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

Resistors have specific values which are indicated using a series of colored stripes on the body of the resistor. Each color is assigned a value, and each stripe position determines its purpose. It takes some practice to learn how to read the values quickly, but here is a guide you can use to help you.

Begin by holding the resistor so that the multiple colored rings are at the top of the body. The picture below is one example.

The first stripe is the first number in the value of the resistor.

The second stripe indicates the second number of the resistor value.

For example, in the picture below, the first stripe is orange, which equals 3 and the second stripe is black which equals 0. So the complete number is 30.

The third stripe color indicates the number of zeros that follows the first two. In the case of the pictured resistor, that value is 0, so no additional zeroes will follow the 30.

This means that this resistor has a value of 30 ohms.

The last stripe, which is usually separated from the others,
indicates the tolerance. In our picture, the resistor has a tolerance of 5%.

30 Ohm Resistor

The Gold or Silver band should always be to the right as you read from left to right. If there is no tolerance band, the side with a band closest to a lead is the first band.

If you cannot distinguish the colors of the bands on a resistor, the only way to read it is by using a multi-meter across the leads.

Do you have any special tricks you use to help you read resistor values? Please feel free to share.

As the vacuum tube gained in popularity and use, competition arose between U.S. manufacturers and European manufacturers to produce new tube types. Out of this competition arose the EL34, the 6L6, the 6550 and the KT88.

The original KT88 was produced by GEC in 1956 also labeled as IEC/Mullard, or Genelex Gold Lion. It was the European answer to the American 6550 tubes, and was specifically designed for audio amplification. The KT88 is also one of the largest vacuum tubes in its class.

JJ-Tesla KT88

Current production of the KT88 is limited to factories in China, Slovakia, and Russia. These include JJ-Tesla, Genalex Gold Lion reissues, as well as Sovtek, Electro-Harmonix and the newly available EAT tubes. Original production, unused (NOS) KT88 tubes in good condition are increasingly rare and expensive, sometimes costing $900 or more a pair. Historically these tubes have been more popular in hi-fi stereo equipment than in guitar amplifiers, since it provides high power and low distortion. Guitar amplifier owners will regularly use them to replace 6550 tubes, particularly if they want less distortion.

The KT88 fits a standard eight-pin octal socket, has can be used in similar applications as the 6L6 and EL34, since they have the same pinout.

From the original GEC description, points about the KT88:

1. The KT88 is a beam pentode with aligned grids for maximum efficiency.

2. 50 watts output is available from a pair connected in the ultra linear circuit with auto bias and an HT line voltage not exceeding 500.

3. 100 watts output is available from a pair connected in the ultra linear circuit with fixed bias and an HT voltage not exceeding 560.

4. 25 watts output is available from a pair triode connected with auto bias and an HT line voltage not exceeding 500.

5. The mutual conductance of the KT88 is 11mA/V

6. An all-glass ring seal replaces the conventional pinch seal giving increased strength, higher rating and reduced dimensions.

7. The valve is mounted on an international octal base and has a heater rating of 6.3 volts., 1.8 amps.

If you use KT88 tubes, tell us what you think. How do you use them? Which brand do you prefer?

We look forward to hearing what you have to say.

by Mary Klaebel

RCA created RCA Radiotron to make and sell radio tubes. Westinghouse manufactured the tubes in their Indianapolis plant, which was already producing more than 20,000 tubes daily. In 1931, fearing a monopoly, the U.S Government stepped in, so GE and Westinghouse broke ties with RCA in 1932.

Still, RCA became a primary tube manufacturer with their Radiotron brand radio tubes. The company’s innovations included the octal base metal tubes and the Nuvistor for television sets. Later, RCA even teamed with Tung-Sol to produce the hi-fi KT88/6550 vacuum tube.

With the Tung-Sol partnership, RCA so completely dominated the tube market that they were able to control the prices in the United States. Only the rise of solid state components in television and radio broke their hold on the market.

In addition to vacuum tubes, RCA began developing technology for television and computers. However, they were taken over by General Electric in 1986.

Today, the original RCA trademark is owned by a French company. Known as the “Meatball,” it is used by companies such as Sony BMG Entertainment and Audiovox.

Authentic RCA Radiotron tubes are still in circulation and in demand. But with no new RCA tubes being manufactured, the prices of certain new old stock tube types are quickly rising.

RCA’s beginnings wind through the creation and development of two other well-known companies - General Electric and Westinghouse Electric. It started with one man’s passion for electricity.

RCA Meatball Logo

Elihu Thomson was born in England in 1853. He grew up in Massachusetts. Even in high school, was aware of the possibilities electricity held for the future. As a student he wrote, “There is scarcely a day passing on which some new use for electricity is not discovered. It seems destined to become at some future time the means of obtaining light, heat, and mechanical force.”

Educated in science, Thomson became a professor at Philadelphia’s Central High School. In 1880, he and fellow science professor Edwin Houston established Thomson-Houston company, selling arc lamp systems. With their initial success they expanded into new markets, purchasing Sawyer & Man Electric Co. in 1886. They made incandescent lamps using the Sawyer-Man patents.

Over time, they built a leading electrical company of the nineteenth century. In 1889, after German physicist Heinrich Rudolf Hertz publicly released his initial work with electro-magnetic waves, Thomson addressed their use for signaling through fog or and solid bodies where light could not penetrate. At the time, Thomas Edison preferred direct current technology, and considered alternate current too dangerous. Thomson disagreed. It was his experiments with alternating current that led to it becoming the U.S. standard. In fact, Elihu Thomson’s system was lighting streets in Kansas City, Missouri six months before Thomas Edison opened his first power station in New York.

By 1890, Edison’s company, Edison General, and Thomson-Houston were two of the big three in the American lighting industry which also included Westinghouse Electric Co. In 1892, John Pierpont Morgan, who financed such enterprises as Federal Steel Company and Carnegie Steel Company, engineered a merger between the Edison interests and Thomson-Houston. He named the new company General Electric.

In his five-decade career, Elihu Thomson received 696 U.S. patents on numerous electrical inventions, including arc lights, generators, electric welding machines, and x-ray tubes. He even created a practical method for measuring the electrical consumption of homes and businesses, the wattmeter.

During World War II, in order to keep radio patents under American control, General Electric was asked to take the lead in organizing an American radio company. GE agreed, and the Radio Corporation of America was formed in October 1919. Though originally meant to merge the interests of both private corporations and the government for the development of wireless communication, RCA soon moved in a new direction. They entered the world of consumer electronics.

Within six years, RCA’s consumer radio sales were bringing in ten times the revenue “wireless technology” generated.

Westinghouse, an RCA manufacturer, became the first commercial broadcaster in 1920. Their station KDKA aired results of the Harding-Cox presidential election. RCA aired the world heavyweight boxing championship by the next summer. This ushered in a new era of technology in homes around the country.

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.