WARNING: ELECTRICITY CAN AND WILL KILL YOU. CAPACITORS HOLD THEIR CHARGE LONG AFTER AN AMPLIFIER HAS BEEN TURNED OFF. Tube amps are especially dangerous! THIS IS NOT A TUTORIAL. WORK OF THIS NATURE SHOULD ONLY BE ATTEMPTED BY A QUALIFIED TECHNICIAN.
Here's a quick little repair of a Jet City PicoValve.
The user complained that the amp only produced sound when he turned the Gain knob to 9 or greater. He elaborated that the volume when the amp started to produce sound was about as loud as one would expect at 9 if the amp were functioning correctly. I tested the amp and verified the user's complaint. Everything else on the amp seemed to be working as intended.
Here is a photo of the head with the tubes and the mesh cover removed. I tested the tubes just because I could. They were fine. This amp comes from the factory with one 6L6 and a pair of 12AX7s. The 6L6 power tube can be swapped out with different octal power tubes to achieve different tones.
Although this isn't the best view, one can observe that there is basically one main board in this amp that includes the power supply, preamp, and output sections of the amp.
I ohmed the Gain potentiometer out from the top and got some weird readings. In circuit the pot read OPEN. At that point it was time for the main board to come out so I could access the bottom and pull the pot out of circuit.
After I carefully removed the board, I used a large ceramic resistor to drain the filter caps as a safety precaution.
WHOA! Look at that hole in the chassis for the Bass pot. What a dent! At first that might not seem important. After all when I tested the amp the bass control seemed to work just fine. That dent tells us that the amp took some kind of hard, physical hit. With cheap parts like the ones in this amp, a hard hit can break the wafer or wiper inside a potentiometer. Even high quality parts can be broken by a hit that is hard enough to bend the amp chassis.
The Gain, Bass, and Master pots were supposed to all be the same value: 1Meg. I decided to pull all three and compare readings out of circuit with some new 1M pots to make sure they were all reading correctly.
That's the Gain pot. I double checked where I placed my meter leads, but they were right on the outside legs of the pot. That should read 1M, but it's completely open. At a certain point in the travel of the pot, I read a resistance lower than 1M. It seems we've figured out why the Gain had to be turned up to 9.
For some reason both the Bass and Master pots read about 850k ohms. That's only 15% off, but the brand new pots I had in stock read almost dead on 1M. I went ahead and replaced all three 1M pots in the amp.
I resoldered all the suspect connections on the board: the tube sockets, pots, switches, et cetera. After testing the amp through my Light Bulb Limiter, I put it all the way back together and tested it. While the amp's tone leaves a lot to be desired in the opinion of this country-rock guitar picker, it works just like it should.
Thanks for following along. Feel free to post any questions in the comments or shoot me an email at ABToneGroup@gmail.com.
- Andy -
I am an avid listener of podcasts. I first started listening to them in a self-help sort of way. They were originally all personal finance and investing shows, but one can only take so much financial literacy at a time. Life is about balance. I find it by listening to the Parcast Network's Conspiracy Theories.
The format of the show involves two episodes per topic. The first presents the official narrative and other important contextual information. The second explores a few different conspiracy theories and rates their credibility on a scale from 1 to 10.
While working on a tube amp last week, I listened to the Conspiracy Theories episode on Amelia Earhart. Officially she and her navigator Fred Noonan got lost, were unable to receive radio transmissions, and ultimately crashed into the sea while attempting to circumnavigate the globe.
While finishing up the amplifier I heard something that caught my attention. At 28:03 in episode 87 "Amelia Earhart Pt. 1" the hosts discuss Earhart's radio receiver issues that occurred a few days prior to her disappearance:
The [Lockheed] Electra took off early in the morning on June 27th. They spent the night on Koepang on Timor island. Then on the 28th continued to Port Darwin, Australia. There the blown fuse of Amelia's radio receiver was replaced - an important fix since a radio receiver needs to be in good working order for a pilot to receive messages and direction finding radio signals.
It's worth pointing out here that a blown fuse is often the result of a deeper mechanical issue. So while the Australian mechanics fixed the receiver's immediate problem there may have been an underlying issue that was still unresolved and could lead to the fuse blowing once again.
I speculate that the technician who worked on Earhart's radio receiver did bad work which lead to the deaths of her and navigator Fred Noonan. C.L.A. Abbott describes the tech's troubleshooting process in this letter from August 3, 1937.
His letter hardly describes the nitty gritty of troubleshooting the receiver, but we can conjecture that the technician stopped the troubleshooting process at the point that he replaced the fuse. Knowing that Earhart was unable to receive radio transmissions before her Lockhead Electra went down, we can guess that the fuse did blow again. The receiver possibly had some sort of intermittent problem that didn't present itself in Australia. An intermittent problem can be incredibly difficult to find especially if all you do is replace a blown fuse. You have to roll your sleeves up and dig in.
Here we arrive at the point of this post. Customers often explain their issue by telling me some variation of "it's probably just a bad fuse." I am here to tell you that IT IS ALMOST NEVER JUST A FUSE. Something causes a fuse to blow. It is a symptom - not the problem.
The stakes aren't nearly as high when it comes to repairing pro audio equipment versus flying over water. A non-functional Fender Twin Reverb isn't going to cause your airplane to crash, but it could cause you to cancel a gig, lose a job, et cetera. Please remember your issue is probably not just a fuse. It might save your livelihood - if not your life.