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.
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!
How a burst of flame and a transistor radio led to a life exploring amp design.
Hi. I’m Steve Carr of boutique guitar amp company Carr Amplifiers. Premier Guitar has been kind enough to ask me to write a little something for you each month about guitar amps. My hope is to entertain with tube insights plus a few illustrative near-topic stories. As a student, I avoided English classes as a rule, desperately substituting any math or science class. My hat’s off to the (for now) willing PG editorial staff!
And now, two early electronics lessons.
Around age 10, back in 1973, I lived in northern Indiana. Once a week, given a good reception night, I would tune in to the Dr. Demento Show broadcast from Chicago. The channel faded in and out on my small 9V AM/FM portable radio. It was a funny show and I really wanted to listen, but my weak long-distance reception made it a struggle. Hey, if the radio is just barely receiving at 9V… imagine the reception at 120V! A donor lamp cord later, and I held a live AC mains wire below the 9V battery socket on my radio. If you’re wondering where my parents were, all I can say is, man, it was the ’70s…. I lowered the socket to the wires and my radio burst into flames! No music, no Dr. Demento, no more radio. I don’t recall getting in trouble, so I must’ve put out the fire and my parents were none the wiser.
Ten years later, in 1983, I was a sophomore at Purdue University and in a band. My amp was a late-’60s blackface Fender Bandmaster I bought from a guy for $40 in high school. This fellow was a smoking guitar player whose day job as a city garbage collector had certain perks. He found the Bandmaster in someone’s trash, so the bargain $40 price seems a little fairer than you might think!
In the late ’70s, when I was just starting to play, the guitars and amps we consider vintage today were just last year’s models. Most people wanted the new stuff if they could afford it. During band practice one night, my Bandmaster started buzzing loudly. It still produced sound, but with a terrible buzz. I didn’t know what to do and did not really understand amps could be fixed, so I decided to buy a new one.
Luckily, I had some money saved from my busboy job at the Lafayette Howard Johnson’s Cock and Bull Lounge. Hard to believe I lost track of the black polyester uniform vest with a gold embroidered Cock and Bull logo. It might not fit anymore, but style is timeless. Anyway, as a young guitarist I wanted the amp my favorite guitar player at the time used: Adrian Belew of King Crimson played a solid-state Roland JC-120. Wow! I would be great with that amp! The JC-120 cost $500—worth it for a dream amp.
After my exciting JC-120 arrived, I used it for weeks and just could not warm up to it. It was not my style. It was very stiff and cold with no overdrive or touch-sensitive vibe. Belew sounded great, but I did not. This was a real eye-opener, and ever since I’ve been careful with my gear choices. Try before you buy, if possible, and be truly honest with what you like. Let that be your guide, along with the ideas you glean from your guitar hero’s gear.
So, after many weeks trying to love the JC-120, I opened up the Bandmaster and there—without any understanding of circuits or electronics—I saw a wire sticking up from a solder point on the chassis. You could see where it had been attached and vibrated free. After twisting the wire around another similar point—no solder skills, yet—I plugged the Bandmaster in and it worked fine! From that point forward my bandmates referred to the JC-120 as the “$500 mistake.” This experience started my love and fascination with tube amps.
It takes confidence to follow our own paths while recognizing the gear influences of our favorite guitarists. During my amp prototyping process, I try hundreds of permutations hoping to get features to work together correctly and achieve tones I want. Many times, I’ll think of a circuit or variation that, on paper, is very clever and exciting—only to find it does not work or, worse, it sounds bad. Here’s my point: The process is the import thing. You work through the sonic blind alleys, but if you don’t try them all, you may miss something good.
So, finally, where did the “$500 mistake” end up? I sold it to a drummer. (Insert your own joke here.)
Next time, bright caps.