Written Sept. 1997
Rev 1.1 Jan. 2001
THE REAL McTUBE
by Fred Nachbaur, Dogstar Music ©1997, 2001
1: INTRODUCTION
With the phenomenon of the "retro" movement in music electronics, there has
been renewed interest in the vacuum tube. Several manufacturers are again
building tube amplifiers and other devices, with claims of "vintage sound" and
"warm tube distortion." Here's your opportunity to experiment with these
interesting devices by building a tube pre-amplifier and distortion unit.
The differences between "The Real McTube II" and the original design are:
- The power supply has been modified to use the parts inside inexpensive and commonly
available 12-volt, 500 mA. "wall-wart" DC power supplies. You will need two of these.
- Some users reportedly had trouble with excessive hum. This possibility has been greatly
reduced by using DC for the tube filaments.
- The relay circuit for switching modes via an external foot-pedal switch has been replaced
by an on-board push-on / push-off button switch, simplifying the circuitry and making the unit
a self-contained "Stomp-Box".
- This construction article has been re-written and expanded, especially as regards to
techniques for noise- and hum-avoidance. The topic of proper grounding has been covered in
greater detail.
Cost will depend on how many parts you already have in your junk box, but it
shouldn't exceed about $50 if you shop carefully. The crucial part, of course, is the
12AX7 or 12AX7A vacuum tube (and the 9-pin miniature tube socket to go with it).
These are again quite commonly available, both from online sources and from many electronics
jobbers. If you get stuck, call around to musical instrument repair shops that specialize in
vacuum tube gear, but don't let anyone give you the "expensive because they're rare"
line.
The classic 12AX7 was used in a wide range of vintage tube gear, mainly because of
its relatively high gain and reasonable linearity. As tubes go, it is relatively quiet;
notwithstanding the high degree of thermal noise inherent in all vacuum tubes,
the 12AX7 (especially its European equivalent the ECC83) is at least
mechanically constructed to minimize "microphonics," noise caused by
mechanical vibration of its internal elements.
If you intend to experiment with different tubes to get "just that sound" also
keep an eye out for type 12AU7 and 12AT7 (and their European cousins ECC82 and ECC81), as well
as 12AY7 and 12AZ7. The 12AT7 and 12AZ7 will give comparable gain, but their clipping
characteristics are a little different. The 12AU7 will give considerably less gain, and will
therefore only be generally useful if the overdrive unit is placed later in your effects chain.
The "Tube Sound"
So what is it about tubes that make them sound different from solid state devices
like transistors? There is really nothing mystical to this; in reality, their unique
sound derives more from their shortcomings than from any other consideration.
Being hot, bulky and expensive, manufacturers couldn't afford to make amplifiers
with massive gain, which would have allowed them to use large amounts of
negative feedback to linearize their response and reduce distortion. The
exception was for applications such as studio monitors and similar high-end
equipment. You only have to listen to such tube classics as Dave Brubeck's
"Take Five" or Heart's "Dreamboat Annie" albums to hear for yourself
that tube equipment can sound anywhere as good as, if not better than, top-end solid-state gear.
(If you're interested in building a "middle-end" high-fidelity amplifier, check out
my surprisingly simple yet capable
RA-100 Reference Amplifer design.)
For musical instrument amplifiers, however, the inherent non-linearity and high
degree of even-harmonic distortion of the "raw" tube amplifier (little or no
negative feedback) contributes to the charm of its sound. "Warm" and
"gutsy" are but two of the many adjectives used to describe the tube
"sound."
This harmonic distortion is especially pronounced when a tube is overdriven. Unlike a transistor,
which remains reasonably linear until it reaches cutoff (no current through the device) or
saturation (maximum current through the device), a tube will exhibit a softer, more gradual
bottoming-out at either extreme. This is especially pronounced at the saturation end, i.e. as
the voltage across the tube approaches zero. There is no sharp saturation "knee", and
any attempt to define the point at which a tube reaches saturation is strictly arbitrary.
There are wide variances in these characteristics between tubes of a given
family, and even between specimens of a given type. What's more, a tube's
characteristics change as the tube ages. This can be a real nightmare if you're
trying to design consistent, predictable gear using tubes. However, these factors
all add up to the mystique of the vacuum tube for musicians and experimenters.
I've known musicians who treasure old, worn-out, gassy 12AX7's because of
the particularly dirty distortion of which they're capable.
The 12AX7 is but one of a whole family of dual triodes. Other devices in the
family are the 12AT7, 12AU7, 12AY7, 12AZ7, (along with improved versions, indicated with
an "A" suffix) along with numerous European equivalents.
The filament (heater) requirements are all the same; either 12.6 volts (at around 150
milliamps) as suggested by the first two digits of the type numbers, or 6.3 volts
(at 300 milliamps) depending on how the two filaments are connected. Another
boon is that the pinouts for all of them are identical; going clockwise from pin 1,
they all go Plate, Grid, Cathode (triode 1), Filament, Filament, Plate, Grid
Cathode (triode 2), Filament centre. The circuit presented here will work with any
of the tubes in this family, giving you the opportunity to experiment with different
types to get the exact sound you're looking for.
