TubeDepot.com

by Robert Hull

To make sure we are providing you with the most accurate information available, here is a quick recap on the two types of biasing most often encountered in tube guitar amps:

1. Cathode Bias - Cathode biased amps DO NOT require bias adjustments. They are “self biasing”. To accomplish this, a cathode biased amp has a large wattage (think big) resistor installed between the power tubes and ground. The louder this type of amp is played, the more positive a voltage is developed across the cathode resistor.

This positive voltage controls the overall conduction of the power tube, keeping it within a safe range. In this way, the power tubes in a cathode biased amp are able to “self bias” themselves.

Amps utilizing this form of bias are most often class “A” amplifiers.

2. Fixed Bias - Fixed biased amps DO require bias adjustments. They require a separate negative voltage to be applied to the control pin of the power tubes to determine the level of current flowing through the power tubes. This voltage is referred to as the bias voltage.

This bias voltage can be either preset by the manufacturer (as in Mesa-Boogie and most Peavey amps) or it can be variable through installation of a small potentiometer control (Most Marshall and Fender amps). Either way, this voltage is “fixed” to a specific level according to the desired idle current for the power tubes.

Keep in mind, this bias voltage is NOT the actual bias. It is only a voltage used to control the idle current of the tube in much the same way as a spigot is used to control the flow of water from a faucet. Therefore, by making this voltage more negative, the current through the power tube decreases. Likewise, by making this voltage less negative, the current through the power tube increases.

Amps that utilize this form of biasing are most often class “AB” amplifiers.

If you have questions, let us know.

Crackling, Rattling and Popping

“Snap, crackle, pop.” Great for cereal, but not what you want to hear from your amp. But what if you do? Let’s look at the causes of crackling, rattling and popping and find out what you can do to make them stop.

Crackling

Crackling is like static, only more severe and with more pop.

Troubleshooting
You can find the source of crackling as you would with static. It often has same root causes. It can also result from absorption of humidity by the resistors, from an arcing transformer or socket or from a broken component.

Rattling

Rattling takes two forms - electronic and acoustic. Each comes from a different source.

Electronic

This rattle often originates with the amp speaker, a power tube or the rectifier tube.

Troubleshooting
Use the same methods you would to find the source of static or popping.

Acoustic:

Acoustic rattling is almost always cabinet-related. It can come from loose panels, name plates, mounting screws, or even from fractures in the cabinet body.

Troubleshooting
The best way to locate the source of acoustic rattle is by close inspection of cabinet. Check all joints, screws and other parts that might come loose. Also, inspect the cabinet for hairline fractures or other cracking.

Popping

The sound of popping is similar to crackling, but distinct and intermittent. It can originate from the rectifier arcing, a socket arcing, bad connections or bad sockets, and arcing of components.

Troubleshooting
The best way to isolate the cause of popping is to use stage isolation. You do this by methodically removing and replacing your pre-amp tubes, beginning with your phase inverter tube. You can find more about this technique here.

Rattling, crackling and popping are not unusual in an amp that gets a lot of use. Like any piece of equipment, it will suffer from wear and tear. However, you will save time and money by learning how to isolate the causes of these problems yourself, even if your technician is the one to fix it.

Good luck. Next week is the final installment of this series.

You turn on your amp only to hear a hum coming from it that you weren’t expecting. What can you do?

Hum in an amplifier comes in two types:

60 cycle hum

This type of hum more often than not originates outside of your amplifier. However, it can also come from the filament supply, a burnt bias pot or resistor or from the filament and grid wires being too close to one another.

In a fixed bias amp, 60 cycle hum usually comes from a bad bias supply filter cap.

120 cycle hum

Unlike 60 cycle hum, this type of hum does originate within the amplifier. It most likely comes from bad filter caps or a coupling cap becoming conductive.

Troubleshooting

Rob Hull Custom Guitar Cable

To determine if your amp hum has an outside cause, unplug your guitar cable from the amplifier. If the hum stops, you know the amp isn’t the problem. Instead,  your cable or input jacks and plugs could be the culprit.

If this is not the cause of the hum in your amp, you can do a visual inspection of the components to look for burnt resistors or bias pots.  Also, check the filament and grid wires proximity to one another.

If the hum is 120 cycle, you can find out if a coupling cap has become conductive by checking the plate voltage of the tubes. One of them will be noticeably low compared to the others.

Disclaimer: Unless you are an experienced amp technician or trained in electronics, please do not attempt to make repairs on your amplifier yourself due to risk of serious injury, including electric shock and burns.

Source: Weber, G. (1997). Tube Amp Talk for the Guitarist and Tech, p. 17-25. Kendrick Books: Texas

You want to use your tube amp at this weekend’s gig, but when you sit down to practice or jam with friends, you get a horrible popping noise. The noise goes away and comes back, but no amount of adjusting the knobs or cables gets rid of it. What you have, is a static problem.

Causes of Amplifier Static

Tube Amp

Some of the causes of static in your amp may include:

• Loose connections
• Bad solder joints
• Arcing capacitors
• Bad resistors
• Faulty ground
• Arcing between components

Of course, other causes may exist.

How do you find the cause of static?

The two best ways to find the source are the same as those used to find the source of hum.

The process of elimination

First, you can use the process of elimination. You do this by methodically removing and replacing your pre-amp tubes, beginning with your phase inverter tube.

The chopstick method

Begin with a visual inspection of the components. Sometimes that is enough for you to see if a resistor is burnt or a capacitor is damaged. If you don’t see anything upon visual inspection, you can use the chopstick method. This method allows you to find the source of the problem without the risk of shock or burn. It is also simple and inexpensive to do.

To conduct this test, remove the chassis from the cabinet. Hook up a speaker and turn the amp “on” and set it to “play” mode. Turn the volume controls up all the way. Using a wooden chopstick to probe, gently poke the components at their joint, wire and component connections. If a component is loose, you will hear noise when you poke or tap the resistor.

Make sure you check every component, because the noise may come from the resistor you are tapping, but one close to it. Go through the components in the same systematic way you did with the pre-amp tubes.

Next steps

Once you are absolutely certain you have found the source of your static problem, you can fix it.

If you cannot find the source of the static in your amp, or if you are a novice, take it to a reputable technician for repair.

Source: Weber, G. (1997). Tube Amp Talk for the Guitarist and Tech, p. 17-25. Kendrick Books: Texas

Hissing Noise

This noise usually occurs whether or not there is signal to the amplifier. Additionally, it rises and falls as the volume is raised and lowered. This type of noise can have one of two causes:

Pre-amp tube problem

If you suspect that the problem originates with a pre-amp tube, the best way to rule it out is to exchange the pre-amp tube with one you know does not hum. Be careful that you aren’t simply using a low gain tube, however. Good pre-amp tubes have good gain, good tone and do not hum.

How can you know if it might be the pre-amp tubes?

Start by having the amplifier on and in play mode, then remove the phase inverter tube. This is the pre-amp tube closest to the power tubes. If you no longer hear the hum, it likely originates with your phase inverter tube or other pre-amp tubes. Now, you want to use a process of elimination. Replace the phase inverter tube and remove the pre-amp tube next to it. If the noise continues, it is your phase inverter tube. If not, keep removing and replacing the pre-amp tubes in order to discover which one is creating the noise.

Plate load resistor problem

Begin with a visual inspection. An arcing plate load resistor may look charred or burnt.

The Chopstick Test

If you cannot visually detect the bad resistor, you can use the “chopstick” test. To conduct this test, you first remove the chassis from the cabinet. Then you hook up a speaker and turn the amp “on” and set it to “play” mode. Turn the volume controls up all the way and using a wooden chopstick to probe, gently poke the components at their joint, wire and component connections. If a component, such as a resistor, is loose, you will hear noise when you poke or tap the resistor.

Check every component, because the noise may not originate with the exact resistor you are tapping, but the one closest to it that is reacting to the shock. Go through the resistors in the chassis systematically as you would the pre-amp tubes.

These two methods should eliminate the hum in your amp.

Source: Weber, G. (1997). Tube Amp Talk for the Guitarist and Tech, p. 17-25. Kendrick Books: Texas

TubeDepot.com is proud to announce a new video:

What is a Microphonic Tube?

If you worry that your amp might have a tube that has gone microphonic, but you just aren’t sure, check out this video. In a little over two minutes, you can gain a better understanding of tubes and microphonics to help you diagnose problems with your own equipment.

Do you have suggestions for other videos you’d like to see?

Let us know what you think.

The short answer…between 600 - 700 degrees. But there is more to this than just a number.

For most of us, our first impression is to use as much heat as the iron will provide. Where as this may sound plausible, this isn’t the best case. In my experiences, most technicians will have their soldering irons set too hot or will be using a iron over-rated for the job they are working on. And believe me, more heat does not equate to better connections. For the most part, the exact opposite applies.

Here are a few of the bad things an overly hot iron can do:

1. An iron running too hot can permanently damage electronic components. Electronic components absorb soldering heat and if the temperature is too high, this heat could melt the internal connections in the component. At the least, make the component unreliable and prone to future failure. And unsoldering and removing a component is always harder than putting it in.

2. Soldering tips run at elevated temperatures have a very shortened life span. Where as a properly run tip will potentially last many years, a tip run too hot will often only last a few months before it has to be replaced. And with steel plated tips, they cannot be reshaped, they have to be replaced.

3. Soldering heat will melt most wire insulation. The hotter the iron, the quicker the melting. Some wire insulation will almost instantly drip off with even the shortest application of intense heat. And once the insulation is melted, it can’t be reinstalled. The wire must be replaced.

4. An overly hot iron will instantly vaporize the flux core of most solders leaving a connection without any flux. Flux, whether from the solder core or added separately makes connections flow much better by boiling off the oxide layer on the components and solder. Without the flux, the oxide doesn’t boil off to expose the bare metals underneath. No bare metals means the solder won’t stick, and this is unbearably frustrating.

5. An overly hot iron not only vaporizes not only the flux but also some of the lead and tin in the solder. This instant cloud of fumes is extremely dangerous to inhale (lead fumes / flux fumes) and the smoke screen left behind is difficult to see through for detail work. All I can say is … ewwwww.

So, how can one tell if the soldering iron is too hot?

The quickest method is to turn the iron on and let it reach its highest temperature. This usually will take 20 minutes for most irons. For the larger irons, the time maybe more but usually 20 minutes is sufficient. Now after the iron has warmed up, gently wipe the hot tip across a moist sponge and this should leave the tip clear of oxides. If the iron is too hot, this clean surface will quickly turn blue-ish and then black. A soldering iron should never be blue or black as these are signs that the tip is way too hot. Another indication is that when solder is applied to a tip that is too hot, the solder splatters and quickly smokes. Solder should flow smoothly and consistently onto a properly heated tip and the flux should vigorously boil on the surface and not instantly vaporize into a noxious cloud.

If you find that your soldering iron tip is too hot, here are a few hints:

1. If you own a variable temperature iron, turn down the temperature. Keep turning it down until when you wipe the tip, it doesn’t turn blue. It should just idle, and turn gray a little. You will probably have to adjust your soldering technique.

2. For the plug-in type, set temperature irons, you will have more struggles getting the temperature down. You can install a light dimmer type control between the iron and the wall to vary temperature. However, I suspect the cheaper route is to just purchase a lower wattage iron.

Summary

Soldering is similar to a dance between the connection and the technician’s solder. We hold the solder in our hands, and apply the heat, and the connection either dances with us or turns us down. My goal over the years of this “dance” is to marry the two happily together. I’ve been very successful so far and I hope that my experiences will help you achieve the same kind of success in your dance. As always, let me know of your thoughts.

Small Flush cutters; Small Needle-nose pliers; Wire strippers, 26 awg - 16 awg
A good solder connection is a direct reflection of the skills of the technician and the quality of the tools. These three items are essential for preparing wire or components for soldering together. I strongly encourage buying very good quality tools here. Prepare to spend 20+ dollars on each of these. These three hand tools are the most used and therefore deserve attention to quality.

Needle Nose Pliers

Flush cutters are not the same as normal cutters. Flush cutters are able to cut closer to the surface of a connection than typical cutters. As well they leave a flat surface on the wire when cut (hence “flush”). Flush cutters should have hardened steel edges (for years of use) and should never be used to cut any hardened steel. Never cut guitar strings, beading wire or even thick copper wires with your flush cutters because a dent will be left behind that probably can never be filed away. A 2 dollar guitar string can easily ruin a 20 dollar set of cutters.

Needle nose pliers are used for picking, bending, shaping, anything where fingers aren’t effective. I recommend a long needle nose for reaching into tight spaces and non-scored, smooth jaws to reduce potential damage to wires and components when holding. I usually file off the scoring as well as I file the ends to a very sharp point so as to look like tweezers. These pliers are not used for tightening anything, only for bending and holding wires and components.

The wire strippers should be very solid and designed only for wire stripping purposes. Multi-tools that do stripping and cutting and crimping are to be avoided. The cutting edges should come together very tightly and be noticeably sharp. Keeping in mind, the goal is to cut just through the insulation without cutting any of the wires underneath.

Shop light
Sadly, the average do-it-yourself technician includes this item as an after thought. Good lighting deserves better because good soldering is made under good lighting. Since most do-it-yourself work surfaces are something akin to the dining room table, the standard kitchen lighting is not adequate for the detail needed for good soldering. And as with any good tool, a good light is not cheap. Expect to spend $50+ on a decent light. I recommend a light with a long, spring loaded arm with as many metal parts as possible, especially the clamp and thumb screws. These two high stressed items when made of plastic, will eventually break. And when they break, the light becomes very frustrating if not impossible to use. A magnifier is nice but its size can often get in the way. A light with a shade handle allows grasping without getting burnt. And I still haven’t decides which I like better…florescent or incandescent lighting. They both look great.

Summary
For those reading this, we all share the common joy in making things. Not that the end gizmo isn’t great, but we see the excitement in the construction details as much as the finished product. Soldering is part of that glorious travel and good tools and techniques make the journey enjoyable. So sit back, place your table and seat backs in their upright positions, pull out the soldering irons and flush cutters, and prepare to build some useful stuff. Although, high powered CO2 cutting lasers are questionable, I do like the blinky LED things.

by Robert Hull

Vacuum solder extractor
In addition to adding solder to connections, it is sometimes necessary to remove solder as well. Especially when there is too much solder or more often, when the solder is old and too contaminated with oxides to flow well. This tool is perfect for removing larger amounts of solder from eyelets, terminals, circuit boards etc. The principle is very simple: A small tube with a spring loaded plunger finished off with a heat resistant narrow tip. The plunger is pushed down and locked into place and the tip is placed at the point where the solder is to be removed. When the solder is molten, the button of the solder extractor is pressed in, releasing the plunger creating a vacuum in the tube. This vacuum pulls in the molten solder leaving a semi-clean spot where the solder used to be. The extractor can be reused and cleaned out easily. In conjunction with solder braid, most connections can be easily cleaned of any solder. I recommend getting the metal construction to alleviate any potential for damaging static build up.

Solder braid

Solder Braid

This is the complement to the vacuum solder extractor. The solder braid is for removing small amounts of solder and for cleaning residual solder. This works so well…it just absorbs the liquid solder like a sponge. Leaving behind a clean spot, lightly tinned with the old solder. Afterward, the used lengths of braid are cut off and disposed. I prefer the types with a light coating of resin solder already applied into the braid. This makes the solder flow faster into the braid. Otherwise, it is best to load the braid with just a dab of liquid flux prior to use. I find the .1 inch width (#4 size Techspray) the most useful, and I purchase in lengths of 25 or more feet.

Orange wood stick; dental scraper/probe; hobby knife
These are all items used to either prepare a spot for soldering or inspect a solder connection. These may seem trivial, but when you need something to scrape or poke or trim, these are just the items.

The orange wood stick is useful for moving components and wires to inspect the quality of solder connections. These are made from very durable wood that is non-conductive and long enough to keep fingers away from energized equipment. The orange wood stick is a specific size and length and amazingly strong and thankfully very inexpensive. In addition to the orange wood, I also have various lengths of recycled chop sticks that work well too. And all can be reshaped with a pocket knife as the ends get deformed.

For difficult scraping, such as burnt circuit board or removing the solder mask from circuit runs, I have a couple set of dental tools. I have some scrapers, probes and carvers all in a small vinyl pouch. These are usually all metal and made from stainless steel and can be found at various hobby stores. I do recommend sharpening the edges prior to use as they often don’t come very sharp.

The hobby knife is great for general cutting and scoring. Perfect for shaping the orange wood sticks and sharpening pencils as well as removing PCB material quickly. These are wonderfully sharp and I often use them in conjunction with the dental tools to remove burnt circuit board material or for stripping insulation from instrument and microphone cables.

Heat sink tool
One of the most underrated tools that every soldering bench should have. This tool has saved many a solder job from melted insulation death. The temperatures used during soldering are very high, much higher than most wire insulation is rated for. This is the tool that will allow soldering to inexpensive PVC insulated wire without making a mess. Every time I build a guitar cable with 1/4″ Switchcraft connectors, this tool gets used. Not only will it hold the wire where I want it, the instrument cable is protected from melting like wax. Not bad for an inexpensive solution.

by Robert Hull

Cleaning brushes
There are two types of brushes I use regularly, natural bristle brushes and plastic bristle brushes. Each are indispensable when needed.

Cleaning Brush

The natural bristle brushes also known as acid brushes, come in various sizes. I prefer the 6″ handle length with 3/8″ width bristles. I buy them in bulk and as they get worn they are just thrown away. The bristles can easily be cut shorter or to a particular shape for different cleaning jobs. These brushes are static free therefore they are safe to use around ESD sensitive components. The slightly aggressive nature of the bristle ends make short work of most surface corrosion and flux residues without damaging circuit boards, runs, or components. This is my brush of choice.

The next brush I use regularly is the nylon bristle brush. These are recycled tooth brushes and their soft, thin bristles are better suited for getting into small crevasses and around small details. The bristles aren’t as aggressive as the acid brush and therefore better suited for general cleaning. Keep in mind, their synthetic construction can build up static and therefore I don’t recommend using around static sensitive components.

Cotton Swabs
These can be the drug store cotton swabs but I’ve found the technical swabs much better at holding together while in use. And the long wooden handle is a big help as well. These are usually purchased in lots of 100 or more and can be found as either single ended or doubled ended swabs. Either one is fine. Swabs are great for cleaning various types of input jacks and for absorbing contaminated solvents after cleaning.

Paper towels
These are great to have close at hand for there is always some sort of general cleaning when dealing with electronic equipment. Whether its the outside of a piece of equipment or cleaning off dirt from a face-plate or knobs or even cleaning dirt off of hands, having good quality paper towels is a must. I use paper towels most often to absorb the dissolved flux residues and corrosion while cleaning. I recommend purchasing the most durable shop towel you can afford. Time is wasted cleaning up the shards of disintegrated cheap paper towel after cleaning; it’s just better to get the higher quality towels.