Replacing tubes for Epiphone Valve Junior Combo amp.

April 16, 2020 11:29 AM

Got my hands on a Epiphone Valve Junior guitar amp and a while back it suddenly stopped outputting any sound. It gets power, I replaced all the capacitors, looked for any broken leads-- it's as much as I knew how to do and replacing the tubes seems like a logical, inexpensive next step for an amp I just got for free. What sort of tubes would I need or what resources exist for a beginner learning about amps/replacing tubes?? Thanks!
posted by dr handsome to Media & Arts (7 answers total)

That amp uses an EL84 output tube (most likely of the tubes to fail and cause this symptom) and a 12ax7 preamp tube. It's pretty much just a matter of getting to the tubes, unplugging them and plugging in new ones taking care to index them correctly (lining up the pins with the holes.) Having said that, I wouldn't assume that tube failure is what's causing your issue. Taking voltage measurements would be the way to logically diagnose the issue, but with the high voltages found in a tube amp you need to take serious safety precautions to avoid...death. Verifying B+ voltage, filament voltages and the voltage drop across the output tube's cathode resistor would go a long way towards figuring out what's going on.

Good tube amp forums: https://music-electronics-forum.com/ and el34world https://el34world.com/Forum/index.php
posted by Larry David Syndrome at 11:46 AM on April 16, 2020 [1 favorite]

Best answer: Just for future reference, I would consider replacing tubes much simpler and easier than redoing all the caps - also a better first step. Literally just unplug the old ones, plug in new ones.

According to the Wikipedia, it has just two tubes, a 12AX7 and an EL84. Both are still super common tubes for guitar amps and still in production, available lots of places.

Personally I've gone with TubeDepot.com the last few times I've needed tubes and been satisfied. No need to spend extra money for vintage tubes for that amp, especially if you're not 100% sure the tubes are the problem.
posted by soundguy99 at 11:50 AM on April 16, 2020 [2 favorites]

Best answer: Also re: learning about tube amps, get this book:

How To Service Your Own Tube Amp
posted by soundguy99 at 11:53 AM on April 16, 2020 [1 favorite]

One other note, you will probably read about the need to "rebias" an amp when replacing the output tube(s). As the Valve Jr. is a single ended, cathode biased amp you don't really have to worry about that, you can plug-and-play a replacement tube with no issues. (If this was a push-pull, fixed bias amp the advice to rebias when replacing output tubes would be absolutely appropriate, but it's not.)
posted by Larry David Syndrome at 12:01 PM on April 16, 2020 [2 favorites]

To answer "tubes?" here's The Tube Store, which I've used forever.
https://www.thetubestore.com/

But.

"Suddenly stopped outputting any sound" suggests speaker or speaker leads to me.
Caps sound funny when they're dying.
Tubes make all sorts of audio when they're dying.
Fried speaker suddenly stops making sound.
posted by lothar at 12:42 PM on April 16, 2020 [3 favorites]

Best answer: Yeah, echoing lothar, do you mean it has stopped outputting input signal, or has it ceased making any sounds at all, like power on pop, hum etc?
posted by Chitownfats at 5:47 PM on April 16, 2020 [1 favorite]

Response by poster: Very helpful answers-- there was no sound, not even a crackle. Got some replacement tubes and there was sound again (yay!) but then the same crackling and sputtering before dying again.

Weird thing was, one of the tubes seemed to have shattered at the bottom. I really don't think it was my mishandling of it-- just went to jimmy it out and realized what had happened. Thankfully it all came out in the end.

That book looks like a fun project!
posted by dr handsome at 10:02 AM on April 20, 2020