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Finding faults in guitar amps

Fault finding in audio equipment

Argh! The internet! The most frustrating thing as a professional amp repairer is seeing so much misinformation spread about guitar amp faults. Every time a poor soul with a broken amp pops up on a facebook group there’s a chorus of very confident replies. It’s the caps! It’s the output transformer! Etc etc.

You’ve probably seen these guys below. Don’t get me wrong, I know people are only trying to be helpful, but they aren’t. Not because of their lack of expertise, but because they don’t possess psychic powers. Despite 10 years experience and a very high success rate in amp repairs, when someone brings me an amp, I usually don’t have a clue what’s wrong until I’ve got it on the bench, run a signal through it and stuck some probes inside!

Fault finding on the internet in 2020!

“Sucks to be you mate, it’s an output transformer failure!”

This is the most bizarre one. Output transformers very rarely fail. I repair about 200 amps a year and see maybe one or two output transformer failures. It just doesn’t happen very often!

“Sounds like BAD CAPS to me!”

“It is a truth universally acknowledged that an amp in possession of a fault must be in want of a recap.” So said Jane Austen in her ill-fated 1817 comeback attempt ‘Caps and Incapacity”.

Caps (capacitors) do cause failures. Most commonly issues are related to the electrolytic capacitors that filter the power supply. Electrolytic capacitors have a dielectric that dries out over time.

I’ve heard various rules of thumb – they should be replaced after 5 years, 10 years, 20 years, 40 years or never at all (argh!).

Cap lifespan is definitely a subject for another post. I don’t go by a rule of thumb. I know that there are certain designs where the caps have been designed close their ratings where replacement is advisable. I know that there are certain brands that *nearly always* leak. I still see plenty of working amps with 40 year old caps in them but yes, I do always recommend that they’re replaced at that age. There are some cap brands in some designs that don’t ever seem to fail around the the 20 year mark so I give that honest opinion to the customer and they can choose whether they want to future proof the amp. There are some amps *cough cough* modern Fender *cough* where I recommend that they’re replaced after no more than 5 years as they’re so darned lousy.

How to diagnose guitar amp faults by ear - a simple guide
The truth is, that any fault can lead to a number of different symptoms. And any symptom can have any number of possible root causes.

“My Vox crackled too. I put new power valves in and it was fine.”

Two amps with the same symptoms will not have the same root cause. Right now, in the workshop, I’ve got a crackling Hot Rod Deluxe caused by an overheating LT supply, a Jet City with a crackling preamp valve and a Mesa with crackles caused by FX loop oxidation.

Add to that the huge difference between various designs. An AC30 runs the valves *hard* and goes through EL84 power valves regularly. A Peavey 5150 biases the power stage very cool and doesn’t wear out power valves anywhere near as often.

So what’s the right way to do it?

There’s no one way to do it, but let’s take an example. I’ve got a Blackstar in at the moment with a hum issue. This is what I’ve been doing.

  1. Test all the valves. They’re all fine. Replace the valves.
  2. Turn the master volume down. The hum didn’t go away, so we’re looking at a power amp issue.
  3. Remove the phase inverter. Hum goes. This amp has no NFB so we’re looking at a phase inverter issue.
  4. Check pin voltages on the valve. Anode voltages are radically different.
  5. Remove valve and retest. Anode voltages still different. Smoking gun, this looks like an anode resistor.
  6. With the amp off, meter the anode resistors, one is way out of spec. Replace it.
  7. Turn amp on. Situation is better, but anode voltages are still different. The valve passed test, but maybe the tester missed something. Replace it
  8. Anode voltages fine.
  9. Test with speaker again. Hum gone!
  10. Check bias. It’s way too high. Fix and recheck.
  11. Check output power.
  12. Listening test with guitar
  13. Soak test the amp.
  14. Listening test again.
  15. Reassemble

Common faults

That said, there are times when it is possible for an experienced guitar amp repair to predict the cause of a failure. This is usually where there’s a flaw in the design that causes repeat failures. Some examples are: