weird issue with egnater renegade head

[clearerphish]

Had my renegade head for a little over 2 years, running into an egnater tourmaster cabinet. I've loved this amp, it does everything I want it to do. I may have gigged with it ten times or so, and I use it every week at band practice.

Lately, it has an intermittent loud SKRONK!!! kind of sound and when it happens, it happens in pulses with 3-4 seconds in between each SKRONK! When it is doing this, a light bump to the top of the amp will make the sound if you do it in between pulses. I am assuming some component is loose. I haven't found anything online about this. I emailed egnater earlier tonight but it will be a day or two before I hear back. I am always able to stop it by powering down the amp, powering it back up, moving it to standby, basically just futzing with it. It happens when just the guitar is plugged directly into the amp, when my effects are between the guitar and the amp, or when I pull the instrument cable out of the amp while it is happening, it continues.

Any clue what's happening here? Thanks in advance.

 

[mayfly]

When a tube amp starts doing funny things like that, especially when it can be made to do it with a mechanical bump, it's a tube going microphonic.

I'd get a set of chopsticks, run the head, and tap each tube individually listening for the sound.  That should lead you to the bad tube. 

BTW, if it's doing it in all channels, my money's on the phase inverter or one of the power tubes.

Happy chopping!

 

[stubhead]

Have you tried just pushing all the tubes in to make sure they're seated tight? It's probably easier to do before they're hot and before you've got 600 volts a-just itching to find a wet, meaty ground.  :icon_thumright: I don't know anything about that particular amp but if you can get a good light on it, you may be able to find some visual information. Like, smoldering little brown things may look out-of-place.

 

[clearerphish]

Thanks, folks, I'll try this and report back. In my youtube searches, it seems like a microphonic tube produces hiss and alters the sound of the amp. I'm not seeing that here, but I may just take it in if I can find someone that won't charge an arm and a leg.

 

[mayfly]

Thanks, folks, I'll try this and report back. In my youtube searches, it seems like a microphonic tube produces hiss and alters the sound of the amp. I'm not seeing that here, but I may just take it in if I can find someone that won't charge an arm and a leg.

erg - don't do that.  You can do this yourself.  It just requires a little intelligent investigation and simple troubleshooting.  BTW I've seen power tubes breaking down to the point where little lightning bolts start shooting around inside of them. EL34's are pretty spectacular in this regard.  That makes some interesting noises through the speakers!  I've also seen preamp tubes with failing internal geometry that made fairly regular loud SPLAT sounds as the getter moved around inside the tube (!).  Some mid 90's Chinese tubes had this fun issue.

Check the tubes with the amp fired up and get back to us.  BTW - if you're uncomfortable with electronics, don't power it up with the chassis out.  But this you can figure out with the chassis installed so no worries.  Also have you tried different channels?  have you tried turning off the reverb (if so equipped)?  Tried removing the phase splitter?  and of course, the chopstick test?

The chopstick is the tube amp repairman's greatest tool.  ccasion14:

 

[clearerphish]

Mayfly you're giving me confidence. If I die doing this I am going to come back and haunt you as noise in your axe-fx!

 

[clearerphish]

I didn't die. It's a bad power tube. i isolated it with a chopstick just like you said. Thanks everyone!

 

[clearerphish]

So I just swapped out the old 5881s for a JJ-tesla matched set of6L6's. Biased them and the existing EL34's each to the precise middle of the recommended range, let the amp warm up for 10m, checked the bias again, and it was dead solid nuts on. It passed the chopstick test and everything. Thanks, Mayfly. It really couldn't be easier on the Renegade head, and swapping the tubes was a breeze. All good in the hood. Except for my neighbors.

:rock-on:

 

[mayfly]

very cool.  Glad it's running.