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Voodoo Amps® Basic Trouble Shooting - Unwanted Hum & Noise

 

Welcome to Voodoo Amps Basic Trouble Shooting Guide. The information on this page has been accumulated through years of technical experience, trouble shooting and countless live shows. We hope you find this page helpful and that is aids in helping you resolve the problems you are experiencing. 

 

 Basics

Be certain your amp has a properly grounded 3-prong IEC / AC power cable
Do not use a 2-prong AC cheater plug
Be certain you are using known-to-be-good cables
Plug guitar directly into amp (eliminate all effects, etc from signal chain)
If the problem persists proceed to trouble shooting guide

 

If you are experiencing unusual amounts of noise, hum, buzzing or other unwanted noise issues; We have compiled a list of possible variables. Some examples are but are not limited to;

CHECK LIST

  • PREAMP TUBES
  • POWER TUBES
  • AC CHEATER PLUGS
  • INSTRUMENT CABLES
  • EFFECT PEDALS
  • GUITAR ELECTRONICS
  • IMPROPERLY GROUNDED AC
  • LIGHTS & SIGNS
  • APPLIANCES
  • GROUND LOOPS
  • PROXIMITY EFFECT
  • COMPUTERS
  • CELL PHONES
  • MISMATCHED POWER TUBES
  • IMPROPERLY BIASED POWER TUBES
     

PREAMP TUBES - Faulty tubes are the #1 cause of issues with tube amplifiers

  • New tubes are not infailable and can preamaturely fail
  • Problem - No audio, weak audio, cracking sounds, volume flucations, loss of gain/distortion, etc
  • Solution - Swap out / replace all preamp tubes with known to be good preamp tubes
  • You can replace one preamp tube at a time. Simply remove the first preamp tube and replace it with a known to be good tube. If problem persists proceed to the next tube until you find the faulty tube.
  • Problem - A high pitched frequency / squeal occurs when playing. You tap on the chassis and you hear a "pinging" sound, etc. These are the symptoms of a microphonic preamp tube.
  • Diagnose - Lightly tap on each preamp. One or more tubes will generally "ring" louder than others.
  • Solution - Replace faulty tube
  • Note, preamp tubes are plug & play

 

POWER TUBES

  • New tubes are not infallible and can prematurely become intermittent, fail or short
  • Problem - Experiencing a low frequency ringing or low frequency feedback
  • Diagnose - Tap on the chassis (front / rear) and you hear a low pitched "thunking" sound etc. This is a symptom of a Microphonic power tube. Next lightly tap on each power tube. One or more tubes will be noisier / louder than the others.
  • Solution - Replace all power tubes with a matched set & properly rebias
  • Note - Amps with two (2) or more power tubes - All power tubes must be replaced with a matched set. Replacing just one (1) power tube may result in damage.
     

AC CHEATER PLUGS  Also known as ground lift plugs

  • IMPORTANT  Do not use an AC cheater plug. If you have a grounding issue resolve it
  • Employing the use of a cheater plug can easily result in costly damage to your amplifier
  • AC cheater plugs eliminate the connection to earth ground allows for costly damage to occur
  • Tube amps need to be connected to earth ground
  • Solution - If your amp does not have a 3-prong AC power cable have one properly installed immediately
     

INSTRUMENT CABLES 

  • IMPORTANT FACT  Quality instrument & speaker cables will improve your tone while shielding out unwanted noise & hum.
  • Poor quality instrument cables are a common cause for unwanted hum, noise, RF signals, etc.
  • Broken grounds on an instrument / speaker cable will cause unwanted hum, etc.
  • If you can move your cable against the floor and you can hear it  This is a sign of a faulty or poor quality cable - Upgrade to higher quality cables
  • Note, Do not use a speaker cable in place of an instrument cable & do not use an instrument cable in place of a speaker cable
     

EFFECT PEDALS   As per the Basic Trouble Shooting Guide listed above

  • IMPORTANT FACT  Quality instrument cables will improve your tone while shielding out unwanted noise & hum - especially when connecting pedals together!
  • If you have removed the pedals from your signal chain and the problem was eliminated - Start by introducing one pedal at a time until you find the pedal that is causing the unwanted hum / noise
  • It is common to discover more than one pedal is causing the problem(s).
  • Solution - Purchase a quality DC power supply. There are numerous pedal board power supplies on the market that will help eliminate hum when using effect pedals.
     

GUITAR ELECTRONICS  

  • Improper wiring on guitar electronics is one of the most common causes for unwanted hum and an increased noise floor.
  • We have experienced this problem on brand new instruments with stock factory wiring
  • Solution - Take your guitar(s) to a reputable repair tech & have your tech properly re-wire / ground the electronics.
  • Note: A professionally wired guitar will sound better and the over all noise floor will be reduced.
  • IMPORTANT FACT  Soldering is an art form. Poor quality soldering will not only result in disappointment but also increased hum and noise as well as unreliability. When in doubt take your guitar to a reputable repair tech (even if it requires shipping the guitar).
     

IMPROPERLY GROUNDED AC OUTLETS

  • Do not use AC cheater plugs (see above)
  • 3-Prong AC outlets are ideal
  • If wired properly a 3-Prong AC outlet will allow your amp to see earth ground. Your amp will sound, feel and perform it's best while also being properly protected.
  • 2-Prong Outlets - Older homes / buildings can have 2-prong outlets, which are not wired properly.
  • If your home has a 2-prong outlet we recommend purchasing  a transformer isolated AC voltage regulator & conditioner
  • Note: There are vintage Fender amplifiers that were wired incorrectly from the factory. When in doubt consult with a reputable repair tech.
     

LIGHTS & SIGNS

  • Computer lights / lamps
  • Office / desk lamps
  • Dimmer - Raises / lowers the brightness of the lights in your home
  • Dimmer packs - Connected lights in a venue (par cans, etc)
  • Florescent lights
  • Neon signs
  • Black lights
  • Computer Monitors
     

DIMMER PACKS

  • Commonly associated with lighting consoles in venues
  • Dimmer packs are used to control the brightness or intensity of par-cans / stage lights 
  • Close proximity to dimmer packs will cause unwanted hum, etc
  • Solution - Move your amp / rig away from the dimmer packs. If problem persists
     

COMPRESSORS  Units sharing the same AC circuit (IE: often times numerous outlets share same AC circuit)

  • Generally speaking; Numerous AC outlets will share the same AC circuit. IE: often times plugging into another AC outlet shared by the same AC circuit will cause the same problem
  • Solution - Find an outlet that is on a separate AC circuit or purchase a transformer isolated AC voltage regulator & conditioner  
    Examples of units containing a compressor(s) are but are not limited to;
    • Air conditioners
    • Refrigerators
    • Washers & dryers
    • Hair dryers
    • Potable Heaters
    • Generators
    • Beer coolers (found in clubs / venues)
    • Ice machines (found in clubs / venues)
    • Upright coolers  (found in clubs / venues)
    • Other house hold appliances
       

PROXIMITY EFFECT  can cause unwanted hum, oscillations, etc

  • Guitar too close to amp (pickups interact with transformers magnetic field)
  • Guitar too close to computer monitor
  • Amp too close to computer monitor
  • Cell phone too close to computer monitor
  • Cell phone too close to computer monitor
     

COMPUTERS & MONITORS 

  • Can cause unwanted hum, noise, etc.
  • See Proximity Effect
  • Note, high quality instrument cables will help to shield out RF signals from computer monitors, etc. 
     

CELL PHONES

  • Can cause a series is beeps in lower to mid frequencies
  • See Proximity Effect
  • Solution - Turn off / power down or Move cell phone far enough away to avoid interference
     

MISMATCHED POWER TUBES

  • Can cause unwanted hum & poor tone
  • Important - Tube manufacturers are not properly burning in tubes. If you have non-burned in matched  power tubes installed in your amp they will drift apart resulting in unwanted hum and poor tone. Make sure your powr tubes are properly burned in for no less than 40 hours.
  • If a power tube fails - Turn the amp off, replace the tubes with a matched set and properly rebias. 
     

IMPROPERLY BIASED POWER TUBES

  • Can cause unwanted hum & poor tone
  • Approximately 80% of all tubes amps that we work on or service are biased improperly.
  • To properly bias your amp you must monitor the plate voltage and utilize that to crunch the math properly. This will then allow you to properly bias your amp. Your tone will be better and the amp will feel better to play on. 
     

Disclaimer  All information given herein is for educational purposes only. There is no warranty, implied or otherwise, as to the correctness, suitability, or merchantability of the information for any purpose.

Warning  Any and all changes made to electronic equipment must be done by qualified professional service personal. Even an unplugged amplifier contains High Voltages that are easily lethal even to the experienced electronic engineer. Do not remove the cover or chassis from your amplifier without professional assistance.

Voodoo Amps does not provide free tech support via phone or email.

 


 

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