Sudden Noise in my RM20B

[mindcandyman]

Hey guys I was playing on stage and I noticed when a song ended that there was a buzzing or hissing in my amp that wasn't there before. Normally my amp is dead quiet compared to the other amps. At the beginning of the show my amp was functioning perfectly as well. I thought it was something with the lighting that was putting noise in my amp so I didn't think twice about it. Now when I come home I hear that same hissing and stuff.

Now if I turn the gain all the way up the noise is really really loud. The level of the noise seems to be associated with what gain level I set. I would normally, of course, get some noise when I turned the gain all the way up before but this is far in excess to what I normally hear. With the gain turned all the way up it's just a really loud buzzing.

I can play my guitar through the amp as well but it seems to have much more head room than normal. I am playing with a stock top boost module and it doesn't really get distorted till about 11 or 12 o clock which is not like the top boost module. Normally I can't really get clean tones at all with the top boost.

When I use my voltmeter on the back of the amp...test point #1 reads around 18 mv (normal for RM20B, 15-20 is the standard)...but the second test point only reads around 4 or 5 mv. Does that mean that my 2nd power tube is bad or is about to go bad. The reason I ask this is because I researched this online and I know this kind of noise and such can be caused by preamp tube failure on the module...and also the gain tube (nearest the input) on the amp as well. I don't have the money right now to replace all of the tubes in the amp so I wanted to see what you guys thought.

If it is the gain tube right by the input would I need a tech to replace that for me? I guess all the preamp tubes would need to be replaced by a tech. Thanks for your help I am new to tube amps.

Oh also both power tube lights are glowing in the back. Also I tapped them very lightly with a pencil to see if any strange noises came from them (saw that suggestion on this forum) and no strange noises were heard.

 

[Soulinsane]

Most likely only one preamp tube is going out. Cost maybe $12 to replace. Its rare for several preamp tubes go bad at once. They are super easy to change as well so no amp tech is needed. Just make sure they are cool before you touch them. Its just like changing a light bulb except you pull them out instead of twist.

Just try your new tube in each place until the problem is solved.

 

[mindcandyman]

Cool thanks man. I think I am going to go ahead and replace the power tubes as well since I haven't done that since buying it. Thanks for your help!

 

[JKD]

Cool thanks man. I think I am going to go ahead and replace the power tubes as well since I haven't done that since buying it. Thanks for your help!

Good call, if the power tubes are off by that many mV then it's definately time to replace them.

 

[TheHunter]

Oh also both power tube lights are glowing in the back. Also I tapped them very lightly with a pencil to see if any strange noises came from them (saw that suggestion on this forum) and no strange noises were heard.

If the two LEDs are glowing red above your power tubes then your tubes should not be working at all. Im pretty sure this is the fast blo fuse light that indicates there is a power failure. Not sure but I would get new tubes. The RM series makes it so you dont need a tech to do the repair on your amp. Like Soulinsane said just pull them out and pop a new one in. Bias your Power tubes and your done.

In general its a good idea to replace you power amp tubes every yr with everyday use normal use, and preamp tube 2 to 3 yrs with normal use. If you hot rod your tubes (bias them over there normal mv amount) you would want to replace them more frequently than once a year.

this is just my opinion!

 

[mindcandyman]

Ahh I'm sorry I was misleading with my description. I meant to say that the filament of the tubes are both glowing. I actually don't have a tube failure light on my RM20 . I wish I did have that. I also have an update for you guys as well. Take a look at this. I decided to pull the power tubes out so I took off the backing and pulled the first power tube...check out what it looked like. Because of the angle and placement of the sticker I couldn't see this from the back very easily and I missed it:

Now everything on the inside of the tube looks the same as the "unburned" tube as far as I can tell but I'm assuming I've found my problem. It only looks like the sticker is burnt and nothing else but I don't know what else would have caused that. I checked the connection point for the tube (up in the top of the amp) and there is no charring there or anything like that. I've sucked it up and ordered new tubes (quad matched to have a spare in the future...might as well right?).

I wanted to post these up to make sure I wasn't looking at a more serious problem. Does this look like a typical tube burnout? When I shake the tube it sounds like something really small is bumping around slightly but the other "unburned" tube sounds the same way. What do you guys think? Oh also I pulled my fuse out and it looks like it is still intact from what I can tell. It's slightly at an angle inside the casing but it won't move around when I shake it. If the fuse is blown it will be shaking all around the casing inside right?

While I am getting all these parts I was thinking of buying some fuses as well for backup. Where do you guys normally buy your fuses?

 

[bbp]

If you'd blown a fuse the filament wouldn't be glowing. The fuse is there to protect the tube from too high supply voltages, it's designed to break before something more expensive does and if it was broken the circuit wouldn't be working at all, ie the tube wouldn't glow.

 

[mindcandyman]

Wow yeah that was a dumb question. It's amazing how revealing a little rational thinking is hehe. Can anyone tell I'm not an engineer.

 

[Soulinsane]

Might have just been that particular tubes heater. Some of them get really hot! I wouldn't worry to much because thats just their nature. They can be.... individuals at time.

 

[mindcandyman]

I guess that's the good part and bad part of tubes. Their individuality hehe. I'm glad nothing is abnormal with how that burned out.

On a side note do you think that getting the output transformer modded on my RM20 would make a lot of difference. I know the output transformer they use is better than most combo amps with such low wattage but I was curious what you thought.

I ordered my tubes from thetubestore.com and I'll update you guys when I install them...I guess I'll be able to tell if I need new preamp tubes then as well.

 

[mindcandyman]

Well an update for everyone. I replaced the power tubes and all the preamp tubes and I am still getting the noise. I even replaced the tubes in my module. But I know for sure that it is coming from the module now. The crazy thing is that when I turn up my gain on the module now I am getting feedback??? It didn't do that before but now when I turn up the gain on the module I get a really high pitch feedback. What do you think could be wrong with the module and how much do you guys think it is going to be to get it fixed ...I will have to wait for a long time. I spent all my money on the tube replacement.

 

[mindcandyman]

Another update actually too. If I tap on the module (with the new tubes in it) I can hear a high pitch "ting" that is coming through the actual speaker...then if I keep tapping it a lot the feedback starts to come. Man this is frustrating. . They are new tubes and I had 3 to try instead of two so I know it can't be that. What do you guys think?

 

[mindcandyman]

Third Update...when I jostle the knobs just slightly on the module I get a lot of crackling and weird sounds. My suspicion is that the module is not sitting correctly in the saddle in the back of the amp. Should I try some cleaner or something first? What type of stuff should I use? It almost feels like the module won't push back far enough into that connector if that makes sense.

 

[mindcandyman]

Well I found a custom amp builder here in the Dallas area. His name is David Smith and his company "Dabeck Custom Amps" is located in Richardson, TX. He makes fantastic custom amps...I tried one out at the store. Really nice stuff and he can basically do anything you want. His build quality is impeccable too...fantastic. You want a direct out on your preamp...you want five or 6 preamp tubes...he can do that too. There are lots of reviews on harmony central about him and he does great work.

So I bring my amp in there and he starts looking at my module through a magnifying lens. He told me there were solder points that were loose or coming loose. He said that over time it happens naturally as things heat up and vibrate, etc... He removed both of the preamp tubes in the module and re-soldered the pins or areas where the pins from the tubes connect to the board. Now it sounds good again. I think I am going to take it back to him and have him give the whole amp an overhaul and just check all the major solder points so that this doesn't happen again.

I post all this up here just in case anyone has this same problem down the road. It sounded very much like the preamp tubes in the module were microphonic but they weren't. I could even tap on the faceplate of the module and hear that amplified through the amp very loudly. Here's the sound I was hearing.

http://www.jonathanhardesty.com/tubenoisesound.wav

It was a tricky problem and if Dave wasn't so good I'm not sure he would have caught it. So I just wanted to post this up in case anyone runs into this problem in the future.