A blown fuse can mean your amp has bigger problems. Here’s how to find them.
Hi Jeff,
I own a Fender Blues Junior and recently had to replace a fuse because it blew and the amp wasn’t powering up. A replacement fixed that problem, but another arose. When I max all the controls I can barely get any volume out of the amp. The speaker is good. I removed the verb tank because it was bad. I also put the tubes from my AC15 in it—they are the same tube types and correctly placed. Not too long after turning the amp on, the tubes get really hot and I get a little burning smell. The amp barely puts out any power—maybe .2 watts. What could be the problem?
Thanks,
Micah
Hi Micah,
Often when a fuse blows in an amp, there’s some sort of failure associated with it. Occasionally one may blow due to a problem with the mains voltage, such as a power outage while the amp is on followed by an immediate return of voltage. This could be a whole neighborhood power outage or just some practical joker turning your power strip off and then on again. (“Hey, thanks dude!”) But usually there is some other cause for the failure.
The first major cause of a fuse blowing is an output tube that shorts. Sometimes the tube can remain shorted, and other times, once a replacement fuse is installed, the bad tube can begin working, only to fail again at some point. The best way to ascertain if you have an intermittent output tube is to run the amp for a while sans output tubes. If it doesn’t blow the fuse, reinstall the tubes and see if it blows the fuse again. If so, at least one of your output tubes is intermittently shorting and they need to be replaced. This method is not foolproof, but may help you determine if you need new output tubes. Assuming that your fuse is no longer failing and you already substituted output tubes, we’ll move on.
The next user-serviceable item I would normally check would be the preamp tubes, but you’ve done good substitutions here as well, so we know it’s not a tube issue. What else could it be? Well, my next point of inspection would be the speaker. And while you mentioned that the speaker is good, I have to ask how you checked it. Did you measure the resistance of the speaker with a meter? Was the speaker connected to the amp while you did this? If so, that’s incorrect. A speaker always needs to be disconnected before measuring across its terminals.
If the speaker was connected to the amp and you measured a typical 6 to 7 ohms of resistance, this tells me one of two things: Either the output transformer secondary is open (because you should be measuring a very low resistance, in the area of 1 or 2 ohms) or the speaker cable itself is open. The best way to check to see if a speaker and speaker cable are good is to first disconnect the speaker cable from the amp and then disconnect at least one lead of the cable from the speaker. Once you do this, measure the speaker itself. If the measurement is good, reconnect the cable to the speaker.
Next, check the resistance at the speaker cable plug. If it reads correctly, then the speaker and cable are okay. If you get an open (infinity) reading, the cable is open. If you read close to zero, the cable is shorted. Because of the nature of the molded-end combo speaker cables I’ve seen used on amps over the years, I strongly recommend you follow through with these tests. I have seen my share of open or shorted cables, and either scenario would cause the amp to have little or no output and cause the output tubes to be overly stressed. If all checks out with the speaker and cable, we’ll need to explore a few other possibilities.
Note: While you may be able to verify a problem using the following info, unless you are familiar with working on tube amps—and circuit boards in particular—you may need to have the amp serviced by an experienced tech. If you have any doubts at all, seek out professional help.
Photo 1
A recurring problem with amps of this particular design has to do with the small wires in the ribbon cables breaking. Closely inspect all the ribbon cables (Photo 1) and make sure that none of the conductors has broken where it enters the circuit board. If you’re careful, you should be able to measure continuity using a multimeter. If this all checks out, it’s time to start checking some voltages.
Photo 2
Because you mentioned the amp has some output, albeit very low, I’m going to assume that the high voltage power supply is working. You can quickly verify this by connecting the negative lead of your meter to the chassis and measuring the voltages on the + side of the four large 450V caps (Photo 2). These voltages should range from approximately 400 to 250 volts. If this is good, we’re going to move on to the low voltage supply, as this could actually tie together a couple of problems.
Photo 3
You mentioned you removed the reverb pan because it was bad, but do you know for sure that it’s actually the pan that’s bad? The low voltage supply in the amp not only provides the bias voltage to the output tubes, but also supplies the plus and minus 15V that powers the reverb IC chip. First you need to check the voltage on pin 2 of the output tubes. This would be the second pin counting clockwise in the space on the socket. It can also be measured on the circuit board at the 1.5k grid resistors (Photo 3). This should read approximately -10VDC. If this is not present, check the voltages at the 750-ohm resistors in the low voltage supply (Photo 4).
Photo 4
Each side of each of these resistors should read +15 and -15 volts respectively. If not, repairing the cause of the failure may very well fix both the output and reverb problems with the amp.
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Okay, I plead guilty to having owned over 150 electric guitars in the past 60 years. So, for kicks, with my experience by way of Fender, Gibson, Ricky, Gretsch, PRS, Guild, Teisco, and others, I decided to attempt to make my own axes from scratch. I found that this endeavor was synergistic—much like envisioning, composing, performing, and recording a song. With my long-time San Diego techie, Val Fabela, doing the assembly, I started carefully designing, engineering, and procuring all of the components.
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The Vincent van Gogh Stratocaster, aka “Vinnie,” was the initial project. Starting with a Canadian alder body, an artist in Italy (who wishes to remain anonymous) applied the Starry Night painting to the front, sides, and back. The heavily flamed, roasted maple neck has the typical 21 frets with a 25.5" scale, and sports yellow pearl-dot inlays. After careful consideration of my playing styles, I went with a configuration using Fender ’57/’62 Stratocaster pickups. I used an upgraded, noiseless, 5-position Switchcraft assembly for the switching circuit. Fender locking tuners, a custom-fitted bone nut, and a Kluson K2PTG 2-point whammy system and brass bridge complete the low-action setup. Overall gold hardware completes the look. Vinnie’s fighting weight is 7.1 pounds.
This is what stars look like from further in space, at least as far as this special build is concerned.
I was so happy with this Strat that I decided it needed a brother, so I started on a Tele. Logically, I named the Tele “Theo,” after Vincent van Gogh’s younger brother. Again, with a Starry Night body painted by the same artist, I coupled a Canadian alder body with a lightly roasted, flamed-maple Stratocaster neck. (Hey, if it was good enough for Jimi to experiment with a Strat neck on a Tele body, why not try it?) And, as expected, my techie Val did a brilliantjob of joining the neck to the body.
The Van Gogh Tele, aka “Theo,” built to similar specs as the Strat and also featuring a lightly roasted, flamed-maple Strat neck.
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It's not in a museum, the the Theo guitar is certainly a work of art.
But how do they sound? Magnificent!Throw in my trusty Keeley compressor, Fulltone OCD, and Fender or Mesa/Boogie tube amps, and the van Gogh boys both easily equal or surpass my White Penguin, White Falcon, PRS Custom 22, Lucille 345 stereo, 335, SG TV, Les Paul Standard, Esquire, or Joan Jett.
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Beauty and sweet sonority elevate a simple-to-use, streamlined acoustic and vocal amplifier.
An EQ curve that trades accuracy for warmth. Easy-to-learn, simple-to-use controls. It’s pretty!
Still exhibits some classic acoustic-amplification problems, like brash, unforgiving midrange if you’re not careful.
$1,199
Taylor Circa 74
taylorguitars.com
Save for a few notable (usually expensive) exceptions, acoustic amplifiers are rarely beautiful in a way that matches the intrinsic loveliness of an acoustic flattop. I’ve certainly seen companies try—usually by using brown-colored vinyl to convey … earthiness? Don’t get me wrong, a lot of these amps sound great and even look okay. But the bar for aesthetics, in my admittedly snotty opinion, remains rather low. So, my hat’s off to Taylor for clearing that bar so decisively and with such style. The Circa 74 is, indeed, a pretty piece of work that’s forgiving to work with, ease to use, streamlined, and sharp.
Boxing Beyond Utility
Any discussion of trees or wood with Bob Taylor is a gas, and highly instructive. He loves the stuff and has dabbled before in amplifier designs that made wood an integral feature, rather than just trim. But the Circa 74 is more than just an aesthetic exercise. Because the Taylor gang started to think in a relatively unorthodox way about acoustic sound amplification—eschewing the notion that flat frequency response is the only path to attractive acoustic tone.
I completely get this. I kind of hate flat-response speakers. I hate nice monitors. We used to have a joke at a studio I frequented about a pair of monitors that often made us feel angry and agitated. Except that they really did. Flat sound can be flat-out exhausting and lame. What brings me happiness is listening to Lee “Scratch” Perry—loud—on a lazy Sunday on my secondhand ’70s Klipsch speakers. One kind of listening is like staring at a sun-dappled summer garden gone to riot with flowers. The other sometimes feels like a stale cheese sandwich delivered by robot.
The idea that live acoustic music—and all its best, earthy nuances—can be successfully communicated via a system that imparts its own color is naturally at odds with acoustic culture’s ethos of organic-ness, authenticity, and directness. But where does purity end and begin in an amplified acoustic signal? An undersaddle pickup isn’t made of wood. A PA with flat-response speakers didn’t grow in a forest. So why not build an amp with color—the kind of color that makes listening to music a pleasure and not a chore?
To some extent, that question became the design brief that drove the evolution of the Circa 74. Not coincidentally, the Circa 74 feels as effortless to use as a familiar old hi-fi. It has none of the little buttons for phase correction that make me anxious every time I see one. There’s two channels: one with an XLR/1/4" combo input, which serves as the vocal channel if you are a singer; another with a 1/4" input for your instrument. Each channel consists of just five controls—level, bass, middle, and treble EQ, and a reverb. An 11th chickenhead knob just beneath the jewel lamp governs the master output. That’s it, if you don’t include the Bluetooth pairing button and 1/8" jacks for auxiliary sound sources and headphones. Power, by the way, is rated at 150 watts. That pours forth through a 10" speaker.Pretty in Practice
I don’t want to get carried away with the experiential and aesthetic aspects of the Circa 74. It’s an amplifier with a job to do, after all. But I had fun setting it up—finding a visually harmonious place among a few old black-panel Fender amps and tweed cabinets, where it looked very much at home, and in many respects equally timeless.
Plugging in a vocal mic and getting a balance with my guitar happened in what felt like 60 seconds. Better still, the sound that came from the Circa 74, including an exceedingly croaky, flu-addled human voice, sounded natural and un-abrasive. The Circa 74 isn’t beyond needing an assist. Getting the most accurate picture of a J-45 with a dual-source pickup meant using both the treble and midrange in the lower third of their range. Anything brighter sounded brash. A darker, all-mahogany 00, however, preferred a scooped EQ profile with the treble well into the middle of its range. You still have to do the work of overcoming classic amplification problems like extra-present high mids and boxiness. But the fixes come fast, easily, and intuitively. The sound may not suggest listening to an audiophile copy of Abbey Road, as some discussions of the amp would lead you to expect. But there is a cohesiveness, particularly in the low midrange, that does give it the feel of something mixed, even produced, but still quite organic.
The Verdict
Taylor got one thing right: The aesthetic appeal of the Circa 74 has a way of compelling you to play and sing. Well, actually, they got a bunch of things right. The EQ is responsive and makes it easy to achieve a warm representation of your acoustic, no matter what its tone signature. It’s also genuinely attractive. It’s not perfectly accurate. Instead, it’s rich in low-mid resonance and responsive to treble-frequency tweaks—lending a glow not a million miles away from a soothing home stereo. I think that approach to acoustic amplification is as valid as the quest for transparency. I’m excited to see how that thinking evolves, and how Taylor responds to their discoveries.