guitar amplifiers founded 1986 founder andrew marshall headquarters seattle washington usa area served global products univalve bivalve 30 and flexi 50 amplifier heads hot plate power attenuator slogan any tube any tone anytime . website www.thdelectronics.com thd electronics ltd. is a manufacturer of vacuum tube guitar amplifiers and amplifier accessories and is located in seattle washington . it was founded in 1986 by president and ceo andrew marshall . although it specializes in hand built vacuum tube boutique style amplifiers it also manufactures the world s best selling guitar amplifier power attenuator 1 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 . 2 3 thd s line of products has stayed relatively small despite its 22 year history as a company . of its amplifiers thd currently makes three the univalve bivalve 30 and flexi 50 . contents 1 name origins 2 amplifier basics 2.1 exterior 2.2 vacuum tubes 2.3 printed circuit boards 3 first vintage reissue amplifier 4 thd plexi amplifier 5 2x12 extension cabinet 6 hot plate power attenuator 7 yellow jacket tube converters 8 univalve amplifier 9 bivalve 30 amplifier 10 flexi 50 amplifier 11 notable customers 12 references 13 external links name origins edit 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 4 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 . 4 marshall has said that thd alternatively means two hairy dogs referring to his bearded collies truffles and tobler 5 and with the addition of a third dog piper three hairy dogs . 4 amplifier basics edit exterior edit 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 edit 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 . 6 printed circuit boards edit 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 . 7 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 8 experience is the one thing you can not get for nothing . oscar wilde 9 don t play what s there . play what s not there . miles davis 10 love is a perky elf dancing a merry little jig and then suddenly he turns on you with a miniature machine gun . matt groening 10 wherever you go there you are . buckaroo banzai first vintage reissue amplifier edit 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 3 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 . 1 the 4 10 was in production from 1987 1993 . in 1989 a head only and 2 10 version were released as well . 1 thd plexi amplifier edit the next amplifier in production was the plexi manufactured from 1990 1993 . 1 although it too was physically styled after the fender bassman and was also available in tweed electronically and sonically it was meant to mimic an early jmp marshall . in 1992 guitar player magazine conducted a one to one test with a 1973 jmp marshall 50 watt amplifier head and found the two sounded very close . 11 from 1990 1991 a very limited run of thd 50 watt bass amplifiers was also produced . these bass amplifiers were dubbed the classic bass head . 1 for the last 18 months of the plexi s production several minor circuit changes were made prompting thd to rename it the type o . 1 2x12 extension cabinet edit in 1990 thd also started production on a 2x12 extension cabinet for guitar amplifiers 1 the 2 refers to the cabinet having two speakers the 12 refers to the size of the speakers in this case 12 inches . the back of the speaker cabinet is ported also known as vented which tunes the cabinet to give it substantially more bass response than either an open back or sealed cabinet can . the two inch slot port in the back of the cabinet is designed to offer the air and breath you get from an open back design 12 while giving more bottom end and overall efficiency than can be had from a sealed cabinet . fifteen years after it started production thd 2x12 extension cabinet won guitar player magazine s reader s choice award for best speaker cabinet . 13 although usually covered in black vinyl cream brown and blue paisley colored 2x12 extension cabinets were manufactured as well . hot plate power attenuator edit the hot plate power attenuator was first released by thd in 1994 . 1 it was designed to act as a type of master volume control for tube based amplifiers without sacrificing the distorted sounds such amplifiers make when they are played at full volume . the concept started in the late 1980s as a way to in house test the company s amplifiers without the technicians suffering hearing loss . 14 the hot plate works by being installed between the amplifier and the speaker cabinet if any is used absorbing the majority of the signal from the amplifier and passing only a small amount to the speaker . the remaining power is converted to heat . 14 the univalve and bivalve 30 amplifiers have hot plates built into them ; the flexi 50 has a foot switchable master volume control in lieu of a hot plate . the hot plate is manufactured in five different colors ; each color corresponds to the specific amplifier impedance that the hot plate is optimized for the impedances should be matched . the colors and impedances are gold 2 ohms green 2.7 ohms red 4 ohms purple 8 ohms and blue 16 ohms . the hot plate has a line out jack which can be used to send a di signal to the mixer or to insert equalization time effects and possibly a solid state amplifier between the distorting tube power amp and the guitar speaker . the line out or di signal can be blended with a miked guitar speaker at the mixing console . the load setting enables using the hot plate as a pure dummy load with no guitar speaker . yellow jacket tube converters edit the yellow jacket was also released in 1994 . 1 yellow jackets are a type of adapter that allows the existing power tubes in a guitar amplifier to be replaced with el84 power tubes without rebiasing the amplifier . in 1994 guitar world magazine conducted a test in which yellow jackets were installed in a 100 watt marshall plexi reissue . the resulting tones were similar to those of a vox ac30 although the amplifier was not as loud as it had been before the modification . 15 there are several sets of yellow jacket tube converters their designations relate to the type of amplifier they are converting . univalve amplifier edit the univalve is a 15 watt amplifier head released in 2001 . 1 although it operates with only one output tube it was designed to take a wide range of output tubes without needing to be rebiased . 8 the univalve was the first ever recipient of guitarist magazine s guitarist gold award under their revised scoring standards . 16 17 the etched design on the amplifier s front panel was made to resemble the work of artists and architects charles rennie mackintosh and frank lloyd wright . 18 as of 2004 only around 1 300 univalves had been produced . their serial numbers start with 000 although model number 000 was given to the thd dealer who coined the amplifier s name . billy gibbons of the band zz top requested serial number 007 and model 666 which is written on the back of the univalve chassis in red ink is owned by a former thd employee . 1 bivalve 30 amplifier edit as the name would suggest the bivalve 30 amplifier head is a 30 watt amplifier with two output valves . it was released by thd in 2002 . like the univalve the bivalve 30 s cathode biasing allows it to adjust to individual tubes automatically . it is one of only a very few parallel single ended tube guitar amplifiers ever produced and is the only one whose output transformer has separate primary windings for each power tube accomplishing the unique feat of magnetically merging the outputs of the two power tubes even if they are of dissimilar types instead of simply connecting the plates of the two power tubes to a single primary winding to sum their outputs which requires that the two tubes be of the same type . it too has a chemically etched stainless steel faceplate although thd chose a marine theme for the bivalve 30 . also the bivalve 30 uses screen printing on the faceplate in addition to etching . 19 in 2002 the bivalve 30 received a guitarist choice award from guitarist magazine after being rated a 4.5 out of 5 possible stars . 19 in that test the bivalve 30 was given a slightly lower score in the value for money category and the accompanying written review stated definitely an amp for the well heeled purist . flexi 50 amplifier edit the flexi 50 amplifier head was released in 2003 and made its debut at that year s music live event in the uk . it is a 50 watt class ab amplifier meant to create an idealized marshall plexi tone . 20 as a subtle homage to the plexi the flexi s faceplate is gold plexiglas . 20 like its predecessors the