Magnatone Giveawya

September 2014

Freshening Up a Vintage Ampeg B-12XT

For years I’ve had an internal struggle. On one side, I love vintage amplifiers—the designs, the tones, the vibe, often the quirkiness and yes, even the musty smell. What I don’t like about old amplifiers is the regular maintenance necessary to keep them in top running order and the noise (both electronically and, in combos, the box crackle).

As both an amplifier manufacturer and repairman for over two decades, I have been inside everything from Dumbles and Trainwrecks to the cheesiest department store tubes amps of yesteryear. In my own life as a musician I have a collection of amps ranging from very early Magnatones and weird lap steel amps to modern boutique and, of course, the prototypes of the models that we make at Schroeder Guitar Repair. Every amp has its place. For the most part I use the funky old amps for recording or home play. On stage, only the most reliable amps are used. Often these are amps that have been completely rebuilt by our shop or my prototypes.

When I had the chance to purchase a long time favorite of mine—a ‘60s Ampeg B-12XT in great condition, I had to jump at the chance. The deal was made, the price paid and the amp was in the mail, with a promise of the amp being packed “extremely well.” I believe the parting words from the seller a thousand miles away were, “rest assured, we’re professionals here”. Needless to say, the beautiful Ampeg was packed VERY poorly and the speaker box was nearly destroyed in transit. So… time for a rebuild.



Fixing the Framework
The first and most obvious issue with the amp was the fact that the beautiful old flip-top speaker cabinet (with its matching dolly) that had survived many decades intact was now a parallelogram instead of a rectangle due to the fabulous packing job. It also had many years of grime and dust deep inside the checkerboard covering that formed what may be described as concrete.

Fortunately, the covering of the box had come away cleanly at the corners due to its age and crispiness, so it had not been damaged when the box had its sides separated at the corners. The finishing nails that were used in conjunction with the glue and joints during assembly were still being held in place by the tips. Using these nails in their original holes as a guide, a few carefully positioned clamps were able to return the box to its original rectangular shape. The covering was then glued back into position and taped at the seams until dry. The next step for the cabinet was to clean the thousands of little ridges that made up the checkerboard covering. A stiff-but-soft brush was used to dig out the years of debris, starting dry, followed by a little mild dishwashing soap mixed with warm water. Hours later, with numb wrists, it was time to tackle the electronics.

When purchasing a vintage amplifier, I assume that I will need to add a couple of dollars to my budget to account for any repairs that will need to be done to get the amp back to a safe and playable condition. As the amp had been sitting for some time, I went through it with a fine-tooth comb to get it back to way it was when it was born in the ‘60s.

First on the list was the power and speaker cord. Due to their age the rubber insulation had cracked, leaving bare wires carrying 120 volts on the power cord and a possibility for a short (at the very least a tone robbing poor connection) on the speaker wire. The power cord was replaced with a modern-style using a third prong ground, which was then connected to the chassis. The ground-reversing switch was disconnected at this time as well. The amp was brought up to full power slowly using a variac so as not to wake the old caps too quickly.

Test Drive
Upon the first play I noticed that the amp was lacking a bit in power and had a noticeable 60-cycle hum as well as some ghost notes (likely the filter caps). All of the controls were active and, with only a few scratchy spots, the reverb worked and had that funky, flat and springy type sound. Unfortunately, the vibrato circuit was not functioning. Also, it seed that over the years as the power tubes had gone, the previous owner had plopped in what ever he had around to make it work. I added all of these issues to my to-do list.

Component Check
The electrolytic filter and bias caps were replaced with fresh ones of the same value to try and retain as much of the original feel as possible. Then, I cleaned and tightened the pots and tube sockets, as well as freshened up a few nasty-looking solder joints. The head was powered up once again using a variac to help aid in the forming of the electrolytic caps upon their maiden voyage. Every resistor was checked and, where necessary, replaced.

Fortunately, all of the original coupling capacitors were in good shape, as were a majority of the original preamp tubes. For the power tubes I used a gorgeous, perfectly matched pair of NOS Philips ECG 7581As—a fantastic and rarely-seen tube that sounds amazing in this application. As the vibrato module was dead, I replaced it with one I purchased from Fliptops.net. When replacing one of these modules it, should be noted that the orientation of the modules as well as the leads should be installed exactly the same way the factory did, as they are very susceptible to noise, pulsing and other undesirable sonic FUBARs.

This project was pretty straight ahead and I feel very fortunate that I was able to find such an incredible and desirable amplifier that only needed a basic freshening up and a little TLC to make it one of the favorite pieces in my collection. It has become one of my go to recording amps not to mention looks like a work of art in any living room or studio.


Tim Schroeder is the owner, master luthier and chief designer of Schroeder Guitar and Amplifier Repair in Chicago Illinois. There he oversees the daily repair operations of the shop as well as designs the amplifiers and effects that they manufacture in house.
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