Thursday, February 2, 2017

Vox Valvetronix amp repair

This post is for you techs and guitar players out there. Most of you are probably familiar with the Vox brand of amplifiers, effects, etc. The company goes way back to the early days of rock, and their equipment can be heard on some of the most influential music of the sixties and seventies. It's a well-documented fact that Jimmy Page used Vox amps for recording with both the Yardbirds and Led Zeppelin. Other names associated with this brand are Brian May of Queen, U2, Dave Grohl, and even the Beatles. It seems safe to say that pretty much every pro has at least tried a Vox amp at one point in his or her career.

Vox Valvetronix Amp

Vox produced various models over the years, but the version I have is called the Valvetronix AD30VT. It's a 30-watt combo amp with some very nice solid state modeling. (For those not in the know, this means the amp uses computer-like circuit boards to produce a wide array of sounds through the speaker. This includes effects like reverb and distortion, and also "modeling," which makes the amp sound like a variety of different amplifiers that Vox has made over the years.) The Valvetronix line is unique. They are hybrid tube amps, or valve amps as the Brits say. They use an old-fashioned glass vacuum tube to process the guitar signal before sending it through the modern solid-state path. In other words, it combines the very old tube-style technology of the 50's with the more modern solid-state technology. This allows the amplifier to produce the pleasing warmth that you get from an old tube amp with the instant gratification and flexibility of a modern amplifier.  (it also spares you the heat and fire hazards associated with these tubes!)

Vacuum tubes used in vintage amplifiers

Sounds like the best of both worlds so far, right? Well, it's pretty close. Unfortunately, there were a few issues with these models, especially the earlier ones (like mine). In some cases, they're easily repairable, assuming you have a basic understanding of electronics and soldering skills. I've run into two problems so far, which seem pretty common:

1) Sudden inexplicable loss of power -In many cases, this is caused by a poorly soldered connection on the fuse-holder located inside the amp, or by damage to the fuse itself. The solution here is self-explanatory.

2) Spontaneous effects switching, regardless of the switch position. For instance, you're dialed in to a nice reverb that suddenly evolves into a slap-back echo or flanging effect. -This is caused by a failing potentiometer (the switch itself is broken).

In the first case, you can easily swap out the fuse and resolder the connection on the fuse holder. (Please always unplug the amp and let the tube cool before working on it. And only attempt this if you know what you're doing!) The second scenario is a bit more tricky. The effects switch on a Valvtronix is an 11-position switch. The bad news is that these were specially ordered by Vox and they are no longer produced or stocked. Vox does NOT carry them and cannot help you if your switch goes bad. If you do some shopping around, you'll find these are extremely hard to locate in the right size, and with the right number of positions. At least they were, until now.

After quite a bit of research, I learned that the potentiometer used for this switch is a standard linear-taper potentiometer. In other words, it's just like a guitar's tone knob, except that it has "clicks." Well, these clicks are called "detents" in the electronics business, and although your knob has 11 positions, it only has 10 detents. So what you're looking for is a 10 detent linear-taper potentiometer that matches the size of the one used in your amp. Getting warmer...

After a couple weeks of searching, I never did find a perfect replacement. However, I found an almost-perfect replacement:

The only difference between this pot and the original is the fact that the legs on this one are shorter. That means if you try to put it directly in place of the old one on the board, it won't be tall enough to reach the mounting panel. Best case scenario here is that you remove the old pot by snipping the legs, and solder the short legs of the new one onto the old legs. If that fails, you can solder a piece of wire from the legs to the circuit board. The thing to keep in mind is that you want it to reach the control panel where it will be mounted when you're done.

There is one other minor difference, or at least there was on mine: the shaft is slightly larger in diameter than the original, making it a very tight fight on the knob. I remedied this by hollowing the knob ever so slightly with a drill bit. (If you try this, don't use a drill. Just do it by hand. The knob is delicate and you don't want to break it. The plastic is also quite soft, and will drill out quicker than you expect even when doing it by hand. If you bore it out too much, you'll have to figure out something else to make it fit again).

The best part: This replacement pot is cheap and readily available at For less than five bucks (plus shipping), you get a pack of three! That means you'll have an extra in case the second switch also goes bad, or in case a friend has the same amp and needs the repair.

I'm happy to say that I've done this repair myself (both, actually) and it worked perfectly. The switch positions line up just like before, and you'd never know by looking that it's not the original. The only giveaway is that when you pull the amp apart, then you can see the new pot welded in there. This is great news for you Vox lovers who thought you might have to toss this amp. Despite what I was told by the techs in my area, this amp Can be fixed. And it should be.

Now, if you see one of these on sale for cheap, you can snag a great deal on a practice amp. 'Cause you can never have too many amps. Or guitars...