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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.

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.

by Robert Hull

The Silver Lining
Thankfully, the best tube tester ever built is what we all carry with us … our own hearing. Our ears are truly amazing in their ability to detect even the smallest and most subtle aspects in sound. With the years that I’ve personally invested in listening, as I’m sure you have noticed too, it is our hearing that is often the deciding factor regarding whether a tube is good or bad.

Final Thoughts
Don’t throw out that tester. Have your particular tester inspected and calibrated by a qualified technician. Additionally, put the tubes and the tester through their paces and make decisions based on comparisons with known good units / tubes. The bottom line is, it would be a shame to dispose of otherwise wonderful tubes because of an inaccurate tester or poor testing procedures. And by all means, trust your own hearing. Until next time, keep listening to the music. And if you have a tester, you are lucky indeed!

by Robert Hull

Tube Tester Vs. Nature
As stated earlier, age can be a cruel master when it comes to electronics. These wonderful testers of yesterday have had years of exposure to humidity, heat, cold, dust, debris, anything you can think of. From meters that read incorrectly because of weakened charges on their internal magnets; resistor values drifting because of moisture absorbency, leaking capacitors due to dielectric evaporation, anything is fair game in regard to what can go wrong internally to an old tube tester. Keep in mind, the best measurement is only as good as the worse accuracy of the test equipment used. To insure good results from an older tester, calibration by a reputable repair / refurbishing company should be performed. Otherwise, the results are merely arbitrary. Having once repaired avionics and aircraft instruments, all the test equipment I used was required to have an up-to-date calibration. If a piece of equipment was out of calibration, it was considered automatically broken and its measurements inaccurate. With that said, if a vintage tester is fresh off of an auction site or out of someone’s attic or garage, until it can be proven accurate … it probably isn’t. By having it tested and calibrated, you will save money knowing that the Telefunken tube you are throwing away is really bad, and not just an inaccurate reading.

Tube Tester, Yugo or Cadillac?
In addition to calibration, the methods which older testers employed to reach their results were as varied as there were manufacturers. Some of these methods were incredibly accurate, where as other methods were questionable at best. Here is where it is important to have a good quality tester to begin with. Additionally, having a set of known good power and preamp tubes is critical as well. Referring to an earlier statement, “the best measurement is only as good as the worse accuracy of the test equipment used”, better quality tube testers utilized several individual tests per tube. Multiple tests are always preferable in determining whether a tube is truly good or bad. The tester that just provides a “go / no go” reading isn’t offering much in the way of thorough testing. And those remaining RCA long plate 12AX7’s deserve the best we can offer.

The biggest aspect of power tube testing involves plate voltages. Most testers of the era test power tubes at plate voltages much lower than what would be experienced in actual operation. Often these plate voltages don’t exceed 300 Vdc, usually hovering around the 200 - 150 Vdc range. Power tubes react much differently when exposed to higher voltages. You can imagine how the added stresses of high voltage can make any questionable aspects light up … often literally. With this in mind, any set of power tubes matched on a tester at some lower voltage will probably not be matched when faced with the force of an amp’s full plate voltage! The same is true for emissions testing and “life” tests.

As for preamp tubes, emissions testing is only one step in determining a good preamp tubes. What is needed is good transconductance testing. Where most testers were acceptable for plate voltage levels, they were often inadequate in how they measured transconductance. This measurement is considered critical (as it should be) in judging a tube’s integrity. Some older tube testers were designed for testing transconductance of preamp tubes with longer plate structures. Today’s preamp tubes, especially the 12AX7 varieties, are built with a shorter plate which is good at keeping microphonics to a minimum, but this shorter plate is more difficult to drive with yesterday’s tube testers. This condition leaves many of these testers inaccurately rating newer perfectly good preamp tubes as “bad” or “weak”.

by Robert Hull

Choices, choices …
Today’s tube choices usually fall into three categories. First choice is off-the-shelf new production tubes. Not a bad place to start. Some of the new production tubes are well within NOS specs. Of course, like in by-gone days, varied quality control may provide a weak tube now and again. But for the most part, these tubes are sufficient for most audio applications. Second choice is to pay a premium for guaranteed quality, pretested new production tubes. For very demanding situations and discriminant listening, this provides better peace of mind and is still very reasonable in pricing. The third option is finding new-old-stock (NOS) tubes from the golden era of tube production.

NOS or bust

6V6GT

Thankfully, NOS tubes don’t have a shelf life. They are as good today as they were when first off the assembly line. And with some varieties, the sound is truly second to none. But tubes are a consumable. Therefore, once they go bad or weak, they are thrown away like an old worn tire and replaced. This consumable nature coupled with their finite existence makes NOS choices increasingly more expensive. Even more troublesome is a trend that we have seen lately where NOS tubes that originally didn’t pass their first (or second) round of inspections, are resurfacing on the NOS market again. Many of these are weak, or microphonic or have high noise levels. To the unscrupelous, a noisy $100+ vintage preamp tube can look very inviting to sell anyway. Again, it is a good idea to invest the additional premium for a preselected NOS tubes. And without question, a reputable company with a good return policy is a must.

The above situations are all the more reasons to have not only a good amp to listen to the installed tube, but also a good tube tester you can trust. By having a working tester, you can personally guarantee a tube’s basic integrity. You are lucky indeed if you have such a tester!

by Robert Hull

The Tester
If you own a vintage tube tester, you are part of an exclusive group of tube enthusiasts. At one time every self-respecting electronics repairman had one or more. I fondly remember the tall tube tester in the corner drug store, the bottom filled with tubes. Ahhh….those were the days. Today, finding a tester in good shape is like trying to locate King Tut’s tomb for the first time. Where did they go? They just seemed to vanish. Sadly, most probably ended up in the nation’s landfills. If you have a good tester, you are lucky indeed!

So, with your favorite vintage tester, you settle in to test a batch of new tubes just purchased from your favorite electronics company. Only you find that a few test in the “bad/reject/replace” range! “How can that be?” you may ask, “Aren’t these tubes brand new”? Like you, we noticed the same thing when testing on our vintage tube testers. Well, after some research, we uncovered some answers. Hopefully this will shed light on what is accurate, what is questionable, and what is definitely bad.

Keeping it Real
First off, don’t get rid of your tester. I dearly love test equipment and although the tester may be old, it isn’t dead…at least not yet. Additionally, we don’t want to blindly assume that a tester is always good and its readings are always correct. As with all classic equipment, age can be an unkind master. There is always the fine line of “gray” readings. A singular “Go/No Go” indication never leaves room for “why”. So, let’s find the “why”.

The Golden Era / The Not-So Golden Era

New Old Stock 12AX7

In the by-gone era of electronics, all was ruled by vacuum tubes. When the tube was king, even the lowest common denominator tubes were pretty good. Everyone enjoyed prosperity and all the free electrons were happy. Fast forward to now…

Today’s electronics market is a much different playing field. Gone are tubes; all is now ruled by the field effect transistor (FET) or some native cousin. The tube is relegated to the very edge of obscurity…except in the hearts of the faithful. Sadly, no amount of our faith can change the facts of supply and demand. Tubes are harder to come by and even new production tubes are different than those produced 40 years ago. Not to say they all are terrible, there are some really nice new production tubes out there. However, the horizon doesn’t show signs of the tube ever regaining it’s once lofty mantle.