RM20B Noise

[PhiloBeddoe]

My 1-month old RM20B makes a pretty annoying cycling noise. It does require a guitar signal with the volume on. I've tried several guitars, cords, outlets, power conditioners and it is always there. I own several other amps including the RT2/50 and an RM4 and none exhibit anything like this. I checked the bias voltage and it's 17mV, which is right in the correct range.

Any ideas?

http://home.comcast.net/~mkaducak/PicsforSayers/RandallRM20BNoise.mp3

Many thanks

 

[tung]

1-The 1X12 speaker type combo sometime cannot take the signal overload, causing lots of noise compare to a 2X12 or a 4X12. The noise can come from high gain or effects.
2- Microphonic pick ups, did you try it with different guitars or just one guitar?
3- Bad tubes
Just couple thoughts,
Tung

 

[PhiloBeddoe]

Thanks for the response.

Yes, I've tried 3 different guitars. There are no effects or anything else upstream of the amp or in the fx loops. Just the guitar plugged straight in.

I forgot to mention that it doesn't matter which preamp module I use. Same noise regardless.

The mp3 I posted is with a Tweed module with the gain at about 10 oclock and master volume at 12 o clock. Eqs are all around 12 oclock. Boost is off.

I tried replacing the 12ax7 tubes with some old used ones I had lying around and the noise was still there, but probably at 75% of the volume.

 

[tung]

I had the exact problem with my Crate V30, and it was brand new out of the box, the 1X12 just can't handle the power.
In your case, can it be power tubes? You've tried all the other things, just pop in a new pair of tubes, you can always use the new tubes for spares if that's not cause. If your amp is still under warranty, Randall is responsible for giving replacement, can't remember how long they warranty the tubes for though.
T.

 

[PhiloBeddoe]

Tried a new set of EL84s and also tried plugging the RM4 into the fx return, thereby bypassing the preamp section of the RM20 (I think).

Noise is still there.

 

[JKD]

I take it no other amp does that when plugged into same mains outlet?

That cycling kinda sounds like oscillation caused by a power supply issue..failing capacitor somewhere?

Did you change the pre-amp tubes in the RM20B? If you go in with the RM4 into the loop, you are still using one side of an ax7 in there and of course the phase inverter that drives the power tubes.

 

[PhiloBeddoe]

Well, I think JKD is on to something. I think I have a problem with the power in general. For some mysterious reason, it seemed to be isolated to the RM20 for a few days but eventually I started hearing the same type of noise intermittently in other equipment.

First I was able to get another combo amp to make the same noise when really cranked. Then I heard it in my studio monitors. At the moment it has gone away in everything, including the RM20.

Apparently the basic Furman PL-8 and similar units I have do not do anything for this problem. Does anyone have any ideas for a power conditioner that might help this condition? The more budget friendly the better of course.

Thanks!

 

[stef823]

I have the same problem. Tried multiple modules, new set up tubes to no avail. It only seems to happen when in non-boost mode and medium to high gain setting. At first I thought it was the speaker as it did not have the problem when playing through and external cab with a V-30. I swapped the V-30 with the Greenback. Once the V-30 was in the cab it had the same problem and the Greenback in an external cab was cool.
I think it's some kind of feeback/oscilation) problem between some circuit and the speaker at high gain settings. I wonder if Randall knows anything about this?

 

[MetalMajik]

It could be Microphonic feedback from your power tubes. I know that EL84 tubes have a hard time withstanding the vibration in high gain combo amps. Also try replacing the input tube.

 

[3 Mile Stone]

Does not sound microphonic as much as maybe a problem with clean power. Other things on the same circuit? Dishwasher, other electronic componenets running while you hear it? Do you live in a house or an apartment?

I'd take it somewhere (anywhere, work or a freinds house, where ever) and plug it in there. See if it does the same thing. Or borrow a really good power conditioner from a fellow musician if possible.

 

[stef823]

I've tried all the tubes so that's not it. I have been playing it in the garage with many fluorescent light foxtures. I'll let you know.
Thanks again for the tip.
Stef

 

[3 Mile Stone]

I have been playing it in the garage with many fluorescent light foxtures.

Hmmmmmmmmmmmmmm. That could be your problem.

 

[okstrat]

I have been playing it in the garage with many fluorescent light foxtures.

Hmmmmmmmmmmmmmm. That could be your problem.

If it's in the lights, you should be able to rotate your guitar and find two angles that are much quieter. It sounds strange but it works.

Pete

 

[stef823]

The light were probably effecting it, but the amp can be fairly high and if I just back off a bit it seems to tame it as well.
Thanks for all the comments and suggestions.