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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.

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.

by Mary Klaebel

Western Electric is a respected name in the vacuum tube world. However, there is more to this company’s history than tubes. A lot more.

The company had its origins a dozen years after the introduction of the telegraph. Originally known as Gray & Barton in 1856, the company became a partner with Western Union in 1872 and changed its name to Western Electric.

On the cutting edge of communications for its time, Western Electric’s main role was to produce telegraph equipment for Western Union. However, this was not all that they did. At the Centennial Exposition in Philadelphia, Western Electric won five gold medals for products such as various types of alarms and a mimeograph pen.

In 1876, Western Union turned down the opportunity to own the telephone patent, not seeing much of a future for the new device. Bell ended up with the patent. Western Electric began manufacturing telephone equipment for Bell, and then for Western Union when they established American Speaking Telephone Company.

This led to a legal battle between Western Union and Bell, which resulted in the smaller Bell company winning the patent dispute and also an exclusive contract with Western Electric. Bell formally purchased Western Electric in 1881.

In the ensuing years, Western Electric pioneered a number of products and was one of the first companies to enter into a joint venture with Japan’s Nippon Electric Company.

Even today, Western Electric’s reputation for quality stands.

Robert Hull
Director of Technical Services
TubeDepot.com

10. CMOS Cookbook and TTL Cookbook - Don Lancaster / Sams IC Op-Amp Cookbook and IC Timer Cookbook - Walter Jung / Sams

Actually these are four different books all printed by the Sams corporation. These great authors skillfully take the cryptic nature of typical manufactorer component data sheets and describe circuits with real world applications. Don’t be put off by the digital discussions in some of these books for inside are hidden secrets to analog switching and matrixing. As well as some unusual analog uses for digital circuits. I have used the CMOS Cookbook more than any other, quickly followed by the IC Op-Amp Cookbook. These two being my “must have” of the group.

11. Bonus … the parts catalog.

Could anything be better than free fun? Hard copy catalogs are always good for looking at product pictures and comparing pricing. Nowadays, the internet is my first choice for researching components, however, it is still nice to smell the fragrance of a good catalog and hear the pages turn. Great bathroom reading…

These are a few of my favorites:
Mouser Electronics http://www.mouser.com/
Digi-key Electronics http://www.digikey.com/
MCM Electronics http://www.mcmelectronics.com/
Newark Electronics http://www.newark.com/
Jameco http://www.jameco.com/
MSC Industrial Supply http://www1.mscdirect.com/
Parts Express http://www.parts-express.com/
Magic Parts http://www.magicparts.com/
New Sensor http://www.newsensor.com/
Antique Elect. Supply http://www.tubesandmore.com/
Mojo Electronics http://www.mojomusicalsupply.com/
All Electronics http://www.allelectronics.com/
Allied Electronics http://www.alliedelec.com/

Summary

There are so many good books that I have enjoyed over the years and picking just a few has been quite challenging. This narrowed down list is what I felt a budding reference library would do well to have. I encourage as much exploration as possible to find books that feed your individual interests. And thankfully, since electronics and the basic concepts are universal, old books are often as valuable as new ones. When dealing with tubes, old books may be the best if not only resources. As always, if you find an electronics book that sparks your interests, by all means let fellow technicians know. That way we all benefit.

Robert Hull
Director of Technical Services
TubeDepot.com

4. Essential Characteristics - GE tube manual (reprint)
This is another Tube Manual but from the General Electric (GE) perspective. This is nice because it lists several tubes that aren’t easily found in the RCA manual. I also think the data layout in this book is easier to locate and read than in the RCA version.

5. Handbook of Electronic Tables and Formulas - Howard W. Sams (or equivalent …)
In the electronics book market, there are many “technician handbooks” filled with formulas and conversions for all occasions. Of those books, I’ve found this Sams version the most durable (great covers) and filled with the best information. When there is some circuit I’m working on and I need an obscure equation for some mathmatic resolution, this book will have it. I have similar Navy handbooks that cover many Mil-Spec items of interest but the Sams book is the one I keep on my bench.

6. The Tube Amp Book - Aspen Pittman
Years ago, I remember buying the second edition of this book. Holding this book was like holding the holy grail of tube guitar amp knowledge. I read it over and over until the book literally fell apart. It had just about everything I was looking for all in one place. Today, those angels still sing over this newest version (5th?), a huge hard backed book with enormous easy-to-read schematics, top quality photos, and all the same great information…and with a CD ROM! Yea…we all should have at least one of these.

7. Getting Started in Electronics - Forrest M. Mimms III
This was the first book on electronics I ever remember buying. I “discovered” it at the local Radio Shack in the early 80’s while attending Navy electronics training. Thankfully this book and its writer, Forrest M. Mims, III made the challenging world of electronics accessible and best yet, fun. With the help of this book, I survived the Navy’s training (thank you Forrest!) and it sparked a love for electronics that continues to this day. Especially good for beginners, I strongly recommend this book for anyone looking to feed their electronics interest. In just 128 pages, Mr. Mims with his simple writing style, great explanations and numerous easy-to-make circuits makes the magic of electronics seem possible.

8. Modern Electronics Circuit Reference Manual - Markus / McGraw-Hill
This enormous red book is far from modern (1980), but many of the circuits inside are amazingly relevant. This book, as do many similar styled books, cull together circuits from various manufacturer data sheets and electronics magazines and repackage them into a single source. This book is very helpful for getting ideas flowing when designing something. The components listed are often obsolete so some replacement research will be required. But this isn’t a bad thing because any researching will pay off by strengthening ones electronics understanding. Therefore, I still use this book…torn, tattered, and wonderfully big.

Beginner's Guide to Tube Audio Design

9. Beginner’s Guide to Tube Audio Design - Bruce Rozenblit
This is a relatively new addition to my “at reach” books list. I received this book as a promotional gift with a magazine subscription and was amazed at its quality of deep tube knowledge. I found that once I started reading, I didn’t want to put it down. And by the time I reached the incredible projects at the end, I felt like a soldier prepared and ready for creative battle. Definately worth having.

by Robert Hull
Director of Technical Services
TubeDepot.com

The Calling …
My first tube amp was a 100 watt, orange colored 1971 Marshall Super Lead head. I bought it for $500 in Jacksonville, FL in 1986 from a funky little music store run out of a house/recording studio. At the time, I was in the Navy stationed there and I had outgrown my 35W solid state Fender combo. Wow, that Marshall was beautiful. Orange covering with gold trim fitted with matching glowing tubes. It was in perfect condition except…someone has modified it (badly) for 6L6GC’s. But hey, I thought to myself, I’m an electronics technician working for the worlds greatest Navy…surely I can fix it. So I thought and so began my quest.

The Quest …
As I started out on this tube amp trek, I quickly realized that my newly acquired Navy training had only taken me so far. I needed far more knowledge than I could get from the Navy. Not being an overly bright 20 year old, I began trolling the book stores and libraries for information. In a very short time, I was quickly over-whelmed with the amount of material written about electronics. There was no shortage of books on computer programming and digital electronics, but very little about analog electronics and practically none regarding tube audio. Being undeterred, I took what little money I could scrounge and I began buying books. Anything that looked good…I would take it home. If it had a circuit or two that was interesting, and I had the available funds, the book found its way to my barracks. Sadly, 90% of those books were eventually thrown out or given away (remember, I earlier mentioned my lack of brightness). It wasn’t until a few years later that I made a nervous phone call and asked the question…”what electronics books do you recommend?”

The Help …
The someone I called was Andy Marshall, and his company, THD Electronics, was a front runner in the boutique amp revolution. He had recently written an influential article in Guitar Player Magazine (November 1991) about tube technology so I figured he could answer any tube related question I could ponder. Now, I don’t know why he would have answered the phone, but on that fateful day, he picked up the phone and pleasantly answered my silly, childish questions about tube electronics. Now I don’t remember most of what I asked or even what he said except for my one question…”what electronics books do you recommend?” That question and his recommendations I remember vividly.

And Now
That question, “what electronics books do you recommend?” is the focus of this discussion. This is by no means an all-encompassing list of books. These are books that I have found useful in pursuing my electronics interests. Most of these books deal with solid state electronics, but keep in mind, Ohm’s law works the same for everything. And it is the application of Ohm’s law in both tube and solid state electronics that keeps me, and hopefully you too, interested from this day forth.

Here are the first three books on my list:

RCA Receiving Tube Manual

1. The Art of Electronics - Horowitz & Hill
This is the book that Andy Marshall recommended first, and for all the right reasons. It teaches the often abstract concept of electronics and then turns around and puts real uses on it. Beyond the basics, I learned more from this book than any source. Thank you Andy…

2. RCA Receiving Tube Manual, RC-30

This is the second book recommended by Andy Marshall. There were several versions through the years and the RC-30 version is the easiest to find. The RC-30 version is still in print and can be purchased from various sources. The projects in the back are timeless.

3. RCA Radiotron Designer’s Handbook, 4th edition - F. Langford-Smith
This is the third book recommended by Andy Marshall. The 4th edition being the huge red binder version and the 3rd edition being a much smaller, hard back version. I can’t say enough great things about both of these books. They cover 360 degrees of tube knowledge, with the 4th edition far surpassing the 3rd edition in content if not mere girth.

On Friday, I will continue this list. Stay tuned….