren't really any options around me.

 

Thanks for the suggestion.

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PC mount pots tend to crack their solder joints. If the amp looks similar to the picture I posted, you'll likely need to remove all the knobs and nuts in back of the knobs, then lift the board up to get to the trace side of the board to get to the solder contacts. You just heat the connection and give it a quick dab of rosin core solder and it should be fixed.

 

I will mention, I've had dealings with the kinds of pots that are in that picture. They have plastic housings that are sealed and died within an unusually short period of time. Because the plastic housing is sealed, they cant be cleaned.

 

I had them in a Rocktron unit and I even drilled holes in them to get some contact cleaner inside and it did nothing. If you re-solder the contacts and its still flakey, you have no option but replace it.

 

While you have the board lifted up for soldering, check the writing on the pot, in fact make a diagram of all the pots so you have it for future reference. They should have the value written on the pot some place. Amp pot values are usually in the 1k, 10k, and 100K ohm ranges.

 

Master volume pots for tube amps usually use a dual pot which can be 500K to 1 meg. It will be logarithmic too. That's very important to know when buying a new because you want the volume taper to be correct. Lets hope the thing just has a cracked solder joint. That's by far the simplest to fix.

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