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Diagnosing and Fixing a Tube Amp

How to Listen to and Care for Your Amp When It’s Sick

Guitar amps are, no matter how you cut it, black boxes. You may think you have control over them, but when it gets down to it, you can’t really see inside the black box, and even if you could, there aren’t any moving parts. Electricity is largely non-mechanical. Or in other words, magic. And when the magic stops, most people think all they can do is resort to prayer. Or an amp technician. 1. My trusty Fender Vibro-King was on the fritz, and the suspects were the output tubes and the speakers. 

When your amp isn’t performing up to snuff, there’s still a lot you can do without having an EE degree, or even knowing how to operate test equipment. Here are some holistic approaches—and solutions—you can try yourself, as I did when my tube amp went on the fritz. Warning: Some of the following procedures involve messing around with the components of the amp, so be careful. Electricity can kill you. Proceed at your own risk.

Creeping Death

Failure in a tube amp is often creeping and insidious, more like the wearing of the tread in your tires than a light bulb blowing. Because the changes generally occur over time, you can become inured to little degradations in performance. Then one day, maybe after you’ve been away and come back to it, you realize something’s not right. Such was the case with me and my favorite tube amp, my Fender Vibro-King (see Fig. 1). The following procedures, though, will work on many tube amps. Just swap out the specifics or make the necessary adjustment for your model accordingly. this thing killing?

The first test I made was just to see how loud my amp got, and if it was ear-splitting—as nature intended—at its highest volume. The Vibro-King has no master volume, so this is fairly easy to determine: wind the lone volume knob up to 10, stand the hell back, and play your guitar with the controls full out. Disappointingly, the amp, in its maxed-out state, did not rattle the windows, disturb the neighbors, and risk injury to my eardrums. I knew the amp wasn’t firing on all cylinders even without the benefit of test equipment or knowing which link in the chain was falling down on the job (if you’ll forgive the mixed metaphor). Next step: Just how is it not delivering full power?

The Symptoms: It Hurts When I Do This

The Fender Vibro-King is an all-tube amp containing five 12AX7’s, two 6L6 power tubes, and three 10" Jensen P10R speakers. The amp was definitely underperforming though I really couldn’t be sure why. The manifestations of this poor performance included a distorted sound at a low volume, a generally mushy and ratty/rattly tone, and an overall output that didn’t seem as loud as it once was when the amp was in its prime.

I noticed further that rather than a wholesale volume loss, the amp was plenty loud with the volume knob up to about noon. Beyond that, the amp simply got more distorted, but not appreciably louder. This led me to conclude that while the preamp was pumping more and more voltage through the amp, the power tubes weren’t up to the task. There may have been other factors involved in this, such as the capacitors, resistors, power transformer, and output transformer, but I knew I could focus on the power tubes.

Now, I know that power tubes lose strength over time and that electrolytic capacitors as a rule need replacing after about 10 years. But I didn’t have the wherewithal nor the time to measure the various voltages across the caps and plate resistors, if indeed these were the culprits.

But I looked at it this way: If I were to take this amp into a shop, the tech would most certainly replace the tubes. Since I know I already need tubes—and this would improve the amp significantly (if not completely)—I had nothing to lose by trying new tubes in the amp to see if that made a difference. So acquiring in advance a matched pair of new 6L6’s wouldn’t be a waste of effort nor money (I’d simply supply the tech with my own parts). And replacing the tubes and hearing an improvement would prove that the tubes were indeed among the problems. But I didn’t have a new set of matched 6L6’s in my possession.

From Break Up to Make Up (Gain)

As it so happened, though, sitting right in the same room with me—in fact right next to the Vibro-King itself—was a Peavey 6505+ combo. You wouldn’t think these amps had very much in common. The ’King is a vintage-style Fender that I use for blues, rockabilly, and country. The Peavey is for metal and balls-to-the-wall distortion stuff—and the only amp of mine my teenage son wants to borrow.

But these two amps do have one thing in common: 6L6 power tubes (see  Fig. 2). If my Fender had bad power tubes, all I had to do was  cannibalize the tubes from the Peavey and drop them into the Fender. You  have to be careful about proper biasing when changing tubes, but I was  just going to momentarily A/B the amp with existing tubes vs. new tubes;  I wasn’t planning on sustained and heavy use without proper setup by a  qualified tech. 2. The Fender Vibro-King and the Peavey 6505+ may not look related, but they use the same output tubes: 6L6’s.

A quick swap produced dramatic results. The amp was at once louder overall and cleaner at higher volumes than with previous tubes. Some tubes come rated by where in the gain stage they’ll break up. For  example, Groove Tubes assigns a “hardness” rating whereby lower-numbered ratings (4-5) break up sooner than higher numbers (8-9). This means that if you like tube break-up, but play mid-size clubs and don’t want to cut people’s heads off to get a good tone, you’d opt for a lower This is especially true if your amp, like many vintage  amps—including my Vibro-King—doesn’t have a master volume. For you technogeeks, the substitute tubes I used were Ruby 6L6GCM-STR’s, which are big, tall-bottle tubes modeled after the classic Sylvania 6L6GC/STR387 (see Fig. 3).

Fig. 3. For a temporary fix, I cannibalized the power tubes from the Peavey (Ruby 6L6GCM-STR's) to test the Fender.

Speaker Shiatsu

With the tubes replaced, I had plenty of volume and high-gain clean tone back, but there was still this problem with the mushy tone. Somehow I divined—more through experience as a musician than as a scientist—that the problem emanated from the speakers. Since there are three speakers, I had to determine just which speaker (or speakers) was the culprit. it’s hard to look at a speaker and evaluate performance, I lightly placed my fingers through the basket and onto the diaphragms themselves. I did this both with the amp off and with a helper playing guitar as I felt the speakers move under my fingertips (see Fig. 4). Sure enough, the top speaker felt weird—different from the other two.

Fig. 4. Lightly touching the speakers with the fingertips found the top one to be spongier than the other two. next step was disconnecting the lead wires from the tags, or lugs, of the other two speakers in order to listen to just the top speaker in isolation (see Fig. 5). (Note well that this changes the load on the amplifier and can damage it if you’re not careful. But I was doing this at a low volume and for just seconds at a time.) Then I performed the same operation with each of the other two speakers. Sure enough, the top speaker was buzzy and underpowered compared to the other two. It was the one that was damaged, and its contribution was making the overall tone unfocused.

Fig. 5. Temporarily disconnecting any two speakers by detaching the wires from the tags, or lugs, allows you to hear one speaker in isolation. sponginess, I would find out later from the tech, was because the surround had deteriorated to the point where the cone has separated from the frame slightly but perceptibly in several spots, affecting the cone’s efficiency and dampingfactors. By the way, all of these terms—basket, frame, diaphragm, driver, surround, cone, and damping—are all standard speaker terminology. You should be able to recognize and define them (see Fig. 6).

Fig. 6. A speaker with some of its principal parts labeled.

When in Doubt, Swap 'em Out

As a first response to a poorly performing tube amp, you can’t do better than to swap out the output tubes—especially if you can’t remember the last time you changed them. But I was able to also detect a speaker problem without the aid of special tools just by using common sense. Changing the tubes got me through the gig, but knowing why the amp was misbehaving in other ways gave me insight into how speakers contribute to the tone, and better prepared me for this problem should it arise in the future.

Research & Development

As far as basic amp maintenance, my research turned up some cool tips I’d like share. At a website called QRP Homebuilder (, I found a checklist from David Allen of Allen Amplification recommending steps for repairing or restoring a vintage Fender amp. Again, some of the specifics may vary for your situation, but here are eleven steps to consider:

  1. Re-tube the amplifier.
  2. Replace all electrolytic capacitors.
  3. Replace all plate resistors with new metal-film resistors.
  4. Replace the 470 ohm screen resistors on the output tubes.
  5. Tighten all connections that serve as AC grounds.
  6. Correctly set the DC bias on the output tubes.
  7. Re-tension all tube sockets.
  8. Convert to a grounded AC plug.
  9. Clean all pots, jacks and switches. Replace, if necessary.
  10. Inspect for cracked solder joints and repair as needed.
  11. Consider replacing tone control caps and some coupling capacitors if AC leakage was found.

On the Allen Amplification website ( itself, the company presents a table of common repairs and the corresponding costs for Fender amps. (Be sure to contact Allen Amplification for clarification on these prices.) This also offers a clue as to the kind of repairs you can expect to make (whether you do them yourself or have a qualified tech do them for you):




Replace all electrolytic capacitors

New 12' power cord

New volume pots

New power switches

New octal sockets

New power transformer

New output transformer







$25 ea



Deluxe Reverb






$25 ea



Vibrolux, Pro, Super, Bandmaster, Bassman






$25 ea




It’s a fine line to determine what you should monkey with and what you should leave alone when something goes wrong in a tube amp. Amps are much more difficult than guitars for two reasons: 1) They’re all electrical and not mechanical and electrical; and 2) you’re dealing with high voltage, which is dangerous to you and your amp. But that doesn’t mean that you should be afraid of your amp, just that you need to take the necessary precautions. If your amp doesn’t work and you find out it’s because of a user-replaceable blown fuse, you’re going to feel pretty foolish if you have to learn this from the tech at the shop. That’s perhaps the simplest example, but even being able to swap out tubes and diagnose problems can help you understand better your amp and even lead you to solutions—even if they’re incomplete and temporary. But that’s sometimes all you need to get you through that last gig before you can pay a visit to the shop.

This article was originally published on Harmony Central.  Reprinted by permission.