Unwanted Feedback

Voodoo Amps® Basic Trouble Shooting Guide - Unwanted Feedback

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 a 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 unwanted feedback, high frequency oscillation, high pitched squealing, etc; We have compiled a list of possible variables. Some examples are but are not limited to;

BRIEF CHECK LIST

PREAMP TUBES
POWER TUBES
INSTRUMENT CABLES
GUITAR PICKUPS
PROXIMITY EFFECT

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

New tubes are not infallible and can prematurely become intermittent, fail or short
Problem - Experiencing a high pitched ringing / feedback when playing or when you stop playing.
Diagnose - Tap on the chassis (front / rear) and you hear a "pinging" sound, etc. These are the symptoms of a Microphonic preamp tube. Next lightly tap on each preamp. One or more tubes will be noisier / louder the others.
Fast Solution - Swap out / replace all preamp tubes with known to be good preamp tubes
Slower Solution - Replace one preamp tube at a time. IE: 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.

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.

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, oscillations, feedback, 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 your cables!
Do not use a speaker cable in place of an instrument cable
Do not use an instrument cable in place of a speaker cable

GUITAR PICKUPS

Proper pickup height is critical to good tone and more times than not is over looked.
Problem - Experiencing feedback when you stop playing.
Solution - Lower the pickup(s). All too often the pickups are too close to the strings causing unwanted feedback.
Other comon problems caused by pickups being too close to the strings include but are not limited to; Tuning issues, intonation issues, increased noise, muddy / unclear bottom end (most noticeable when palm muting), smeary / undefined tone, harsh top end, etc.
Pickups with metal covers can contribute to unwanted feedback
Solution - Removing the metal pickup covers will usually resolve the problem
Some pickups are not properly potted causing unwanted feedback. With the guitar volume full up, press on the pickup. If the feedback goes away you will need to have the pickup properly re-potted. This problem tends to occur on cheaper pickups or pickups with a high output.
IMPORTANT FACT Pickup height is critical to achieving good tone. Proper pickup height varies from one guitar to the next.

PROXIMITY EFFECT can cause unwanted hum, oscillations, etc

Guitar too close to amp (pickups interact with transformers magnetic field)
Problem - Experiencing feedback when standing next to amp
Solution - Move away from the amp
Other solution - If you cannot move further away from the amp purchase a noise suppressor or noise gate
Problem - Experiencing feedback when facing the amp or when you stop playing.
Solution - Face away from the amp and turn the guitar's volume knob off when you are not playing.
Other solution - Purchase a noise suppressor or noise gate

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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