RM100 Problem - Crackle, Pop, Blown Power Tube Fuse - Help!

Ok, so I was playing through my 'silent rig' last night, which consists of RM100->THD 16ohm Hot Plate (as dummy load) Direct Out->Computer Interface, etc...

Well, I forgot to switch my RM100 back to 16ohms, from 4ohms (how I need it through my speaker cab). So, was playing it set to 4ohms into a 16ohm dummy load - BIG no no! And, I do crank the master pretty high into the Hot Plate (about 11 oclock).

So, about 10min in, I started to get some loud crackles and pops (from headphones/speaker cab emulation, so output from power amp), but it went away and was intermittent, so I just played on thinking maybe some static discharge or something. Well, it eventually started getting bad so I took a look in the back and noticed one of my power tubes glowing like the surface of the sun! I immediately shut it down. It was then that I realized that I had not matched the impedance at 16ohms... Uh oh...

Gave some time to cool down, switched to 16ohms, and turned back on. The fuse for the glowing tube was blown as indicated by the red LED. So, shut it down, installed a new fuse, and a brand new power tube (SED =C= EL34). Turned it back on, let tubes warm up for a bit, then took off standby, guitar not plugged in. Within 30 seconds, the tube in that same slot started glowing like the sun before supernova again, and the fuse blew (but the tube continued to increase in brightness). So, shut her down again. So, no, the fuse blowing did not stop that runaway heat-up of that tube...

I removed the blown fuse, and removed the fuse for it's matching pair (inside pair), so I could run at 50W on the outer two. Amp operates just fine on the outer two, so it's an issue with the circuit on the inner two, or just that singular tube slot.

I'm assuming I blew a resistor somewhere, or something along those lines. Question is, do any of you diagnosticians out there know pretty surely what I did here? What I should look at when I tear it apart? What parts/electrical components I'm likely to need?

Any help/insight is appreciated.

Thanks.

Having the amp set at 4 ohms into a 16 ohm load is a safe mismatch and the amp will actually run a little cooler. That wasn't the cause of your issues.

My guess is the tube in question failed and took out it's screen resistor. That socket is red plating because it can't bias the tube in it. Unless you are very handy, it a job for a tech to diagnose and repair. Check out my Facebook posts two days ago for a Fender Hot Rod Deville that the owner kept using with this very issue. Scorched the earth and nearly burned clear through the PCB.

Thanks Rob!

I know that typically would be a safe mis-match, but for some reason, it is not the case with a THD hotplate. No idea why, but this same thing has happened before with this same amp, as well as two others (a Bogner Shiva and an Orange Rockerverb). In all previous cases where I failed to properly match the impedances, I caught it before any damaged occurred (the change in tone was significant, and could see the back side of the amp lighting up like a sunrise, lol). Same thing happened in those cases, with one or a pair of tubes on a runaway heat-up...

The tube that was in that socket, works just fine in the working sockets. So, while the tube may have had it's life shortened by such a violent heat-up, it still operates and sounds just fine. Good thing, those SED's are pretty **** expensive these days...

BTW, tube socket 3 is the culprit.

So, that said, I got it apart last night. I could not find any blown resistors, no PCB damage, nothing. All internal fuses in-tact. No evidence of any burning or arcing.

So, busted out the multimeter and started testing resistors and caps in the vicinity of that tube socket. Everything checks out just fine compared to the working sockets, except for the following:

R24 = 140k (vs 220k on other sockets)

C7 = dead, 1.1nF (vs 30nF on sockets 1 & 4)

C6 = 14nF, plus the rubber insulator is cracked/split on the top

I've no clue what purpose these specific components serve in this amp, so maybe this will help diagnose the issue.

Rob, any idea?

Nothin'? Lol. Can anyone tell me if my stated issues match what you'd expect if those particular components are damaged?

Thanks.

Well, just in case anyone ever experiences the same problem as I had, just wanted to follow up.

I replaced all three of the named components (C6, C7 & R24) with new components. Problem is fixed and amp is again running at 100W, like a champ!

Glad to hear you got her fixed up!

Thought about getting rid of that hotplate? :?: :!:

blacksun said:

Thought about getting rid of that hotplate? :?: :!:

Click to expand...

Nope, not for a second, lol. I've been using it exclusively as a load box for my 'silent rig' (direct signal out to interface/speaker cab IR's in DAW for headphone playing). Been using it that way for a good two years. It does the job great. And if all I gotta do is replace a couple components every few years, it's worth it to me.

However, if you know of a better loadbox with direct out, for a reasonable cost, let me know, id be willing to take a look!