Vacuum Tube Audio FAQ

What follows is sort of a FAQ on tube hi-fi. I have been a fan of tube hi-fi for many years. We have owned McIntosh, Dynaco, Audio Research, Harmon Kardon, And Fisher Tube Equipment, and have worked on many others. There are many that feel that vacuum tube amplifiers offer superior sound to transistor amplifiers. Though this isn’t universally true, in many cases tube amps offer a more realistic representation of the original performance.

A short primer

Vintage CJ tube amp

Conrad Johnson MV75

Most of you reading this probably are too young to remember vacuum tube radios and televisions.  Others probably still think tubes went out with eight track tape players.  Over the last fifty years or so, the use of tubes in electronics has been displaced in virtually all applications by transistor devices and integrated circuits. In computers, tubes were ridiculous. Early computers consumed entire rooms. Audio equipment was another matter.  

The more recent rise in popularity of tube audio has, more than anything else, to do with sound quality. Solid state (transistor) amplifiers, at least on paper, produce less distortion of the type that is commonly measured, namely harmonic and intermodulation distortion. The problem is that those two measurements don’t tell the whole story. The type of distortion they produce is different from the kind produced by tubes. Tube amplifiers produce more (total) harmonic distortion, but the type they produce is referred to as even order distortion, and is not as harsh-sounding as the odd order distortion transistors produce. Large amounts of even-order distortion (as high as 1-2%) produces little listening fatigue, and can even be relatively pleasant. Electric guitar players favor tube amps for this very reason. On the other hand, small amounts of odd-order distortion (less than .5%) are audible, even by untrained ears, and make music harder to enjoy.

One of the clearest indicators of the musical value of tube audio gear is the fact that there are more manufacturers of high end tube stereo equipment now than at any point in history.  Many of the new components are expensive, ranging in price from several hundred dollars, to a staggering $180,000 for the most exotic modern tube amplifiers. Some of the better-known brands are Conrad-Johnson, Audio Research, VTL, Cary, Jadis, and Sonic Frontiers, just to name a few.


A less expensive route

There is a wealth of vintage hi-fi gear out there, vintage tube amps are still found at relative bargains. Most of it however, is climbing in price due to increased interest. Dynaco, Scott, Fisher, and others can still be had for a relatively modest price. McIntosh, Marantz, and Audio Research will be much more expensive. You should be aware, though, that the garage sale, or Craigslist/Ebay purchase will almost certainly need a good bit of work. Probably the most common example is the Dynaco ST70, a popular 35 watt per channel amp that was very popular, with many examples still in circulation at reasonable prices. It will likely need a new tube set, electrolytic capacitors, and maybe a resistor or two. On larger amplifiers, this can add up to several hundred dollars.  DIY folks can save some money, but don’t take this on unless you know what you are doing. 



Tube availability isn’t what it was in 1960, but most common audio types are still available. Some companies specialize in vintage or new-old stock, which are unused or slightly used tubes that have been sitting in someone’s warehouse for years, waiting for the opportunity to make music. Quite a few new tubes come from China. The Russians have never stopped making tubes, as they found that tube electronics were immune to the radio waves made by atomic explosions, and used the tubes in military applications. Former Czechoslovakia and Yugoslavia have produced some pretty good stuff in the past, but the supply isn’t what it used to be for obvious reasons. Individually tested and guaranteed tubes from companies like Gold Aero are probably the best available today, but carry a price
to match.

Setting the bias

Some tube amplifiers have a manual grid bias setting. The bias setting of a tube amp is a bit like the idle adjustment on a car. Just as with a car, improper settings will cause problems. It is very important to make this adjustment every time you change power tubes, and at least every six months between tube changes. Failure to do so will potentially result in damage to your amp. McIntosh owner’s need not worry, as mac amps (and a few others as well) don’t require manual bias adjustment. Your amplifier may have a built in meter, led’s, or simply a terminal for an external voltmeter. If you do not know how to properly perform this adjustment, find out how, or have it performed by a shop.


This brings us to an important point, do not attempt to work on electronic gear like these older tube pieces unless you are experienced in doing so. Even when switched off and unplugged, tube amps can store lethal energy. Find a good shop with experience in tube hi-fi. Not every electronic technician has an understanding of, or experience with tubes, so ask questions FIRST. Spending a few dollars on good technical help will get you reliable, good-sounding gear that STILL is far cheaper than exotic, newer units.


Technical Tips

If the amp has not been powered up in some time, power should be applied slowly with a variac. If one is unavailable, make sure the proper fuse size is installed, and proceed. Watch the output tubes for red glowing plates. The red color is NOT a good sign. Either the tube(s) are bad, or the bias supply (which controls the amount of current flowing through the tube) is out. Bias is another important point for those just beginning to work with tubes. Most amps have user adjustable bias, so that the tubes can be adjusted for best performance as they break in and age. A few amps have built-in bias indicators, but many require you to use your own voltmeter. Dynaco amps have what appears to be a tube socket on the front panel. A meter probe is to be inserted into the hole marked “bias set”, and the other lead is touched to the chassis. The bias adjustment on top of the amp should be adjusted to read about 1.5 volts. This is the correct setting for all Dyna models, but other brands may require a different setting. The bias setting is crucial to the performance of the amp, and the life of the tubes. The wrong bias setting will result in excessive distortion, premature tube death, or both.

Even more technical stuff

One of the most common causes of problems in older amps (besides tubes) is the b+ filter capacitors. After 20 years or so, these things go BAD! It doesn’t matter if the amp was never used since from the day it was built, since capacitors just deteriorate with the passage of time. McIntosh used very good quality caps in their amps, and there are many with original caps out there that are still running. Dynaco, on the other hand, used cheaper parts, and as a result, if the capacitor is the original, it’s likely bad. Look for the date code on the cap. A typical example would be something like 6605, indicating it was built in the fifth week of 1966. Pretty old stuff. New ones are tough to find. Mail order is likely to be your only source. Don’t overlook the bias supply capacitor. These are much easier to find, and rarely requires parts with more than 100v rating. Dynaco MKIII’s are a particular problem because they require a hard-to-find 525 volt rating on the B+ capacitor. 

Other likely causes of problems

Open cathode resistors are a common cause of failure. This is usually a result of running the amp with bad tubes, or improper bias setting. Tube sockets with splayed pins or extremely heavy oxidation are also common in old amps. Cleaning may seem tempting, but replacement of the sockets is by far the best route in this case. Newer sockets will have tighter tolerances and better metallic materials for conductors. Some people replace internal wiring with newer, higher quality stuff. Commercially available wire from companies like KimberKable can yield significant audible improvements. If you need to replace sockets, take a good look at the wiring for signs of oxidation. Older wire did not have insulating materials as durable as what is available today, and the purity of the copper is often less than current materials. Oxidation (tarnish) will have an audibly negative effect on high frequency reproduction.


What the heck is single ended?” or “What kind of nutcase pays $5000 for an eight watt amp?”

The first audio amplifiers were of the “single ended” type. A triode tube is one that uses three elements. A single ended amp is one that uses a single power tube (per-channel) that amplifies the entire audio signal. The triode tube is comprised of a filament (cathode), a grid, and a plate (anode). A small signal placed on the grid, will cause fluctuation of a large voltage at the plate of the tube. This basically allows a small amount of signal energy to to control a large power supply in such a way that it creates a larger replica of the audio signal. This is (a very simplified version of) how amplifiers work. In an effort to improve the performance of the audio amplifier, other elements were added to the vacuum tube. Tetrode (four element) and pentode (five element) tube followed the triode in an effort to
improve amplifier performance, and design.

Another early design advent to improve performance was the push pull amplifier. In push pull mode, the audio signal amplification job is divided between two (or a multiple) of output tubes. One tube amplifies the positive portion, and one the negative portion of the signal. This design produces more power than the single ended configuration. Most of the tube amps built over the last 40 years are push pull types. Most of those use tubes in tetrode or pentode mode, this configuration produces the greatest power output of all.

In recent years the single ended amplifier has enjoyed a resurgence in popularity. In spite of the fact that most designs are based on amps created over 60 years ago. Some (many?) believe the sound produced by these amps is simply better that later designs.  One possible reason is the absolute simplicity of these amps ensures a short path for the signal to follow. Also, since the signal is not split between two (four etc.) tubes it does not suffer degradation from being re-combined. One important point these amps drive home is: power output has absolutely nothing to do with sound quality. I can honestly say that I have heard eight watt amplifiers that sound better than numerous hundred watt per channel amplifiers.