While the Arthur Loesch audio designs are famous for being straightforward and natural sounding, a simple way to modify the sound to one's individual taste is by swapping or "rolling" the tubes (valves). By trying different brands or types, it's often possible to fine tune your component to the room, the recording, or other associated equipment.
 
In the sections that follow, we'll review specific tubes that we like or supply with our line of components. While we have our reasons for preferring certain examples, this shouldn't be taken as the last word on the subject. If you have the time and the patience, making comparisons on your own can either be a gateway to Sonic Nirvana or an exercise in audio neurosis. As with all things audio, we urge you to keep an open mind. But first, some technical advice.
 
Keep Them Clean!
 
Unless gold plated, most vintage and modern tube pins will oxidize over time, inhibiting their contact properties. A good all around pin and contact cleaner is Caig DeoxIT.^® It is non-toxic and does not conduct electricity. We buy it in a larger can and decant it into a small bottle with a dropper for everyday use. The dropper makes it easy to apply and a soft toothbrush is used to scrub the pins clean without bending or damaging them. After cleaning, we dry the pins with a paper napkin or soft cotton cloth. Afterwards, a protective coat of DeoxIT Shield on nickel or other common plating or DeoxIT Gold over gold or silver plating is optional. Useful anywhere else that an audio connection is made.
 
It's also a good idea to keep fingerprints off the glass, too. Over time, acids from your skin can etch the glass, weakening the envelope and the vacuum inside. Either wear cotton gloves when handling your tubes or carefully wipe the glass after inserting them into their sockets. When pulling larger tubes from their sockets, such as rectifiers or power tubes, hold them by their base, not the glass. Pulling on the glass could pry them loose from the base, causing problems or even failure. A pin straightener for miniature tubes (shown upper right) is also handy to have around.
 
Testing and Matching
 
Because of variations in the manufacturing process, vacuum tubes, even those fabricated on the same line, on the same day, can exhibit different measured characteristics. More so if they have been used for any length of time. If you only need a single tube in a single circuit, such as a mono FM tuner, that's nothing to worry about. However, in high-end stereo gear, poorly matched tubes can produce slightly different volume levels in one channel or the other. In the case of push-pull or parallel SET amplifiers, the result is higher distortion, premature aging, or less than optimal power.
 
Enter the tube tester. The most common test for strength or age is "mutual transconductance," also called Gm. It's unit of measurement is the microMho (that's micro + Ohm spelled backwards), sometimes abbreviated as µMho. Rather than give a true reading, some brands, like the Hickok TV-7, use their own reference scale. Simpler models have meters that simply deflect into zones labelled "Good" or "Bad" or "Aging Quickly." Either way, that's only half the story. For a true match, you also have to measure emission or plate current, expressed in milliAmperes or mA. The minimum standard for matched sets of tubes places them within 5% of each other for both emission and transconductance. Of course, 1% or even identical readings, are the ideal. Closely matching the emission of each tube when used in multiples within the same circuit greatly improves performance and long-term reliability.
 
The Triplett 3444-A Tube Analyzer
 
Our personal reference device is the Triplett 3444-A Tube Analyzer, pictured above, but no longer manufactured. A versatile instrument, the solid state 3444-A and its predecessor, the tubed 3444, offer a wide variety of features:

• Measures Mutual Transconductance and Plate Current, perfect for true tube matching.
• Has four Plate Voltages: 12V, 30V, 100V, 250V,
• Two Plate current ranges: 5 mA & 50 mA,
• Four Gm (transconductance) ranges: 1000, 3000, 10,000, 30,000 microMhos,
• Five Screen Voltages: 12, 30, 45, 100 & 250V, and
• Two Bias Ranges: 0 to 5V and 0 to 50V.
• Short/Leakage tests are made directly on the meter.
• Gas test up to 250V, making this much more sensitive than with most other analyzers.
• Dual Section test allows the user to switch between both halves of a dual triode without having to reset the selectors.
• Rectifiers tested under load.
• Sockets for every popular tube: 4, 5, 6, 7 prong, octal, loctal, 7 & 9 pin miniature, 7 & 8 pin subminiature, nuvistor.

• Triplett 3444-A Instruction Manual
• Triplett 3444-A Calibration Manual

• Amplitrex AT1000 Vacuum Tube Tester Manual

• 1st Stage
• 2nd Stage
• 3rd Stage

• Rectifier
• Input
• Driver
• Output

Phono Preamplifier : 1st Stage
 

Western Electric 417A, plus
Amperex, Raytheon, and Ericsson 5842's

• 417A / 5842
   
  A miniature triode, the best examples were made by Western Electric, but must be custom selected for low noise (virtually all are microphonic). Amperex, Raytheon, Ericsson, and RCA, among others, also manufactured this tube as the 5842 or CK5842. While examples made by Amperex are typically quieter than other brands, they are not as musical, sounding somewhat dry and distant. The Raytheons, however, are almost as nice as the Westerns for a fraction of the price. Unsorted WE 417As typically sell for $30 to $50 each. However, it can take as many as 20 examples to find one that's quiet enough to use in the front end of a phono stage without a head-amp or step-up transformer. Westerns of pre-1970 manufacture, with the logo printed in the cursive lightening bolt font, tend to be quieter than later ones which have the name printed in squared-off type. Expect to pay $10 - $30 each for other brands.

Phono Preamplifier : 2nd Stage
 

6GK5 by RCA, Sylvania, Pro/Comm, GE, and Mullard

• 6GK5 / 6FQ5A
   
  The standard tube in the 2nd Gain Stage is the 6GK5, also known as the 6FQ5A (many are labeled with both numbers). RCA is the brand of choice here, being especially rich in midrange tone and detail. Though they can be almost too rich and caramel sounding, American made GE tubes are a close second. Beware of GE tubes that are made in Great Britain -- these are actually rebranded Mullards. The latter also show up under the Pro/Comm label, too. Examples by Mullard, Sylvania, Hitachi, Magnavox (which are rebranded Hitachi tubes), and others, are less musical and thinner sounding. Tubes by RCA and GE sell for about $6 each, Hitachis are typically $2, and Mullards are often $15.

Phono + Active Line Preamplifier : 3rd Stage
 

Tung-Sol 5687, Amperex 7119, and Bendix 6900

• 5687
   
  Originally developed by Tung-Sol for military communications and guidance systems, the 5687 is a dual triode which was manufactured during the 1940s, '50s, and '60s. Prior to the invention of the transistor, thousands were often used in giant mainframe computers (remember the Univac?).
   
  The 5687 also has the potential to amplify an audio signal with great integrity and is popular because of its low price and consistent build quality. Its sound is on the warm side, emphasizing the midrange of the audio spectrum and is a perfect match for most typically thin or bright popular recordings. Often relabelled, they're available under many brands for about $5-$10 each. Sonically, Tung-Sol is still the most widely regarded. Of these, variations include the black plate (the best sounding), gray plate (still good), and copper plate (not as musical to our ears as the other two).
   
  
• 7119 / E182CC
   
  The 7119 (known in Europe as the E182CC) is another computer tube which was developed by Amperex in the 1960s. Electronically equivalent to the 5687, the 7119 has much larger plates and can provide an additional 4dB of gain over its smaller rival. All things being equal, it's actually more neutral than the 5687, but suffers on bright (mainly popular) recordings and can compound the edginess of many commercial loudspeakers, especially those which are too bright to begin with. In our preamp, it performs best with recordings of classical music, especially RCAs, Deccas, and Mercurys from the Golden Age (1957-1964), as well as on well-engineered popular albums. Virtually all were sold under the Amperex brand and were either made in Holland at the Philips plant or in the USA at the company's Hicksville, Long Island, factory. Typical retail is $15 each.
   
  
• 6900
   
  The 6900 is a rugged version of the 5687 that was originally developed circa 1954 by the Bendix Corporation of Red Bank, New Jersey, for a US Air Force project. In the 1960s, the remaining stock was purchased and rebranded by MU Engineering, which continued to manufacture it in small runs at their Oceanside, California, plant. Only a few thousand were made and partly because of its scarcity, it's acquired a mythical aura.
   
  In our designs, its sound is closer to the 7119, being highly detailed and dynamic, though somewhat on the bright side. Compared to both the 5687 and 7119, it lacks the warmth and musicality usually associated with vacuum tubes and sounds to us somewhat like a solid state device. While some listeners say that it represents the "last word" in this family of dual-triodes, it's sound is very circuit dependent. When you can find it, the 6900 is pricey and sells for $100 each or more. It should be noted that MU still has the ability to fabricate them, but there is a 1,000 piece minimum production order with a price of $100 each. You do the math!
   
  

Model 302 Amplifier : Rectifier
 

JJ Electronic, Philips (Eindhoven, 1955), Mullard (branded Eico),
and Philips ECG [formerly Sylvania] "Fat Bottle" (USA)

• 5AR4 / GZ34 / CV1377
   
  The 5AR4 is a full-wave rectifier that was (and still is) available from many makers in a variety of styles. It's slow turn-on buffers the audio circuit against the initial shock of inrush current and because of its popularity in guitar amplifiers, it is still in high demand. The most musical ones were made by Philips (Amperex) of Eindhoven, Holland, in the period from 1954 to 1957. These tubes have a distinctive golden metal band around the base and are stamped with factory codes rS1 (1954-55) or tv2 (1956-57), as well as a date code on the second line. The earliest ones have flat, gray plates, while later examples have three vertical holes on each plate, as well as other minor differences. Dealers typically ask $300+ per tube, but they often sell at auction for $150-$250. Rebranded Mullard, Amperex, or Mazda, among others, this tube presents the fullest, richest mid-range of any ever made and is our current reference.
   
  Almost as good are the ones manufactured by Mullard, itself, in the late 1950s and early 1960s. These usually have a black or brown plastic base, were often rebranded GE, Eico, or Dynaco, among others, and sell for about $50 to $100 each. The CV1377 is a select military version that often sells for a 50% premium.
   
  While vintage Philips and Mullards are still the best choice, of the current production, those from JJ Electronic (formerly Tesla) of Slovakia, are affordable substitutes with reasonably good sound.
   
  Update, February, 2008: We recently revisited our collection of rectifiers in the Model 302 Amps and made some interesting discoveries. When paired with vintage Western Electric 300B's, the reference Philips (Amperex) metal-base GZ34 was clearly the most musical of this type. But, to our surprise, the Philips ECG* (USA) "Fat Bottle" out-performed the vintage metal-based tubes when paired with the Emission Labs 300B, mentioned below. Apparently, there is a higher degree of synergy going on between different tube combinations than we had previously thought.
   
  * Philips ECG (Electronic Component Group) is the successor to Sylvania, suggesting that Sylvania branded "Fat Bottle" 5AR4's may be equally good.
   
  
• GZ33 & 5R4
   
  
  
  Mullard GZ33 & 5R4 types by GE, Chatham, Sylvania, and RCA
  
  In the Loesch 302 Amp, "tube rollers" can safely substitute GZ33 and 5R4 types, which will yield a slightly different sound. Of these, the Chatham 5R4WGA "Potato Masher" (pictured in the center, above) is remarkably close to our reference metal-banded Amperex GZ34 and Philips (USA) 5AR4, though slightly lacking in transparency when compared to the latter two. Commonly available for $20 each, they represent a solid bargain. At $60-$80 each, the Mullard GZ33 is not as kind on the ears in our amps and should probably be avoided. Still popular in tubed guitar amps, NOS prices for other brands range from $20-$100 per pair.
   
  

Model 302 Amplifier : Input
 

Left to Right: RCA, RFT, JJ Electronic, Electro-Harmonix Gold,
Telefunken, Tung-Sol (Black Glass), and Amperex

• 12AU7 / ECC82 / CV4003
   
  Popular in guitar amps, as well as in home audio, by far the most musical 12AU7 is the original Tung-Sol version made with a black glass envelope. Closely behind are the early long-plate RCAs that have white lettering on the glass and the West German Telefunken ECC82. Genuine Telefunkens have a diamond embossed in the glass on the bottom of the tube between the pins. When Telefunken ceased production, later versions were manufactured by RFT of East Germany, as well as by Mullard of Great Britain. While these were relabeled Telefunken, they are not the real thing. However, along with RFT and Mullard branded types, they are still among the best, followed by Amperex. Of those currently manufactured, the Electro-Harmonix Gold has a detailed, open sound, but is still a runner up to the older types. Later RCAs, as well as the current JJ Electronic, are bold and dynamic, but not as rich in the midrange.
   
  

Model 302 Amplifier : Driver
 

 
6350: Sylvania, GE, and RCA

• 6350
   
  This is another dual-triode from the 1950s designed for use in mainframe computers. Though uncommon in home audio, if the circuit is right, it offers unmatched musicality and sonic "rightness." The major manufacturers were Sylvania, GE, RCA, and Tung-Sol. These were often rebranded with Hewlett-Packard, IBM, and Raytheon logos.
   
  While they are all good performers, the Sylvania is the smoothest from top to bottom. The RCA offers the most robust bass and may be preferred for rock music, while the GE is somewhere in between the other two. We have not had a chance to hear the harder to locate Tug-Sol and cannot comment on its sound. Sylvanias are easy to distinguish from the others by the rounded plate tops which project above the insulating spacer (see detail, below). Prices typically range from $6 to $25 for NOS.
   
  
  
   
  Detail of the Sylvania 6350

Model 302 Amplifier : Output
 

Original Western Electric 300B, Emission Labs 300B, RCA Globe UX-250,
RCA 50 (ST-19), Sylvania 50 (ST-16 style 6A3 remanufactured as a 50)

• 300B
   
  The 300B has a long history of high performance and great sound. The original 300A was first manufactured in 1935 by Western Electric and was replaced in 1937-38 by the 300B. The main differences were changes in the internal construction to make it more reliable, as well as rotating the guide pin to allow it to substitute for other power tubes. When Western introduced this tube, it revolutionized audio amplification with its 7 watt output, a vast leap over the 1.5 watts of the 45 and the 3.5 watts of the 2A3. It was mainly used in theatrical applications and, ironically, was discontinued in 1988 just as the single-ended triode (SET) revival in home audio was beginning. As a result, demand for vintage 300Bs has driven the price as high as $1,250 each. Luckily, the 300B is back in production from almost a dozen manufacturers worldwide, including Westrex, the successor the Western Electric's tube manufacturing division.
   
  The 300B's sound is bold, dynamic and detailed. While the original "engraved base" Western Electric 300B is still the standard against which all others are measured, the latest Emission Labs 300B (our current reference) actually exceeds it in dynamics and low noise for a fraction of the price. Today, a pair of NOS vintage WE 300Bs from the 1950s could set you back as much as $2,850. Double that for a pair of 300As.
   
  
  
• 50
   
  Also developed in the 1930s, the 50 power triode was the largest tube used in home receivers at the time. In contrast to the 300B, the 50 has half the power and somewhat less detail, but offers what many consider to be a higher degree of musicality -- always a good antidote to the sound of digital playback. For many listeners, the globe or balloon-shaped 50 represents the pinnacle of the classic tube sound.
   
  At auction, vintage ST-19 examples from the late 1930s-1940s sell for $125+, while "mesh plate" or globe shaped styles from the early 1930s can command twice as much or even more. In the 1940s, the 6.3 volt 6A3 was sometimes beefed up to use a 7.5V filament supply, relabelled, and sold as a 50, but these are not the real thing and should be avoided. You can easily tell them by the smaller ST-16 shape. Whether globe or ST, a true 50 is the same size as a 300B. Although the 50 is now manufactured again by TJ Full Music of Tianjin, China, we have not auditioned them and cannot comment on their sound. Anecdotally, we have heard that these are actually 2A3s which have been modified for 7.5 volt filament supplies.