How to troubleshoot preamp and rectifier tube problems and solve other common tube amp concerns
In Part 1 of Keeping Your Tube Amp Healthy, we talked about ways to prolong tube life and select new power tubes when they’re needed. In this tech tip, we’ll help you identify preamp tube issues and other factors that may be hurting your amp’s performance.
How to spot preamplifier tube problems
Usually when something’s amiss with your tube amp, it’s the tubes themselves that are the culprit. When you peer in the back of your amp it’s easy to distinguish between the power and preamp tubes. Power tubes are those big suckers; the preamp tubes are significantly smaller.
As we mentioned in the previous tech tip, preamp tubes usually last much longer than power tubes. Also good news is the fact they cost much less and can be swapped out one at a time rather than as a group as is recommended with power tubes. Since they’re relatively cheap, we recommend keeping an extra set on hand.
To avoid burning yourself or risking an electric shock, be sure to unplug your amp and let it cool down before getting into its innards. One further risk for shock are the filter capacitors used in older tube amps. Resembling small cans or batteries, they can store a lot of voltage that will discharge if you touch a contact—even when the amp is unplugged.
Some signs that your preamp tubes are on their last legs include:
Erratic EQ controls: If twisting any of your EQ knobs doesn’t seem to have an effect on your tone, it’s likely a preamp tube problem since equalization circuits reside in the preamp stage. Old capacitors (addressed below) can also affect the response of tone, volume and gain controls.
Unwanted distortion: If your amp is breaking up even at its cleanest settings with the gain and volume backed off, preamp tubes should be suspected. Usually the “driver” preamp tube located closest to the power tubes is the likeliest suspect. The output from the other preamp tubes cascade into this one, so replacing it may provide a quick fix. Guitarists who dote on heavily distorted tone all the time can probably disregard this concern.
Weak and noisy output: The first suspect is the preamp tube located closest to your instrument input since it handles the initial signal boost from your guitar.
Dead or erratic reverb: The reverb on older amps is handled by the preamp circuit and is the likely cause.
Other tube amp problems
Rectifier tube: Pre-1968 amps use rectifier tubes that will eventually fail. More modern tube amps use a solid-state rectifier that rarely fails. If you have a rectifier tube, it will usually be located at the far end of the power tube array furthest from the preamp tubes. The symptom of a dead rectifier tube is not subtle: there is no sound output whatsoever.
Amp/speaker mismatches: Swapping out your original tube combo’s speaker for one with a different impedance rating is likely to cause trouble. The same goes for routing your amp’s output to a speaker cabinet with a mismatched impedance rating or using an undersized connector cable.
Learn more with our expert guide: How to Choose the Right Guitar or Bass Amp Replacement Speakers
Weak capacitors: These inexpensive little components can lose their effectiveness over time. A competent tech can quickly identify and replace any that don’t test to specs.
When to hand it off to a pro
If you’ve swapped out suspect tubes, gently cleaned your control pots, checked that your tubes are well seated, and your problem persists, then it’s time to turn your loved one over to an amp tech. These folks have the test gear, tools and experience to restore your amp to its original performance, and can also make suggestions on currently available replacement tubes and other components that will help restore its sound and performance.
Shop the big selection of guitar amp tubes at Musician’s Friend.
Learn more with our Instrument Amplifier Buying Guide.
Got your own tips and experiences in dealing with tube amp trouble? Join the conversation below.