0
Shopping cart
0
Cart is Empty

Mercury Rising: Making a Tube Fuzz

The Mercury fuzz was born out of pure curiosity: an experiment to investigate the possibility of making a fuzz pedal with tubes. What might such a strange beast sound like? Would its glowing thermionic bottles, operating at temperatures of 800°C, and high voltage spice up the tone, add some fire and fury to the fuzz effect? Would the huge voltage gain and non-linearities inherent in the tube circuitry add richer harmonic colouration, smoother compression and more sustain to the guitar? Would guitarists even care about the inner machinations of such a marvellous and novel contraption? Well, just in case you do, here is the story of how the MERCURY fuzz, the world’s first tube fuzz pedal, was made…

It's Not Astrophysics…

…it’s ‘Astrotone’! The Mercury pedal was inspired by the old Astrotone fuzz box made in New York City by Universal Amplifier Corp back in 1966-1968. The Astrotone is the earliest example of a diode ‘clipping’ fuzz pedal effect and utilised what were at the time the latest innovations in solid-state technology.

The Astrotone pedal creates the fuzz effect by first boosting the guitar signal to high levels using a silicon transistor gain stage. Next, a ‘shunt’ arrangement of two silicon diodes cut (or clip) the peaks from the top and bottom halves of the guitar signal waveform. This alters, or distorts, the waveform shape so that becomes flattened—the circuit is a “crystal diode wave shaper“. This circuit made its first appearance in Sylvania Electric’s booklet titled, “40 Uses for Germanium Diodes” many years earlier in 1951.

Technically, clipping generates higher order harmonics, which are mixed in with the original signal. In use, when applied to electric guitar, clipping creates a coarse, gnarly, rough-sounding distortion, a.k.a. fuzz. Subjectively it makes the guitar sound more complex and more interesting to listen to. But this really is understating the significance of fuzz—the effect it had on guitar was hugely transformative. The instrument became something new and exciting. Fuzz changed the way guitarists, such Keith Richards, Jimi Hendrix and Jimmy Page played. Their playing became more riff driven, more hard-edged, more grinding and more unacceptable to the more mature, responsible members of the general public. Blues and rock ‘n’ roll rapidly evolved into rock, and then heavy rock. The music of the Rolling Stones, Jimi Hendrix, Led Zeppelin and countless other artists would not exist as we know it without fuzz. It’s no exaggeration to say fuzz was breakthrough for guitar players and for musical self-expression.

Incidentally, for any staunch enthusiasts interested in constructing an accurate replica (as opposed to a copy) of the Astrotone the parts list is shown below. Incredibly, a few of the genuine NOS parts can still be obtained through vintage electronic part suppliers. Even more incredibly, American companies, such as Switchcraft and Chicago Telephone Supply (CTS) are still operating today and manufacturing the components they made back in the 1960s. Getting hold of one of the original custom made diecast enclosures will prove challenging though! A schematic can be found on David Morrin’s excellent website. Finally, here’s a high resolution scan of the Astrotone instruction manual.

Astrotone logo

PARTS LIST

C1, C2, C3, C4—50nF, 20V, ceramic disc capacitor (Central Radio Laboratories, a.k.a. CentraLab or CRL)
R1—3.3 megaohms, 350V, ±10% ½ watt, carbon composition resistor (Ohmite)
R2—22,000 ohms, 350V, ±10% ½ watt, carbon composition resistor (Ohmite)
R3—1.2 megaohms, 350V, ±10% ½ watt, carbon composition resistor (Ohmite)
R4—470,000 ohms, 350V, ±10% ½ watt, carbon composition resistor (Ohmite)
R5, R6—1,800 ohms, 350V, ±10% ½ watt, carbon composition resistor (Ohmite)
P1—100,000 ohms solid shaft potentiometer (Chicago Telephone Supply, a.k.a. CTS)
P2, P3—10,000 ohms solid shaft potentiometer (Chicago Telephone Supply)
D1, D2—1N330 general purpose silicon diode (Fairchild)
Q1, Q2—2N3566 general purpose NPN silicon transistor (Fairchild)
SKT1—¼” stereo, open frame jack socket (Switchcraft)
SKT2—¼” mono, open frame jack socket (Switchcraft)
SW1—Double pole double throw latching foot switch (Carling)

The electronic components are mounted on a 4″ × 2½” FR-2 (Flame Resistant 2, a.k.a. ‘Paxolin’) circuit board, which is laminated with copper foil on one side only.

Ah, the Sweet Sound of Germanium

The MERCURY operates on precisely the same principles as the ‘Astrotone’ fuzz box. However, instead of transistors operating from a 9V ‘PP3’ zinc-carbon battery, it employs vacuum tubes running at 300VDC to boost the guitar signal. The signal is amplified by two triode grounded cathode gain stages to massive levels—far greater than in the Astrotone—before it hits the clipping diodes. In fact, the signal voltage swing is so great that a matrix of series germanium diodes need to be used to handle the high voltages.

Vintage point-contact germanium diodes are utilised, rather than more modern silicon p-n junction types as they possess a ‘non-linear’ (or ‘soft-knee’) clipping characteristic—in contrast, silicon diodes have a ‘hard-knee’. This smoother, more curved transition into clipping results in the generation of lower levels of high order harmonics. Subjectively this sounds less “buzzy” or “fizzy” than hard-knee silicon diode clipping, and the tone is more natural and quite forgiving, even when recording direct.

Hard and soft knee clipping — 'Hard' and 'soft' knee clipping.

In use, silicon diodes perform more like an ultra-fast signal limiter and germanium types act more like a very fast compressor. But the smooth clipping action of germanium diodes helps to preserve dynamics and the original character of the guitar at lower input level (fuzz) settings. This is useful for adding richness and body to the instrument for a fatter, more ‘bluesy’ tone. Moreover, germanium diodes make for a more expressive or touch sensitive fuzz than their silicon counterparts: dig in harder and they generate a thicker, more driven tone; ease off and the sound cleans up adding just the barest amount of “hair” to the guitar.

At the other end of the distortion spectrum, germanium diodes can create colossal fuzz sounds, especially when they’re driven with tubes. Vacuum tubes can amplify the guitar signal to many tens, even hundreds of volts. Hitting the diodes with such a massively boosted signal pushes them way past their clipping threshold into generating complex, harmonic splattered saturation and endless, shimmering sustain—signal dynamics are ruined and crushed to oblivion—quite beautiful really. Such heavy clipping effectively shapes the signal into a square-wave, which results in the guitar sounding more like Moog synthesiser. All this from an old germanium diode.

Sylvania 1N34 germanium diode — Germanium crystal diodes were a compact, lightweight and rugged replacement for the old vacuum tube types.

So, semiconductor diodes play a vital role in creating the fuzz effect in the Astrotone (silicon 1N330) and MERCURY (germanium 1N34) pedals. On a philosophical note, semiconductors seem to possess a certain, unique sonic signature that is associated as being part of the fuzz sound. Whether this sound is intrinsic to crystalline nature of semiconductor materials and/or related to the topology of solid-state circuitry has been, and still is, a matter for great debate amongst sound engineers and musicians. It’s not so much astrophysics, as quantum physics: it would certainly make an excellent basis for a PhD thesis. A full-on, proper scientific explanation of how diodes work is complicated… very complicated, however here’s a quick, canned explanation of how they work.

Diodes For Dummies

cats_whisker — The "cat’s whisker" is what makes radio ("the wireless") possible

Crystal diode technology began its development in the early 1900s, where wireless receivers utilised a thin wire in mechanical contact against the face of a crystal, typically a piece of coke (carbon) or galena (lead sulphide). The wire had to be manually adjusted to find the ‘hot-spot’ on the crystal for best radio wave detection. The hot-spot is a small region on the surface where current can pass in one direction only, flowing from the ‘electron-rich’ metal to the ‘electron-poor’ crystal. Current cannot flow from the crystal back to the metal and so rectifies the received carrier signal providing a D.C. voltage to drive the headphones. Within a few years germanium/galena “cat-whiskers” were being used by amateur radio enthusiasts and in early commercial radios.

With the development of radar systems during WWII there was an urgent need for a more reliable, high frequency, low-noise detector/mixer—the cat’s whisker was too fragile and finicky. Fortuitously, advances in manufacturing and a greater understanding of physics made it possible to miniaturise the cat’s whisker and encase it within a tiny glass envelope, a.k.a. the ‘point-contact’ crystal diode. The minute cat’s whisker assembly was made from the tiniest slither of high purity, perfectly crystalline germanium and a fine S-shaped tungsten wire. Millions of these minuscule devices were manufactured in the 1940s and it was Sylvania Electric who pioneered the use of germanium for diodes, with the introduction of the 1N34—the first commercial germanium crystal diode.

Sylvania 1N34 diode — In 1946 Sylvania Electric introduced 1N34 diode [Photograph by José Gustavo Sánchez González]

Metalloids on the periodic table — Metalloids inhabit that strange boundary on the periodic table where chemical elements sometimes behave like metals and other times like non-metals.

Scientifically speaking, germanium (Ge) is what’s known as a ‘metalloid’. A metalloid might sound like some psychotic alien robot from an atompunk sci-fi novel, however it’s actually a chemical element that exhibits some properties of metals and some of non-metals. For example, metals are conductors of electricity and non-metals are insulators. Therefore metalloids can behave as electrical insulators, conductors or… somewhere in between. Electromagnetic radiation (heat, light, x-rays, etc) or electric charge can cause a metalloid to change from one state to another, from conducting current like a copper wire to insulating like glass—miraculous stuff. To quote Dr Manhattan, “it’s like turning air into gold.”

Metalloids are ‘semiconductors’. Other metalloids, such as selenium (Se) were also once employed in the manufacture of electronic semiconductor devices such as photocells and diode rectifiers. However, for the most part, silicon (Si) is utilised for the construction of diodes, transistors and microchips today. Modern silicon diodes are somewhat different in construction to the old germanium types. Instead of a tiny cat’s whisker, they’re made up from two silicon wafers. Each wafer has different chemical impurities added to create either electron-rich (negative, or ‘n-type’) or electron-poor (positive, or ‘p-type’) silicon material. Such devices are more accurately described as a silicon ‘p-n junction’ diodes. But enough about semiconductors…

…What About the Tubes?

Sylvania 6021 tube — Sylvania/Philips subminiature vacuum tubes were the zenith of thermionic technology.

Well, there’s a tiny Sylvania ‘6948’ tube within the electronic guts of the MERCURY pedal. This high-mµ double triode plays a vital part in creating the fuzz effect. The tube is wired to operate as two grounded cathode gain stages (Fender employed this type of tube circuitry in their ‘Deluxe’ and ‘Twin Reverb’ amplifiers). The first gain stage is ‘hot’ biased, which adds richness and warmth to the sound and boosts the input signal level 30dB. The second, and this is the clever bit, has variable biasing. This enables it to clip the signal symmetrically or asymmetrically and in doing so alters the sensitivity, feel and character of the fuzz.

The ‘BIAS’ toggle switch on the rear panel of the MERCURY selects the biasing of the second tube gain section. In the ‘up’ position the tube is biased in its linear region creating symmetrical clipping distortion. In the ‘middle’ position it is ‘cold’ biased generating asymmetrical clipping of the top half of the wave. And in the ‘down’ position the tube is ‘hot’ biased creating asymmetrical clipping of the bottom half of the wave.

The distortion generated by the non-linearity of the second tube stage blends beautifully with the clipping diode distortion to create a rich, composite fuzz sound saturated with complex harmonic overtones. Further, when biased for symmetrical clipping, the MERCURY is operating at maximum gain generating maximum distortion and the smoothest, most sustaining fuzz tones. Conversely, ‘hot’ and ‘cold’ biased asymmetrical clipping results in slightly reduced gain and a rougher, grittier distortion.

In closing, were it not for the staggering technological innovations that Sylvania Electric made back in the early part of the 20th century the MERCURY fuzz pedal would not exist. It’s operation depends on the 1N34 crystal diode and, more critically, the ‘6948’ tube, which was originally developed by their boffins for guided missiles. These tubes represent the absolute pinnacle of thermionic technology and are unquestionably the finest tubes manufactured by any company, ever. Hats off to Sylvania!

Cookie	Duration	Description
_GRECAPTCHA	5 months 27 days	This cookie is set by Google. In addition to certain standard Google cookies, reCAPTCHA sets a necessary cookie (_GRECAPTCHA) when executed for the purpose of providing its risk analysis.
cookielawinfo-checkbox-advertisement	1 year	Set by the GDPR Cookie Consent plugin, this cookie is used to record the user consent for the cookies in the "Advertisement" category .
cookielawinfo-checkbox-analytics	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Analytics".
cookielawinfo-checkbox-functional	11 months	The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional".
cookielawinfo-checkbox-necessary	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookies is used to store the user consent for the cookies in the category "Necessary".
cookielawinfo-checkbox-others	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Other.
cookielawinfo-checkbox-performance	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Performance".
JSESSIONID		Cookie used to allow the Worldpay payment gateway on the website to function.
machine		Cookie used to allow the Worldpay payment gateway on the website to function.
viewed_cookie_policy	11 months	The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. It does not store any personal data.
wordpress_logged_in_		Users are those people who have registered an account with the WordPress site. On login, WordPress uses the wordpress_[hash] cookie to store your authentication details. Its use is limited to the Administration Screen area, /wp-admin/ After login, WordPress sets the wordpress_logged_in_[hash] cookie, which indicates when you’re logged in, and who you are, for most interface use. WordPress also sets a few wp-settings-{time}-[UID] cookies. The number on the end is your individual user ID from the users database table. This is used to customize your view of admin interface, and possibly also the main site interface.
wordpress_sec_	1 year	Provide protection against hackers, store account details.
wordpress_test_cookie		Test to see if cookies are enabled.
wp-settings-	1 year	WordPress also sets a few wp-settings-{time}-[UID] cookies. The number on the end is your individual user ID from the users database table. This is used to customize your view of admin interface, and possibly also the main site interface.

Cookie	Duration	Description
_ga	2 years	The _ga cookie, installed by Google Analytics, calculates visitor, session and campaign data and also keeps track of site usage for the site's analytics report. The cookie stores information anonymously and assigns a randomly generated number to recognize unique visitors.
_gid	1 day	Installed by Google Analytics, _gid cookie stores information on how visitors use a website, while also creating an analytics report of the website's performance. Some of the data that are collected include the number of visitors, their source, and the pages they visit anonymously.

Cookie	Duration	Description
mailchimp_landing_site	1 month	This cookie is used to keep track of newsletter sign ups and client emails at checkout, Mailchimp utilises cookies to store information captured from user input for remarketing purposes.
mailchimp_user_email	1 month	This cookie is used to keep track of newsletter sign ups and client emails at checkout, Mailchimp utilises cookies to store information captured from user input for remarketing purposes.
mailchimp_user_previous_email	1 month	This cookie is used to keep track of newsletter sign ups and client emails at checkout, Mailchimp utilises cookies to store information captured from user input for remarketing purposes.
mailchimp.cart.current_email		This cookie is used to keep track of newsletter sign ups and client emails at checkout, Mailchimp utilises cookies to store information captured from user input for remarketing purposes.
mailchimp.cart.previous_email		This cookie is used to keep track of newsletter sign ups and client emails at checkout, Mailchimp utilises cookies to store information captured from user input for remarketing purposes.

Shopping cart

Mercury Rising: Making a Tube Fuzz