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Guyatone FLIP VALVE MM-1 Metal Monster (Serial 94090420)Estimated 1994 (based on serial number and component analysis)Confirmed 1994 Japanese productionThis listing is for a Guyatone FLIP VALVE series MM-1 METAL MONSTER, a Japanese-made tube hybrid distortion pedal from the 1990s.Carefully cleaned, inspected, and fully tested, this unit is fully functional and retains the aggressive hybrid distortion character that made the MM-1 one of Guyatone’s distinctive tube-equipped distortion pedals.This particular example includes its original box, original warranty card, and original instruction manual — a notably complete set that is becoming increasingly difficult to find.Based on serial number structure, footswitch codes, and internal component consistency, this pedal is confidently identified as a 1994 production example.The MM-1 represents Guyatone’s Japanese approach to hybrid tube-based distortion design, combining vacuum tube tonal coloration with flexible active EQ control.Key Features & ConditionModel: MM-1 METAL MONSTERBrand: GuyatoneSeries: FLIP VALVESerial number: 94090420Made in JapanBuilt-in FLIP 12AX7A vacuum tubeCompatible replacement tube types include: 12AX7 / ECC83 / 7025Users may also experiment with compatible lower-gain variants such as 5751 or 12AT7 / ECC81 at their own discretion, though gain structure and tonal response will vary.Original box includedOriginal warranty card includedOriginal instruction manual includedFully cleaned and carefully inspectedTested and confirmed working properlyDC 9V center-negative adapter operation onlyBattery operation is not supportedPots are clean with no noticeable scratchiness during testingVisible scratches, rust, dirt, paint wear, and cosmetic aging consistent with real vintage usePlayer’s conditionPlease refer to photos for exact cosmetic condition details.Tonal Design / Circuit CharacterThe MM-1 METAL MONSTER combines solid-state distortion architecture with integrated vacuum tube coloration, creating a distinctive hybrid distortion design.Its control layout is notably advanced, offering extensive active EQ flexibility centered around detailed midrange shaping.Control layout and function:DistortionControls overall gain intensity. Turning clockwise increases distortion saturation.MiddleActive midrange EQ control with center-zero operation. Clockwise boosts midrange frequencies, while counterclockwise cuts them.Middle FrequencySelects the target frequency range affected by the Middle control. Turning clockwise sweeps approximately from 200Hz to 5kHz.LevelControls overall output volume of the distortion circuit. Turning clockwise increases output level.HighActive treble EQ with center-zero operation. Clockwise boosts high frequencies, while counterclockwise reduces them.LowActive bass EQ with center-zero operation. Clockwise boosts low frequencies, while counterclockwise reduces them.The MM-1’s dual Middle / Middle Frequency system functions similarly to a semi-parametric midrange EQ.This allows players not only to boost or cut mids, but also to precisely choose which part of the midrange spectrum is being shaped — from low-mid body enhancement around 200Hz to upper-mid attack and presence approaching 5kHz.This broad tonal range makes the MM-1 capable of:Heavy scooped metal tonesFocused cutting lead soundsLow-mid thick rhythm texturesUpper-mid aggressive presence shapingKey design characteristics:Built-in 12AX7A / ECC83 tube hybrid architectureReplaceable with compatible 12AX7 / ECC83 / 7025 typesHigh-gain distortion voicingActive High / Low EQCenter-zero active Middle EQSweepable 200Hz–5kHz Middle Frequency targetingSemi-parametric style midrange shapingTube-swapping potential for tonal experimentationUnlike many distortion pedals that rely on simple tone roll-off, the MM-1 offers more sophisticated EQ architecture, making it especially useful for amplifier matching, cabinet compensation, and player-specific tonal sculpting.This combination of tube hybrid circuitry and flexible frequency control makes the MM-1 both a practical performance pedal and a distinctive hybrid pedal design.Serial Number / Estimated Production PeriodSerial number: 94090420Estimated 1994 (based on serial number and internal component analysis)The dating estimation is based on multiple production indicators with strong consistency across major components.Production Dating Evidence (Primary Source)Serial Number Analysis:9409042094 = 1994The first two digits strongly indicate 1994 production.While the remaining digits may represent batch, production sequence, or internal factory coding, there is insufficient verified documentation to confidently assign precise month or lot interpretation based solely on those numbers.For accuracy, this listing identifies the unit as a 1994 production example rather than claiming a confirmed month.Footswitch:FPD63 — Marking: 4E21This code is broadly consistent with 1994-era component production and supports the serial-based 1994 estimate.Operational Amplifiers:NJM4558L JRC 3165G (x6)These components are fully consistent with mid-1990s production and align with the estimated 1994 manufacturing period.Taken together, the serial prefix, footswitch code, and internal IC consistency strongly support this unit as a 1994 Japanese-made MM-1.Other Major Components (Documented for Transparency)Vacuum Tube:FLIP 12AX7ACompatible standard replacement types:12AX7ECC837025Optional lower-gain compatible alternatives (user discretion advised):575112AT7 / ECC81This tube appears period-correct for the model and production period.While tube branding alone is not used as primary dating evidence, it remains fully consistent with the estimated manufacturing era.Condition DetailsThis unit is fully functional and operates properly as intended.The pedal shows visible signs of age and use, including scratches, paint wear, rust, dirt, and general cosmetic aging consistent with vintage player-grade equipment.Controls were checked during inspection, and no significant scratchiness was detected.The pedal has been thoroughly cleaned overall while preserving its original character.Please refer to photos for detailed condition.Power Supply (Important)This model operates via DC 9V center-negative adapter only.Battery operation is not supported.Adapter is not included.Production BackgroundThe MM-1 METAL MONSTER is a Japanese-made hybrid tube distortion pedal that reflects Guyatone’s design approach to combining vacuum tube tonal coloration with practical pedal functionality.Its combination of integrated tube architecture and flexible active EQ design makes it a uniquely feature-rich distortion pedal for its era.This model remains notable for its broad tonal shaping capability and distinctive engineering approach.IncludedOriginal boxOriginal warranty cardOriginal instruction manualNote: DC adapter is not included.Shipping / Import Duties / Address RestrictionsShipping costs (if applicable) are determined according to the individual listing and destination.If free shipping is offered, it applies to outbound shipping charges only.Import duties, taxes (including VAT), customs clearance fees, brokerage fees, and any additional charges imposed by the destination country or shipping carrier are the responsibility of the buyer and may vary depending on the destination country.Please note:We cannot ship to P.O. BOX, POB, APO, or FPO addresses.Shipping eligibility may vary depending on the destination country and local customs regulations.Certain countries may restrict delivery to either private individuals or registered business entities.If shipping restrictions apply to your address type, we may contact you for clarification or cancel the order.Please ensure your shipping address is complete and accurate before purchasing.Orders that do not meet these conditions may be cancelled.About RETUNE WORKSRETUNE WORKS specializes in careful inspection, cleaning, and preservation of musical equipment while respecting original circuits, tone, and historical integrity.For this MM-1, major components appear period-consistent and no major circuit modifications have been identified.This unit has been carefully cleaned, inspected, and tested to ensure reliable operation while maintaining originality and preserving its authentic character.Our mission is to ensure that quality vintage gear continues to be played, enjoyed, and passed on to the next generation.