If you work on repairing tube amps that have odd intermittent issues, you know that one of the most valuable diagnostic tools on your bench is not a multimeter, oscilloscope, signal generator, signal tracer, distortion analyzer, or even a soldering iron. It’s a chopstick.
Not a fancy expensive chopstick, either – a simple wooden chopstick from your favorite Chinese takeout restaurant will do just fine. Audiophiles and gear heads: you’ll be disappointed to learn there is no “tone wood” chopstick or germanium-tipped chopstick that does the job any better.
When is a Chopstick Test Needed?
The electronics in amplifiers – especially guitar amplifiers used by working musicians – can get jostled around and sometimes minor damage sustained by this movement can cause intermittent problems. Solder joints can break. Connections can get tenuous through corrosion or weak connection points.
These intermittent problems can be maddening to diagnose. An amp can be making a certain noise – and then the noise goes away. Now you can’t track down the problem because the issue is gone! Or a problem appears for an amp owner but when they shut the amp off and take it to a tech, the problem magically disappears – only to reappear later when they take the amp home thinking it fixed itself. For cases like these, the chopstick test can save hours of frustration trying to diagnose an intermittent issue.
What is the Chopstick Test?
The chopstock test is simple. You take a simple non-conductive wooden chopstick and start tapping around the inside of the amp while it is on.
WARNING: Tube amps that are opened up and turned on contain very high voltages that are quite dangerous. If you’re trying this on your own at home you are doing so at your own risk.
The idea is to gently jostle the components, the solder joints, the PCB surface or turret board, and any connection points to see if slight mechanical pressure will cause the issue to surface. Listen for crackles, hums, and other noises caused by the tapping.
Remember: we’re talking about gentle taps – no more forceful than a finger flick.
Sometimes with very subtle issues, having a signal generator injecting an audible sine wave and an oscilloscope connected to the amp output can help give you a visualization of the changes to the signal that you might not hear as clearly.
An Example: How a Chopstick Fixed a Fender Twin Reverb
Here as an example is a Fender Twin Reverb amp that was in the shop last month. The amp had a weird issue – channel 2 was introducing some intermittent noise and distortion into the signal. After spending an hour testing tubes, switching tubes, testing voltages, and tracing the signal, I got out the old chopstick.
I found that I could trigger the noise by playing a C major power chord with my guitar’s neck pickup engaged. This for sure was a red flag that a connection issue was at fault, as the high amplitude bass frequency was probably vibrating a connector and causing it to shudder a bit.
For this test, I injected a sine wave around the same frequency as C4 on my guitar (~262 Hz) At first, it seemed like a bust – components and solder joints all seemed fine. But then – as I tapped on a ribbon cable connector between the two PCB boards, I hit the jackpot.
Results of the Chopstick Test
In this case, the ribbon cable connector was found to be not making solid contact. Connectors sometimes will need to be adjusted to ensure contact is solid and secure. In this case, however, it would seem the issue was simply a bit of corrosion, as a few squirts of DeOxit solved the problem immediately. I waited until the next day and tried again – and the problem did not resurface.
Other Common Results
- Bad solder joint
If a solder joint has cracked, tapping on it will break and re-establish the connection and you should hear a noisy crackle when you tap on it. This is an easy fix: Just re-flow the solder on that connection.
- Bad Connection
Connectors can sometimes not have a solid connection – and this is true not only of wire/cable connectors, but also tube sockets. If a tube socket and/or tube pins are corroded, this could cause some trouble. Tapping on tubes (or simply wiggling the tube with a gloved hand) can reveal intermittent connections on the socket. Some DeOxit should easily sort this out – but I’ve also had cases in really old amps where the socket connectors need to be adjusted as they’ve gotten too large for tube pins to make a solid connection with.
- Faulty Component
In some cases, tapping on a component itself might reveal noises, crackles, or buzzes. This could be indicative of a bad solder joint on one of the legs of the component, or it could reveal a component that should probably be replaced.
Have you used a chopstick in your tech work on tube amps? Let me know your experience in the comments.