A dead amp bought for next to nothing might—with a few inexpensive fixes—become a new workhorse.
Alamo Fury Combo
Once again I’m going to stray from the typical format for this month’s column. Instead of attempting to answer a reader’s question, I’m going to feature an amp that came in for servicing. Okay, if you really need a question, we’ll go with, “Hey Jeff, I just acquired this cool old amp. I don’t know too much about it, but it’s dead. I’d like to know if it’s repairable, but I don’t know what it’s worth or how it sounds, so I don’t want to invest too much in the repair.”
The amp in question is an Alamo Fury combo. Remember the Alamo? No? Me neither. I don’t know if I’ve ever seen one before, and if I have, it wasn’t this model. This combo has two 7868 output tubes and appears to have a single 15" speaker. But wait—there’s only one 12AX7 tube. Hmmm, a couple of possible scenarios here, so let’s see what’s going on under the hood.
Because the output is directly wired to the speaker, the first move is to remove the rear panel of the speaker enclosure and disconnect the speaker. This reveals a stock 15" speaker (Photo 1) stamped 67-7307. This would be an Eminence speaker (67) that was manufactured in the seventh week of 1973 (7307).
So now it’s on to the electronics. Upon pulling the chassis, the question of a circuit with only one 12AX7 is put to rest. That tube was used as a phase inverter, which means that the front end of the amp is solid state. I would have already known this had I taken the time to look at the tube chart first (Photo 2), which is on the inside wall of the cabinet. It’s actually a “hybrid” chart and happily touts the fact that the Fury’s preamp is powered by a 2N5210 transistor! It also appears to tell us that the amp has a solid-state rectifier, as denoted by the diode symbol. Okay, mystery solved. Now let’s see why the amp will not power up.
Multimeter in hand, I did a quick check of the fuse. It shows continuity, although I don’t think a 10-amp fuse belongs here. That could easily cause transformer failure, but let’s continue. After installing a more appropriate fuse, I began measuring voltages starting with the AC line voltage, which we find stops right after the fuse holder. By process of elimination, if the fuse isn’t bad, the fuse holder is. Replacement of the fuse holder yields … an illuminated pilot light. Progress!
A quick signal at the input and load on the output shows that the amp is working, but barely. More issues. A sound check with a guitar and speaker reveals that the amp has weak output with hum. Okay, this sounds like a filter capacitor issue. Seeing as how this amp has a chassis mounted multi-cap can, I opted to reduce the time and expense by installing discrete filter caps.
One important thing to bear in mind when doing a repair this way is to keep all the capacitor ground connections as close to the original ground point as possible, as this is the way the amp was designed. It may be easier to install the capacitors in other locations, but this often results in increased hum. Now, let’s see what the amp does.
Power on, plug in, turn up and … we’re good. The amp seems stable and, while it’s not going to set the world on fire, seems to function as intended. Could the amp benefit from a new set of tubes? Probably. And maybe a new, more efficient speaker? Probably. But I’ll leave that to the customer, should he wish to invest further in his acquisition. But hey, while I have the amp on the bench, let’s learn a bit about it and see if it could benefit from any improvements.
The preamp board is designed around not one measly transistor, but two (Photo 3). The first is used as the initial gain stage for the inputs and drives the tone controls. The second re-amplifies the signal from the tone stack and drives the phase inverter tube. Surely there must be some way to enhance the performance of these devices—and there is. Both transistors use a 1k emitter resistor (Photo 4).
Placing a capacitor across these resistors is the same as placing a bypass capacitor across a cathode resistor in a tube amp. Too large a value here might be too much of a full range boost for this little amp to handle, but because this was designed as a bass amp, a bit more highs may add some needed sparkle for guitar. I found that a value of 0.1 µF added a noticeable increase in high end.
Placing it across the Q1 resistor gives the increase before the tone stack, which means it’s controllable, while placing it across the Q2 resistor gives the boost just prior to the phase inverter. It’s a matter of sonic preference. Also, Photo 5 shows the tone stack bass and treble caps, which can further alter the amp’s tone.
There you go. Now maybe this amp will be an Alamo to remember!
All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.
Looking for more great gear for the guitar player in your life (yourself included!)? Check out this year's Holiday Gear Finds!
D'Addario XPND Pedalboard
DR-05X Stereo Handheld Recorder
Wampler Pedals Ratsbane
This full-amp-stack-in-a-box pedal brings a new flavor to the Guitar Legend Tone Series of pedals, Missing Link Audio’s flagship product line.
Adding to the company’s line of premium-quality effects pedals, Missing Link Audio has unleashed the new AC/Overdrive pedal. This full-amp-stack-in-a-box pedal – the only Angus & Malcom all-in-one stompbox on the market – brings a new flavor to the Guitar Legend Tone Series of pedals, Missing Link Audio’s flagship product line.
The AC/OD layout has three knobs to control Volume, Gain and Tone. That user-friendly format is perfect for quickly getting your ideal tone, and it also offers a ton of versatility. MLA’s new AC/OD absolutely nails the Angus tone from the days of “High Voltage” to "Back in Black”. You can also easily dial inMalcom with the turn of a knob. The pedal covers a broad range of sonic terrain, from boost to hot overdrive to complete tube-like saturation. The pedal is designed to leave on all the time and is very touch responsive. You can get everything from fat rhythm tones to a perfect lead tone just by using your guitar’s volume knob and your right-hand attack.
- Three knobs to control Volume, Gain and Tone
- Die-cast aluminum cases for gig-worthy durability
- Limited lifetime warranty
- True bypass on/off switch
- 9-volt DC input
- Made in the USA
MLA Pedals AC/OD - Music & Demo by A. Barrero
Sporting custom artwork etched onto the covers, the Railhammer Billy Corgan Z-One Humcutters are designed to offer a fat midrange and a smooth top end.
Billy Corgan was looking for something for heavier Smashing Pumpkins songs, so Joe Naylor designed the Railhammer Billy Corgan Z-One pickup. Sporting custom artwork etched onto the covers, the Railhammer Billy Corgan Z-One Humcutters have a fat midrange and a smooth top end. This pickup combines the drive and sustain of a humbucker with the percussive attack and string clarity of a P90. Get beefy P90 tone plus amp-pummeling output with the Railhammer Billy Corgan Z-One.
Patented Railhammer Pickups take passive guitar pickups to a new level with rails under the wound strings lead to tighter lows, and poles under the plain strings offer fatter heights. With increased clarity, the passive pickup’s tone is never sterile.
Railhammer Billy Corgan Signature Z-One Pickup Demo
For more information, please visit railhammer.com.
Designed for utmost comfort and performance, the Vertigo Ultra Bass is Mono’s answer to those who seek the ultimate gigging experience.
Complete with a range of game-changing design features, such as the patent-pending attachable FREERIDE Wheel System, premium water-resistant and reflective materials, shockproof shell structure and improved ergonomic features, the Vertigo Ultra Bass takes gear protection to the next level.
The Vertigo Ultra Bass features:
- Patent-pending FREERIDE Wheel System that allows for wheels to be attached on the case in no time, giving you the option to travel with it seamlessly
- Upgraded materials, including a water-resistant 1680D Ballistic Nylon outer shell, plush inner lining and new reflective trim for maximum backstage and night visibility
- Enhanced protection with a shockproof shell structure and heavy-duty water-resistant YKK zippers for protection from the elements
- Improved ergonomics and functionality including added back support and load-lifting detachable shoulder straps with side release buckles
- Flexible storage options with added space for touring essentials