Common Guitar Amp Issues and Troubleshooting

by Philip Fisher/15 May, 2020

Hum

Hum can be heard in the audio signal of amplifiers due to the addition of an AC signal from AC mains power or magnetic coupling. This can occur due to a fault with a component or grounding or the proximity of audio signal carrying wires to high AC carrying component. Depending on where the source of hum is generated the frequency can be different i.e. AC mains frequency 50Hz (60Hz US) or post rectified DC of 100Hz (120Hz US).

Magnetic Coupling

Remove all valves and power up the amp. If there is hum then the likelihood is that a transformer is not tightly bolted to the chassis or the transformers are magnetically coupling.

Check all nuts that are fastening transformers to the chassis are tight. If any require tightening then do so and then turn the amp on again to see if there is any change in hum level.
For transformers that are coupling undo one of the bolts to the output transformer, power on the amp and swivel the OT on its axis to see if there is any difference in hum level. If there are further transformers such as reverb or choke then a similar process may need to be performed. If this makes a difference in hum level then consider repositioning the transformer on the chassis.

General Note

Any wires containing unfiltered AC are a potential source of hum. Examples include heater wires, transformer wires and wires leading to and from the the power switch. All should be tightly twisted and kept away from signal wires as much as possible. Where they do have to cross they should do so at right angles.

Power Section

If there is no hum with all tubes removed then put both rectifier and power tubes back in the amp. If there is hum when the power tubes are back in the amp then perform each of the steps one by one and test the results (these checks apply to 50 or 60Hz hum). Each test should be performed without the amp switched on, plugged in (unless otherwise stated) and filter capacitors fully discharged:

Visually inspect all transformer wires for proximity to audio carrying wires or components. Power on and very carefully use a non-conducting chopstick or similar tool (not a pencil) to move the transformer wires or audio wires away from each other and see if this makes any difference to hum levels.
Visually inspect all AC carrying wires such as mains, output transformer HT out, rectifier 5V (6.3V) out, heater 6.3v out and make sure twisted and away from any component that they may couple with. An example could be a 5F1 champ dual power and volume switch which has AC power to the back of the switch - this setup can result in a certain amount of hum if not fitted properly.
Remove the rectifier and power tubes and visually inspect the tube sockets. Check to see if they are dirty or any of the metal pin retainers are lose. Use contact cleaner to clean them if they are dirty. Use small implement to bend back into place if lose.
Replace the power tubes with tubes that are known to be working correctly and power on to check if hum is reduced.
Visually inspect all ground points in the circuit for bad or cold solder joints or cracks. Resolder where applicable.
Follow a grounding scheme scheme such as star or bus and minimise ground points which do not adhere to this i.e. if there are several ground points in an amp then there may be a risk of creating ground loops.
Make sure that all points in the circuit where there is supposed to be a ground that there is. Cross reference with the amp schematic.
Visually inspect components in the circuit for any cracks, burns, bad solder joints and replace or resolder where applicable.
Make sure that the heater wires are tightly twisted and are placed away from any audio carrying wire feeding into the power tubes. As before power on the amp and very carefully use a non-conducting tool such as chopstick to move wires to see if this makes a difference in hum levels.
Make sure that the heater wires are in phase for both power tubes i.e. the heater wire from pin 2 of one power tube should be attached to pin 2 of the other power tube. This also applies for pin 7 of one power tube to pin 7 of the second power tube. If they are not in phase then remove and resolder so that they are.
Make sure that the heater wire centre tap is either grounded properly to the chassis or grounded on an elevated DC supply such as power tube cathode.
If there is no heater wire centre tap then make sure that there is a virtual centre tap made up of two small value resistors (generally 100 ohm) with one side attached to each of the heater wires (pilot light connections or pins 2 and 7 of power tube as an example) and the other side to ground (either chassis ground or elevated DC ground as above).
A humdinger can be used instead of centre tap or virtual centre tap to allow for a more precise level of balancing of both heater wires and reduction of hum. Make use of a 100-250 ohm pot with either side attached to heater wires (as mentioned above) and centre wiper attached to ground or elevated ground.

If the hum being generated is 100Hz (120Hz for US) when only the power tube(s) are installed then the following checks should be performed:

Visually inspect the filter capacitors for any bulges, damage, bad solder joints or cracks. Replace or resolder if applicable and test again.
Alligator clip an extra 20-40uF cap in parallel with the first filter capacitor temporarily and test to see if this reduces the level of hum. This should be performed with extreme caution and at no point during power up should you touch any wires or components. If by increasing the level of capacitance at the reservoir capacitor decreases the hum the following options are available 1) increase the capacitance of the reservoir capacitor (make sure that this is within the limits for valve rectification – SS do not have these sort of limits) 2) Add another RC node prior to the first node and use a resistor with a low value such as 100-250 ohm so as to not drop the HT voltage too much. The capacitor value can be similar to that used in the reservoir capacitor. Remember to remove the HT B+ wire from current position and solder to the new node 3) Replace the first node resistor with a choke (this provides a high level of inductance which will let DC and only low AC frequencies pass).

Preamp Section

When all tubes are in place if the hum increases and decreases with the volume then the source of hum is prior to the volume control.

Visually inspect all wires for proximity to each other and make sure that if they do cross each other that they cross at right angles.
Replace the preamp tubes with tubes that are known to be working correctly and power on to check if hum is reduced.
Remove the preamp tubes and visually inspect the tube sockets. Check to see if they are dirty or any of the metal pin retainers are lose. Use contact cleaner to clean them if they are dirty. Use small implement to bend back into place if lose.
Visually inspect all ground points in the pre-amp section for bad or cold solder joints, cracks. Resolder where applicable.
Visually inspect all components in the preamp section for damage, cracks, burns, bad or cold solder joints. Resolder or replace where applicable.
Power on the amp and very carefully use a non-conducting tool such as chopstick to move wires and tap wires/components to see if this makes a difference in hum levels or induce any sort of noise. Replace components (where applicable) when there are large microphonic noises generated.
Make sure that all points in the circuit where there is supposed to be a ground that there is. Cross reference with the amp schematic.
Make sure that the heater wires are tightly twisted and are placed away from any audio carrying wires feeding into and out of the preamp tubes.
Make sure that the heater wire centre tap is either grounded properly to the chassis or grounded on an elevated DC supply such as power tube cathode.

When all tubes are in place if the hum does not increase or decrease with the volume then the source of hum is after the volume control.

Visually inspect all ground points in the section after the volume for bad or cold solder joints or cracks. Resolder where applicable.
Remove any tubes in the section between the volume control and power section and inspect the tube sockets. Check to see if they are dirty or any of the metal pin retainers are lose. Use contact cleaner to clean them if they are dirty. Use small implement to bend back into place if lose.
Much like the preamp section check all ground points and components for damage, burns, cracks and cold solder joints (this includes pots). Replace or resolder where applicable.
Make sure that heater wiring is not close to any of the components in this section.

Squeal or Motorboating

Sometimes it is possible for a positive feedback loop to form in an amplifier that results excessive heat in tubes (without noise), temporary signal dropouts, or an audible noise such as squeal or motorboating. The name give to this is parasitic oscillation.

Detection

If the amp has an audible sound sounding like phut putt putt putt i.e. motorboat.
If the amp has an audible sound that sounds like feedback.
If you have access to an oscilloscope then put an audio signal through the amp and trace the signal. When there is a parasitic oscillation the sine wave should appear as a fuzzy worm.
An amp with oscillation is normally very sensitive to hand position. Try moving your hand closer (but without touching any component) and further away from all tubes when the amp is on. If there is a change in output then there may be oscillation.

Solutions

If the amp has a negative feedback loop then it may be the case that the output transformer plate leads need to be swapped around. This is a common fault with new builds.
Are there any tubes without grid stopper resistors? All tube grids should have grid stoppers to help RF signals and oscillation. Add grid stoppers with one side of the resistor attached to the tube pin and the other to the lead coming in.
Sometimes putting higher gain tubes or bad tubes into the amp can trigger oscillation. Try replacing preamp tubes with other same gain and/or lower gain tubes to see if this makes a difference in oscillation.
Look at the positioning of wires in relation to one another and other components (lead dress). Power on and very carefully use a non-conducting chopstick or similar tool (not a pencil) to move wires going into tubes away from other wiring and see if this changes the level of squeal or motorboating.
It is possible to reduce oscillation by the addition of components

Add plate bypass capacitor in the pF range to tube causing oscillation to reduce icepick highs and remove oscillation.
Reduce the bypass capacitor value to reduce the amount of frequency/boost going through the tube.
Reduce the value being used for the coupling capacitor which feeds the signal into the tube which has the oscillation.

Motorboating can also be caused by a conductive component board - dampness.

Popping

Amps that have a popping sound could have issues relating to rectifier arcing, tube socket arcing, intermittent component connecting, component arcing, bad tube socket or transformer arcing.

Detection

If an amp has an audible sound like popping.

Solutions

Power on the amp and visually inspect rectifier and power tubes for any signs of arcing i.e. flashing. Carbon deposits on tube sockets are evidence of where there has been arcing. Replace tubes where applicable.
Power on the amp and very carefully use a non-conducting tool such as chopstick to move wires, tap wires/components and tap tube sockets to see if this creates a popping sound. Replace components and/or resolder where applicable

Static and Crackling

Amps that have a static or crackling sound may have issues relating to bad connections or solder joints, arcing in coupling capacitors, bad tubes, bad resistors, dirt switches, component arcing, faulty pot or other causes.

Detection

If the amp has an audible crackling or static sound when powered on.

Solutions

Remove the preamp and inverter and power up the amp. If the crackling or static sound remains then visually inspect all components and wires after the inverter tube. If any components look burnt then replace them; if any solder joints look bad or are cracked then resolder. Visually inspect power tube sockets and clean with contact cleaner. Also visually inspect tube socket retaining pins and make sure they look tight - if not tight enough then use a small tool to pinch the clip together so will tightly hold the tube pins. Power on the amp and very carefully use a non-conducting tool such as chopstick to move wires, tap wires/components and tap tube sockets to see if this makes the crackling / static sound worse.
If no crackling exists with only the power tubes in then power down, replace inverter tube, and then power up again. If the static or crackling starts then perform the same visual inspections, replacement and resoldering as mentioned in the previous step. Visually inspect volume / master volumes and clean with contact cleaner. If this does not work then replace the inverter tube with a known working tube.
If no crackling exists with the power tubes and inverter tubes in place then power down, install preamp tube(S), and then power up again. If the static or crackling starts then perform the same visual inspections, replacement and resoldering as mentioned in the previous step. If this does not work then replace the preamp tube(s) with known working tube(s).

Hiss

Hiss in amplifiers is a common thing to have but differs in magnitude from amp to amp based on the component types, number of gain stages, use of negative feedback and level of gain the amplifier has.

The choice of resistor type can make a huge difference in amount of hiss (johnson thermal noise) an amp generates. Carbon composition resistors are the noisiest type of resistor followed by carbon film. Thin film, metal foil and wire wound resistors have better noise characteristics than any other type therefore should be used exclusively for low noise amps or placed in locations where there are large voltage swings. You will find that many hand built amps do not have metal film resistors throughout because this potentially leads to an amp with a more flat and clinical tone. Metal film will be judiciously placed at different locations to reduce as much noise as possible without impacting the tone they are looking for.
Addition of negative feedback into the amp circuit can reduce hiss and increase headroom.
Replacing a 12AX7 preamp tube with a lower noise 12AX7 or lower gain tube such as 12AY7 can reduce hiss.

Power

Coming Soon ...