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.
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!