Even though it dates back from the 1930s Golden Age of Radio, but can the Type 85 vacuum tube work as a heart of an audiophile grade fuzz unit?
By: Ringo Bones
To some, an “audiophile grade fuzz unit” may sound oxymoronic because after all they are primarily designed to produce harmonic distortion at relatively low volumes in electric guitar amplifiers. But given that vacuum tubes and electric guitar electronics had been going hand-in-hand for over 60 years with exceedingly good sounding results, should electric guitar electronic effects designers explore underutilized vacuum tubes for possible use in the hopes of designing one that offers both versatility and good sound quality?
Fuzz units or fuzz pedals as they are originally known were originally introduced back in the 1960s and were readily embraced by rock and Blues electric guitar players. It makes use of germanium signal diodes to clip the electric guitar signal originally produced by the magnetic pickups to produce a chainsaw-type distortion. Germanium semiconductor diodes have a lower voltage drop, Vf = 0.2 volts, in comparison to silicon semiconductor diodes at Vf = 0.6 volts. This means that the signal is clipped at a much lower level. The sound produced by this electric guitar effect / processor is quite versatile. It can sound warm and fuzzy to nasal and grainy. The most famous example of this effects unit is the Dallas Arbiter Fuzz Face. Originally, they are composed of solid-state semiconductor parts – i.e. germanium signal diodes and germanium transistors during the 1960s to operational amplifiers by the start of the 1980s, but will a better-sounding one be made around the Type 85 vacuum tube?
The Type 85 vacuum tube is a duo-diode triode vacuum tube in a single enclosure designed for use as the first-stage detector, automatic voltage control and first stage audio amplifier in 1930s AM radios / AC line operated receivers. It was also used as the phase inverter in several 1930s era public address amplifier designs. It is electrically identical to the octal base 6V7. The Type 85S is a spray shield type made by Majestic. The Type 85 vacuum tube has a maximum plate voltage rating of 250 volts DC but it is typically run at 135 volts, a maximum plate current of 8-milliamperes but typically it is run at 3.7-milliamperes, a maximum grid voltage of -20 volts but it is typically run at -10.5 volts, a filament or heater voltage of 6.3 volts and a filament current of 300-milliamperes. The Type 85 vacuum tube has an amplification factor or mu of 8.3 and a transconductance or gm at 750 and it has a plate resistance of 11,000-ohms.
The Type 85 vacuum tube is a multiunit vacuum tube that is it is a vacuum tube containing several independently acting tubes in one envelope. The electron stream is divided into several parts, each part being acted upon by one set of electrodes. The top metal cap is electrically connected to the ground and therefore does not represent a high voltage electric shock hazard. But can the two vacuum tube signal diodes enclosed in the Type 85 vacuum tube be used as “clipping diodes” for it to be used as an “audiophile grade fuzz unit”?
Even though hobbyists and DIY-ers well versed on how the Type 85 vacuum tube works are still relatively rare, Kara Chaffee of deHavilland managed to design and produced one in current commercial production called the deHavilland Mercury preamplifier. Although she didn’t use the two built in vacuum tube diodes and left them unconnected. But given the Type 85 vacuum tube’s robust tone and crystalline clarity, would it be a great vacuum tube not just for electric guitar preamplification and distortion work, but also for acoustic guitar amplification given its robust tone can make a typical acoustic guitar’s built in piezoelectric Fishman Transducers sound as if they were 3,000 US dollar Condenser Tube Microphones?
Even if the Type 85 vacuum tube’s big size might intimidate the contemporary 21st Century guitarists, there are now pack-of-gum-sized lithium polymer rechargeable batteries with nanostructured anodes sold in Airsoft shops that can hold enough juice to power a Vietnam War era AN / PVS-2 starlight scope with its 15,000 volt photomultiplier tube for 15 hours straight before recharging, so battery operated fuzz pedal units using the Type 85 vacuum tube is technically feasible this day and age. And given that the Type 85 vacuum tube was used as a automatic voltage control unit in AM radios during the 1930s Golden Age of Radio, with its built in vacuum tube diodes, it could also be useful as a gold channel or audiophile grade compressor-sustainer unit, noise gate and limiter.