Problem with RM20 combo - loud hum/unstable on certain notes

[UltraLow]

I am having a problem with my RM20 combo, any help is appreciated.

At louder volumes, on certain notes, especially high B if I let it ring out, the amp intermittently cuts out to a loud hum when I play the note. The hum sounds like I pull the cable out of my guitar and touch the jack while its plugged into the amp, then cuts back in when I play another note

Here are some things I tried:

1. I swapped out the tubes, but still had the same problem.

2. The problem exists with several different modules (ie, Plexi, Ultra Lead, Top Boost)

3. If I plug the module into an RM4 preamp, and patch it to the effects return of the RM20, the problem seems to be pretty much eliminated.

Because of #3, it would eliminate the power amp section of the RM20. It is probably not the module, since it happens across several modules and does not happen with the same module in the RM4.

It would have to be related to the preamp section of the RM20, or a vibrational problem when the module is in the RM20B is is eliminated when the module is in an external enclosure.

I know this is kind of a quirky problem, but if anyone has any experience with solving a problem like this, any help is greatly appreciated.

Thanks!
Jim

 

[rickgk]

what about trying different leads, or it could be the output jack of your guitar also.

 

[Chad]

Do you have a new preamp tube you can try? If you're just switching around the same tubes, it could be the same bad tube causing a problem.

 

[UltraLow]

I swapped in all different tubes (two different sets), same results.

Also, its not a loose connection or jack, but it almost sounds similar to what it would sound like if the jack or cable was loose and cutting in and out. The weird thing is it mostly happens on only one note (high B played on the 2nd string 12th fret). It will happen on other notes to a lesser degree, but it is really loud in that note.

I will record the sound it makes and post it. It is kind of hard to describe.

Thanks,
Jim

 

[3 Mile Stone]

You've tried every option except this one. Plug the amp into a different cabinet with a different speaker. You sort of covered this with #3 but for total isolation of each component you could give this a try. Speakers can vibrate the tubes in a close proximity combo where everything is so tight. EL 84's are quirky little dudes too. Could explain why it's only that certain note because the vibration from that particular frequency freaks out the tube with sound waves.

 

[Chad]

Good point. Bob at Eurotubes goes as far as calling some combo amps tube "torture chambers" and he notes that EL84's are especially susceptible. Check out this FAQ and especially #9:

http://eurotubes.com/euro-i.htm

 

[MitchSchaft]

Did you swap the preamp tubes out, too?

 

[UltraLow]

Yes, I swapped out all the tubes in the RM20 (three 12AX7's and the two EL84's)

I didn't swap out the tubes in the module, but tried 3 different modules with the same results.

I am thinking maybe the socket itself may need to be tightened up, maybe certain notes hit a resonant frequency of the chassis and is vibrating the connector to the module. I will try to retention the receptacle and see if that fixes it.

If it doesn't I'll try to get a recording of what it sounds like tonight...

Thanks,
Jim

 

[UltraLow]

Hey 3 Mile Stone, somehow I missed your post last time I checked in... You hit the nail on the head...

Prior to reading your post, I had retentioned the tube sockets & module socket, but still the same problem.

But tonight I tried an external cab as 3MS suggested, and problem solved! So I guess it is most likely a tube problem, although I guess it could be a loose connection/component on the PCB.

What a shame, the 1x12 combo is voiced so well I love the tone from that thing. I will try holding each tube with a rag to see if I can pinpoint a tube. If its a tube then I'll just buy a few new sets to see if that solves the porblem.

I recorded what it sounds like in case anyone is curious:

http://www.outerdrive.com/mp3/rm20noise.mp3

Thanks everyone for your input!

Jim

 

[UltraLow]

Also, good link Chad.

Not sure if it is tube rattle, I tried holding each tube with an oven mitt and did not really find any reduction of noise, so I don't think those clamp on tube dampers will do much, at least for the tubes that are in there.

I am going to open her up & see if I can find any loose components while a few sets of new tubes are in transit. I could use some more preamp tubes and another set of EL84's anyways.

Thanks,
Jim

 

[3 Mile Stone]

I just had a picture in my head of you trying to play a massive power cord on the guitar while holding a tube with an oven mitt.

Ha! sorry I just thought that was funny.

:lol:

 

[UltraLow]

Hehe... luckily it consistently happens on one note (high B), so I capo'ed the guitar in a stand and could somewhat easily pluck the string with one hand while holding the tubes with an oven mitt in my other hand. Maybe that makes it even funnier... ;-)

 

[Chad]

Just as a note, for tests like that, a pedal with a looper is invaluable. I've used a Digitech Digidelay, Boss DD-20, etc.

 

[bruce egnater]

Do any other B notes do it? Also, try "whacking" the top of the RM20 with your fist and see if the amp makes any noise similar to the B note noise.

 

[UltraLow]

Hi Bruce,

Thanks for your reply... I can now get it to do it on other notes (certain E's and A's when tuned slightly flat). When it first started, it was *only* the high B, and if I bent that note sharp just a tad, it would go away.

I do hear it now on the low B, but it is much more faint.

Maybe (hopefully) a component is slowly failing and it will be easy to find & fix the source of this.

I tried hitting it a few times as you suggested, and it didn't make any noises at all. It only happens when I am playing a note.

If you were able to listen to the mp3 I recorded of the problem (posted above), it is almost like there is another note playing when I play certain notes, and the noise I'm hearing is the two notes creating a low beat frequency.

Is it possible that there could be some sort of low oscillation happening that is inaudible, but when certain notes interact/resonate, there is some sort of audible beat frequency created? That's what it sounds like to me.

I am ordering a brand new set of tubes just to rule that out before I bring it in to a local authorized repair shop. Even though I already swapped the preamp tubes out with two different sets of lightly used tubes, I would like to replace them with brand new tubes just to make sure.

Thanks,
Jim

 

[bruce egnater]

Early in your post your said when you plugged the preamp into the return it "seems to be pretty much eliminated". Is it 100% eliminated or does it still do it, just not as bad?

 

[UltraLow]

Well, I tried plugging the RM4 into the return of the RM20 again to listen more carefully, and while it seemed to be eliminated compared to the way it sounded before, there was still a faint hint of the problem.

I probably wouldn't have noticed it by itself, but there is an interference that I would have detected as a lack of sweetness or sustain. It causes the note to be "choked" a little bit on the initial attack, if that makes sense.

Thanks Bruce...

Jim

 

[UltraLow]

Another update... I replaced all the tubes with a brand new pre-tested set from the Tube Store (all 5 tubes). No change in the problem, so its defintiely not tube related.

Thanks,
Jim

 

[Chad]

I'm really inclined to think it might be a resonance issue. Check out this thread from the M/B forum:

http://forum.grailtone.com/viewtopic.php?t=11849

Might be worth looking into some dampeners to see if that would help.

 

[UltraLow]

Thanks, Chad.

I don't think it is a tube resonance problem. I tried playing while holding each tube with an oven mitt to see if the problem would be reduced at all, and it wasn't.

Thanks,
Jim