THD Electronics, Ltd. is a manufacturer of amplifier accessories, and a former manufacturer of vacuum-tube guitar amplifiers. It is located in Seattle, Washington. It was founded in 1986 by President and CEO Andrew Marshall. At one time specializing in hand-built, vacuum tube “boutique”-style amplifiers, it continues to manufacture the “world’s best selling [guitar amplifier] power attenuator,” the THD Hot Plate. The company is well known in the boutique amplifier market, and is often credited with the creation of this market in the United States. THD's line of products stayed relatively small despite its 22-year history as a company. Of its amplifiers, THD made three: the UniValve, BiValve-30, and Flexi-50.
Name origins
Copyright issues prevented Marshall from naming THD Electronics after himself, as Marshall Amplification had been established in the 1960s. Marshall also decided against naming the company after his first name because, in his words, “Who…is going to spend $2,500 on an ‘Andy Amp?’" Instead he chose THD, the acronym for “Total harmonic distortion”.
'(Total harmonic distortion) is one of the ways that amplifiers are rated,' says Marshall. 'And in the hi-fi world, where people are quite conscious of total harmonic distortion, everybody goes for the lowest possible amount of distortion. In the guitar amp world, people tend to go for the most. So it was slightly tongue-in-cheek, but more of an inside joke.'
Marshall has said that THD alternatively means “Two Hairy Dogs”, referring to his bearded collies, Truffles and Tobler and with the addition of a third dog, Piper, “Three Hairy Dogs”.
Amplifier basics
Exterior
THD amplifier heads have a unique perforated steel cage, which is removable (via captive thumbscrews) if the amplifier is added to a rack or installed in a cabinet. Aside from the different faceplates, the UniValve, BiValve-30 and Flexi-50 look nearly identical. These amps are also available in a "box head" version. Although THD does not offer a hard-sided carrying case, they do make a soft-sided case for their amplifier heads.
Vacuum tubes
All THD amplifiers utilize vacuum tubes for their preamp and power amp sections. Until the release of the Flexi-50 amplifier, they were all class A (the Flexi-50 is class AB). This is why early THD UniValve prototypes were sometimes referred to as the “Pure Class A Head.”
Printed circuit boards
Unlike the vast majority of high-end, “boutique”-style guitar amplifiers, THD amplifiers are hand-built, but not hand-wired “point-to-point”. Instead, they utilize printed circuit boards (PCBs). There is some controversy in the guitar amplifier community about using PCBs as opposed to point-to-point wiring in tube amplifiers. THD escaped much of this criticism by using thick boards with large ground planes to reduce noise, as well as through-plating and use of Teflon wire for durability. THD also does not mount their electro-mechanical components like jacks, potentiometers, switches and tube sockets to the board.
An identifying feature of THD boards is the addition of famous quotes to the printing on the board. These circuit board quotes include: “‘A lie can travel halfway around the world while the truth is putting on its shoes.’ –Mark Twain” “‘Experience is the one thing you cannot get for nothing.’ –Oscar Wilde” “‘Don’t play what’s there. Play what’s not there.’ –Miles Davis” “‘Love is a perky elf dancing a merry little jig and then suddenly he turns on you with a miniature machine gun.’ –Matt Groening” “‘Wherever you go, There you are.’ –Buckaroo Banzai"
First vintage reissue amplifier
In 1987, THD Electronics put the first vintage reissue vacuum tube guitar amplifier into production, the 4-10 amp. It was modeled after a 1959 5F6-A "tweed" (a misnomer: the cloth is actually twill) Fender Bassman and, unlike the Peavey Classic Amplifiers of the 1970s (which were covered in twill-patterned vinyl) the 4-10 was covered in authentic cotton twill, the first since Fender and Gibson stopped production of twill-covered amplifiers in 1960. The 4-10 was in production from 1987-1993. In 1989 a head-only and 2-10 version were released as well.