#1
 
Hello all. 
I am brand spankin' new to the world of tube amps, so I know very little. Hoping there are some kindly experts here who can guide a newbie.

I just got a Hughes & Kettner Tubemeister Deluxe 20 last Sunday. It's a fantastic little amp head, but after a few days of rocking, I've run into a problem. If I stop playing or turn my guitar volume all the way down, the amp goes silent, and will not wake up until I cycle the power. As soon as I do, I'm back in business. I haven't noticed a degradation in tone or volume. It just shuts down when it loses input signal.

This amp has a system called Tube Safety Control, which consists of LEDs on the back of the amp that indicate the health of the power tubes. They stay dark while I play, indicating normal operation. When the cut out happens one of them begins to blink, indicating over-current, and the other lights solid, indicating no current.

So I'm thinking I have power tube issues. I started watching the tubes (visible through the amp's glass front). One of the two EL84's glows much brighter than the other. Both tubes glow faintly near the top all the time, but as I play, one of them develops a glow toward the middle of the tube. When I cut the volume and the cut-out happens, the brighter one immediately loses that secondary glow and looks just like the other one. This is the tube that corresponds to the LED indicating overcurrent. 



If I had to make a (very uneducated) guess, I'd say that brighter tube has gone goofy on me. Then again it could be something else making it act that way. Has anyone run into something like this before?

Really hoping I have bad tubes because I wanted to replace the stock ones anyway, and it's not that expensive a fix.

Thanks in advance for any help!
#2
Try swapping the two power tubes and see if the problem stays in the same socket, or if the problem follows the tube. If it follows the tube, it is just a matter of getting a new matched duet. If it stays in the same socket, tech time.

By the way, the image of the tube looks really neat. Remove the text and you have a sweet computer background.
Last edited by Will Lane at Mar 25, 2017,
#3
Great suggestion, Will. I will try that. Thanks for your comment on the pic. Took it with my DSLR and a macro lens when my iPhone couldn't get a decent image.
#4
jamesm601 did you try to plug yourr guitar directly to "Return" jack-in on the back of the head... If it ok, you amp's power section is good... Nothing to worried about that maybe your preamp sections has probs... Usually if the sound quite or faded out, you should change amp's preamp tube
#5
datresults Hadn't thought of that. This is a great way to troubleshoot. I guess I could also patch the amp's send to another amp and see if I still get signal after this cut-out happens. That would indicate that the pre-amp section is ok, right? 

After reading a bit more about Hughes & Kettner's Tube Safety Control system, it really seems like at least one of the power tubes has gone bad. I didn't know this before, but the TSC system is designed to shut down damaged power tubes. That seems to  be exactly what it's doing. What I don't understand is why the tubes function while being driven hard, but then crap out when I stop playing. That makes no sense to me, and makes me worry that something non-tube related might have gone sideways.

Thanks for the reply.  
#7
Will Lane Did this today and the problem followed the tube. Swapping the tubes actually made the problem worse by introducing a huge amount of microphonic noise while the amp worked, but the cut-out still happened when I cut the guitar volume. The LEDs that indicate tube health switched they're behavior though, so I'm confident the issue is tube-related. I also noticed that one of the nine leads (or whatever they're called) that go into the amp seemed to be disconnected in the tube that reported no current. Pretty open and shut case now. 

Thanks for this tip. Really helped me zero in on the problem.