Call him Captain Tubes! For his final column, amp builder Steve Carr urges readers to dive into the world of tube replacement without hesitation or kid gloves.

Don’t be afraid to use some carefully applied force to get results—on vintage Teles or funky tubes.

I was visiting a friend who is a well-known luthier, with 40-plus years of guitar making under his belt. The plan was to have him give me an introductory lesson on guitar fretwork and setup, while evaluating a funky Tele of mine. Before we started, he asked if I could look at an old Ampeg Reverbrocket guitar combo that needed new power tubes. Those 7868s are strange tubes. They have a 9-pin base, like a 12AX7, but in a much bigger socket diameter. The problem, he said, was that the tube pins were too thick, and he had tried filing one of the 7868s down to fit in the socket. Hmmm, I say…. I grabbed an un-whittled 7868 pentode, lined it up, and pushed. Thunk. It’s in and secure. Next one in, and job done! It turns out my highly skilled friend was being too timid—worrying about breaking the amp. Next up, it was my turn. He takes my vintage Tele, quickly unbolts the neck, and starts taking the truss rod assembly apart. Hey, wait! Can you do that? Is it wise? Slow down man! Where are my nitroglycerin pills?

Turns out I had something to learn about swallowing my fear and getting the job done, too. This story leads to some basic troubleshooting tips, good for anyone with a tube amp.

Grumbly preamp tube syndrome. If you have a tube that is making a grumbly crackling sound without you playing, here is a way to track down which tube it is. Generally, amps are made with the input guitar signal traveling from the preamp tube furthest from the power tubes toward the power tubes. That’s good sense, since you want the most sensitive part of the amp that deals with the tiny guitar signal far from the heavy-swinging and somewhat-noisy power section. On an amp like a blackface Fender, we can see this route by looking from right to left, where the power tubes are, generally.

If you’ve never removed a tube, do not fear. Anyone can learn to comfortably replace tubes.

Put the amp on standby and remove the first (furthest from the power tube) preamp tube. Set the amp to play and listen. Is the noise gone? You can pound the top of the amp with your fist, just to stir things up in case the bad tube decided to be good for a moment. If the noise remains gone, replace that first tube (probably a 12AX7) with a new known-good tube. (Some tubes are bad out of the box.) Play through the amp a bit. If the noise is still gone, it’s fixed.

If the noise is not gone with the first preamp tube out, put the amp on standby and return that tube to the socket. Then remove the second preamp tube. Repeat the steps as necessary. Once you get to the power tubes, stop. If the amp is still making noise with that last preamp tube out, the problem could be a power tube. They can make crackling noises, become microphonic, and do weird things. You want to change all of those at once. But power tubes are a bit trickier, so let’s leave that for now.

Tube-wrangling tips. If you’ve never removed a tube, do not fear. Anyone can learn to comfortably replace tubes. Things to note: Some tubes get very hot—even some of the small ones, and especially 12AT7 reverb-drive tubes. You can use a cloth to protect your hand, and a number of venders sell tube gloves—rubberized tube-pulling, heat-resistant sleeves. Another good tool to have is a pin straightener, which will realign preamp tube pins if they get bent. With preamp tubes, firmly grasp the tube between your curved first finger and thumb, almost like holding a TV remote. Gently wiggle in a less than 1/2"-diameter circle while pulling down. Be ready to slow your hand once tubes come free, since you do need to use some force on the down-pull. And note that when putting 9-pin preamp tubes back, there is a pattern to the pins with a gap in it, so you need to line them up correctly with the socket holes when you push them back in. You don’t need to wiggle much when putting them back in. Now go change some tubes!

Hey, this is my last column. I’ve really enjoyed writing these, plus it was super nice of PG to invite me. I truly hope they were fun and informative. Thanks for reading!

Read More Show less

In a new band? Here’s how to fit in sonically—and do it in style.

Starting or joining a new band is a blast! There’s new music to create, new chums to hang with, and new sounds to explore. Which new sounds? In original music bands, in particular, this is a quandary that defines your role and the vibe of the band. If you are like me, you do a bit of everything at home: clean, classic-rock overdrive, jazzy mellow, over-the-top fuzz. It’s all fair game when it’s just for you. But once you get in a group of musicians, all listening to each other while morphing parts, how each person’s sound effects the flavor of the soup is worth considering in defining your approach. This is fun and challenging.

Do you want to play clean or distorted? How distorted? Naturally, this is a fine opportunity for amps and other new gear, depending on where you want to take it. Most of us are musical omnivores. I’ve found that after being in a band for a while where I’ve played mostly overdriven, I want to play clean atmospheric/textural guitar in the next group I play with. It’s great to mix it up, to stay fresh. Plus, to be honest, this starts to feel like a plausible justification for having so many guitars! I need my Les Paul for the rock, my Strat for slinky funk, and so on.

Some players take the opposite approach. There is a thing they do and love. They stay with that thing and continually refine it. These men and women are called … professionals!

I’m kidding a bit, but it’s also true! That said, most of us play in bands purely for fun. It’s a chance to get out, socialize, and create music. There’s no downside there. So, in that spirit, here’s a short guide to band practice and playing out with friends:

• Be a good host. Snacks and drinks are always welcome. You have a handful of musicians coming to your place after a long day of work. Perhaps they’re driving a while to get to you. Creature comforts go a long way, making for smiles and relaxed creativity. Plus, remember why you got together. We all love music and playing instruments. Shrug off the day’s stress. It’s time to rock!

Most of us play in bands purely for fun. It’s a chance to get out, socialize, and create music. There’s no downside there.

• Listen. Did the bass player just play the wrong notes … but it sounded great? Go with it! Ask him to do it again. Are you playing lead riffs over the vocals? Pull back and find the punchiest place to step out. Music is as much about space as it is about the notes you play.

• Do your part: Help move gear. One group I was in had a guitar player/singer who would leave the stage immediately after our sets to greet the adoring crowd, leaving me and the rest of the band to move our gear and his offstage so the next band could start. Occasionally a star is born, but until you have roadies, you better do your share!

• Bring what you need. Do you have your strap, picks, extension cords, spare strings, drummer?

• Don’t drink too much at practice or onstage. Stages are usually hot, thirsty places. By the time you realize you are a bit too jolly, your rhythm and chord-change memory are fading. Please don’t ask how I know.

Here’s an anecdote: Years ago, a drummer friend’s band was on a college-town club tour. Two weeks in, they found themselves the only band playing in a town they’d never been to before. It’s 10 p.m. and not a soul is there. So they figure, “Hey, it’s been a push so far, let’s relax tonight. No one is coming so we’ll get loose, open-bar style.” So they do. At 11:15—when the band is now very well lubricated—people start showing up. Lots of people. The place is packed. The bartender decides this is the time to tell them no one shows up till 11:30! Thanks fella. Our boys then have a two-hour knuckles-dragging set to perform. I can only imagine how it sounded, but my friend said playing that set was the most difficult, demoralizing, self-inflicted pain the band ever endured.

• Clear the stage when you’re done. And on a final note, to keep in the good graces of local clubs and the bands you may open for, when it’s time to get off the stage, do so graciously and quickly. I love being the opening band. Hey, I’m in my mid 50s. Being done with our set by 11 p.m. or even earlier is the greatest!

Until next time, rock hard and pass the snacks.

Read More Show less

Can your amp take the heat—literally?

Let’s continue our conversation about filter chokes, from last month’s column.To my ears, filter chokes sound smooth and very clear. They open the sound when used in the power supply and hold up under heavy use. It’s important to remember that the power supply components (rectifiers, filter caps, filter choke, dropping resistors) are what we’re listening to and feeling in an amp. Once your guitar pickups connect to the first preamp tube grid, everything else is a copy of that signal via modulated power supply voltage/current. Each gain stage creates an analog of the signal it’s fed, so the power supply plays a massive role in the sonics and touch-sensitivity of any amplifier.

Most modern higher-power amps will use a filter choke in the power supply between the power tube B+ voltage and the next stage down the line, which is the power tube screen supply. Then the voltage flows through dropping/feed resistors to the preamp filter caps.

What about just using a power resistor instead of a filter choke? Resistors are much cheaper, and that’s why you see them in low-budget, typically low-power amps from the ’40s through the ’70s. Some of these amps are great sounding, with raw, primitive bite and caveman-like swagger: Valcos, small tweeds, Silvertones. Here’s a rule of thumb: Generally, for the purest sound you want a filter choke, but opt for a resistor between the plate and screen supply if you want a sound like those bargain-bin classics. That sounds does provide a fantastic vibe in a mix!

After completing a new build, I like to run through the circuit with a pen in hand. I start from the input jack and move with the signal path to the first gain stage, making sure it’s correct, and then I continue to follow the signal path as it goes through the amp all the way to the output jack. You can find a lot of mistakes that way.

My first build was from the tweed-style schematic in the Angela Instruments Catalog that I mentioned in last month’s column. It used a tube rectifier and an off-the-shelf “universal” power transformer. This power transformer had a lot of windings, so you could build many types of amps using it. That gave me plenty of room to get in trouble, it turned out.

After completing a new build, I like to run through the circuit with a pen in hand.

Before I explain how, it’s tube anatomy time. Vacuum tubes have either directly or indirectly heated cathodes. With directly heated cathode tubes, the cathode and the heater/filament are the same element inside the tube. (The filament is the part of the tube that glows and gets the tube hot enough to work.) Indirectly heated tubes have a separate filament and an independent cathode element inside the tube. This helps with noise from the heater winding and provides a slow start, as the cathode needs to heat up enough to conduct current. 5Y3 and 5U4 tubes are directly heated, while the 5AR4/GZ34 is indirectly heated. The latter’s cathode is internally connected to one side of the heater.

Often windings on power transformers have a center tap, which is a wire in the middle of the coil. Typically, this is connected to ground for lower noise and more efficiency with balanced/full wave operation. My universal transformer’s 5-volt tube rectifier filament supply winding had a center tap that I connected to ground. Seemed like the correct thing to do, since the other windings had center taps connected to ground.

I’d turn the amp, which used a 5AR4 rectifier tube, on … and 30 seconds later the fuse would blow. I rechecked all the wiring and it seemed good, but again and again the fuse blew after 30 seconds. Talk about depressing. The next day I started looking at every schematic I could find, eventually noticing rectifier filament windings on the old Fenders did not have a center tap to ground. They had a “floating” AC winding.

Was this it? I snipped the 5V winding center tap and threw the switch. Success! What I learned is the cathode of the rectifier is the voltage output point for the power supply. If a tube rectifier’s cathode is connected to the heater and the heater supply is center tapped to ground, then you’re shorting out the power supply. Got that? The fuse blew once the rectifier heated up enough to conduct current. Remember, rectifiers turn our AC to DC for the amp to run on, but only once the rectifier tube is hot enough to work. (Note: There is one common tube rectifier that is indirectly heated with no internal cathode-to-heater connection that is safe to run with a grounded center tap filament winding. It’s the EZ81/6CA4, found in the Vox AC10, Marshall 18-watters, etc.)

If this seems a little overwhelming, rest assured that building your own amp makes everything about amplifiers less intimidating, and your skills and knowledge will grow as you go.

Read More Show less

A tale of filter chokes, rectifiers, 60-cycle hum, and the perils of self-navigation.

It’s never been easier to build an amp of your own. You can find scores of websites with a ton of classic-amp-model options. These kits range from minimal to complete, including cabinet and speaker. Sounds fun, but it can be a bit scary. It takes a fair amount of time and follow-through, plus deep regard for electrical safety. If I had a wire for every time I was shocked, blew up a voltmeter, red-plated a tube, or smoked a resistor … I could string a harp. Follow the instructions! Most of my trials came through freestyling—building without a schematic and sort of winging it. But if you have the interest and patience, building an amp is a very rewarding endeavor.

When I first became interested in guitar-amp electronics in the early ’90s, I wanted to apprentice with my local amp repairman, Rich Bogart. He had a hole-in-the-wall shop in Chapel Hill called The Tube Farm. Repair work was a side interest for Rich, who was an IBM mainframe tech, on call 24/7. Sometimes he went weeks between service calls, so The Tube Farm kept him busy. Rich didn’t go for the apprentice idea, but told me to do what he’d done 25 years earlier: DIY-build the simplest Fender—a tweed Champ. Locate parts, figure out the schematic, assemble, light fuse, and step away!

Rich gave me a copy of the Angela Instruments Catalog, the size of a small-town phone book. It was full of listings for tubes, transformers, vintage radio parts … a ’20s to ’60s tube-tech Rosetta Stone. And it did have a Champ project schematic. Buy our stuff to build this!

But what about you and your amp build? If you’re lucky, unlike me, when you put it all together it will work the first or second time. Modern kits take most of the guesswork out, which is super helpful. Imagine staring at a pasta bowl of wires trying to see the one lead in the wrong place, or find the backwards capacitor. That’s how you receive the extra education that makes the next build easier. For my first build, there were two items that had me flummoxed. One was a schematic symbol I couldn’t figure out, and the other was an issue when I turned on the amp. It would blow the fuse after about 40 seconds every time.

Imagine staring at a pasta bowl of wires trying to see the one lead in the wrong place, or find the backwards capacitor.

Look at the mysterious symbol highlighted in yellow (Photo 1). It looks like a transformer, but it only has one side and it’s hooked up funny. The appendix of the amp? No. It’s a filter choke. Like our appendix, an amp can live without it. Cheaper models often use a power resistor in place of the expensive half-transformer filter choke. (The ’50s Champ has a resistor.) My Angela Instruments schematic had one of these mysterious extra bits, unidentified, so I had to go back to Rich for guidance.

The filter choke helps smooth ripple or hum in the power supply and is most often found between the main filter cap, which feeds the output transformer/power tubes, and the screen supply filter cap. Let’s back up a little. Tube guitar amps run on DC voltage. The electricity in our house is AC, so outlet voltage swings up and down 60 times a second. You’ve heard of 60-cycle hum? That’s this alternating wall-voltage signal: a low 60 Hz tone infecting the electronics. So, since we need DC for our amps, how do we convert AC to DC?

That’s where rectifiers come in. They can be tube or solid-state. Rectifiers allow current to flow in only one direction. When the AC power from the wall swings up (positive), the rectifier lets current through to fill the amp’s filter caps. When the AC power swings down (negative), the rectifier blocks the negative current flow. I’m oversimplifying—and omitting the power transformer, which takes the wall AC voltage and transforms it to higher or lower AC voltage for the amp—for now.

So, that’s how we get DC for our amp, but the DC is pulsing with the wall-cycle frequency 60 times a second, so that 60 Hz hum is riding along with our new DC flow, modulating the amp’s power supply. Filter caps work to smooth out this hum, and the filter choke—see, I got back to it—is simply another tool to remove the ripple or DC power supply fluctuation. A filter choke has dynamic properties that bounce and can actively assist the filter caps when the amp is drawing current through tubes. A much cheaper resistor can only help reduce the 60 Hz hum, not remove it. (Most amps use full wave rectification, which yields double the current pulsing for a ripple frequency of 120 Hz, but let’s keep it simple for now!)

Stay tuned for next month, where I’ll describe the sound of filter chokes versus resistors, plus tell the story of why my first build almost became a doorstop.

Read More Show less
x