Dear Enthusiast,
Here is one of the better explanations I have seen... by Joe Mizera,
a respectable "Seeker" like perhaps yourself! Hope you enjoy it!
Respectfully,
Steve Deckert
A short Primer
Younger folks reading this may not remember vacuum tube radios and televisions,
and others probably still think tubes went out with eight track tape players,
since over the last 25 years tubes electronics have been displaced in most
applications by transistor devices. In computers, tubes were ridiculous.
Early computers consumed entire rooms. Televisions also suffered size and
design limitations when they used vacuum tube devices. In TVs, controls
had to be adjusted frequently, since tubes change as they age. Adjustments
(vertical hold, for one) had to be fiddled with regularly to get a watchable
picture.
Hi-fi audio is another matter altogether. Back in the days when all
stereos(monos?) were all vacuum tube, we didn't have those cute little
mini stereo systems the size of a bread box that are so popular today.
Still, as the average consumer has gotten access to better and better sound,
there are ever-increasing numbers of people who demand better fidelity
in their audio systems. The 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 inter modulation distortion.
The problem is that those two measurements don't tell the whole story.
Early transistor audio gear sounded absolutely awful. If you're skeptical,
find a piece of early 1960's-vintage transistor gear and listen on modern
loudspeakers for as long as you can stand it. 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 about $800 to a staggering $180,000US
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, some of it is still
quite a bargain. Most of it however, is climbing in price due to increased
interest. Dynaco, Scott, Fisher, and others can still be had fairly cheaply.
McIntosh, Marantz, and Audio Research will be much more expensive. You
should be aware, though, that the $100 bargain from a garage sale or flea
market will almost certainly need work. Probably the most common example
is the Dynaco ST70, a popular35 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, a filter capacitor, and maybe a resistor or two. Expect
to spend $120-$250 on tubes, depending on the quality of the tubes you
use, $25 or thereabouts for the filter capacitor if you can install it
yourself, and $5-25 for damaged resistors. On poorly kept units, tube sockets
may need cleaning or replacement, and power cords should be replaced it
they are cracked or otherwise damaged.
Tubes
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. Despite their low prices and easy availability,
these are the last resort in my view. Better stuff comes from Russia, made
by Sovtek and Svetlana. 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. 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.
A word of caution!
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 un-plugged, 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 most 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. Macintosh used
very good quality caps in their amps, and there are many with original
caps out there that are still running fine. 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. Placing two capacitors of double the desired
capacity in series that have about a 300 volt rating will work. Both caps
should have a 100k 3 watt resistor in series as shown below. If you are
replacing a four section cap (as in the MKIII) you will need four of these
(a total of eight caps).
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.