DC-5 Bad Hum

[eeviac]

My Dc5 combo has recently developed a bad hum on the lead channel. Its very noticeable at even quiet volumes and gets louder as the volume increases. Its present on the rhythm channel too but not as bad. Ive not changed anything in my set up recently so its a bit strange. I have a noise suppressor just before my signal hits the amp and it makes no difference to the hum so i suspect its something to do with the amp. I also run a parametric eq in the loop. when i adjust the mix pot the hum gets worse then better then worse again. Taking the eq out of the loop helps somewhat so im thinking it could be something to do with the loop. plugging the guitar strainght into the amp also makes no real difference. The actual sound of the lead channel is unaffected - the hum can only be heard when not playing.

Please can anyone help or point me in the right direction - its doing my head in - the amp is a late dc5 serial 31k something

if it helps i can put a vid on you tube

cheers

 

[MJ Slaughter]

I've had this problem a couple of times. Usually turns out to be a bad preamp tube but on occasion I think old failing capacitors can be the culprit. Try swapping your preamp tubes one at a time with a known good one starting with V1. That's where I had my problem.

 

[eeviac]

cheers dude

i tried swapping out each preamp tube in turn with a new tube - this made no difference to the hum.

Then i pulled each power tube in turn to check if one of those was causing the problem and the hum was same.....

not sure where to go next!

 

[Monsta-Tone]

Could be filter caps or a bad ground connection somewhere.
Have any of the pots been loose?
Has it been dropped?

Also, have you tried different cables, different guitar, etc.?
It's frustrating enough at times just trying to figure out what is wrong with your amp, so the best place to start is to eliminate everything else.

You could also narrow it down to preamp vs. power amp by unplugging the return cable from both the amp and the pedal.
This will definitely let you know if the sound is before or after the Loop.

 

[ces]

I've experienced something similar. I only hear it when I'm not playing. It's like a low rumble.
Ever since i bought this amp used 2 months ago.
My luthier loaned me some used, but healthy original stock Mesa power tubes.
They made a difference but did not totally remove the problem.

Luckily, I live near Petaluma, CA where Mesa is located.
I've been meaning to take my amp in for over a month.
They said it would cost between $120-$170 to fix, no matter what.
Sounded good to me...

 

[eeviac]

i wish i lived down the road from mesa at times like these!!

monsta-tone - the amp is moved around every few weeks but nothing major and i take care of it - ive never dropped it and there are no loose pots either -i have tried different cables but yet to try another guitar....

ive made a tone test video trying to show the problem with clean (wet/dry) and distorted tones (recorded with a room mic)

im playing through my whole rig but the directly connected guitar with nothing in the loop has the same effect

as i mentioned above the hum is present on the clean channel but worse on the lead channel and doesn't seem to affect the tone. The noise suppressor is just before the amp input and you can hear the hum when its toggled which makes me think its the amp / fx loop.

http://www.youtube.com/watch?v=wr6AF-vk4X4

 

[Monsta-Tone]

Well, shoot!
If it still hums after a tube swap, then it is usually filter capacitors or a bad ground somewhere.

Is there anyone in your area that you can trust to take it to?

 

[eeviac]

I'm not sure if there is anyone around here i can take it too - i will have to do some searching

are the filter caps an easy job?

thanks

 

[wizard333]

Easy in terms of the soldering etc.

The biggest part of the labor in any amp, especially a pc-board amp, is not the soldering. Its pulling the **** thing apart and pulling the boards so you can get to what you need to get to.

 

[Monsta-Tone]

You won't have to remove the circuit board to replace the caps. It has solder pads on the top. I usually cut the leads off of the old ones and pull them out with pliers. They are glued to the board. The hard part is not breaking anything, and then, not burning anything with the iron when you're putting new ones in.


You should definitely try some new power tubes before replacing caps though. Just in case!

 

[wizard333]

What about the cathode bypass caps and bias filter? I generally like to hit those as well as long as I'm in there; if some electrolytics are going bad, may as well do them all. I've never been a fan of "partial" cap jobs because a week later another can go bad. If they are at that age, replace all and not have to worry about it. Haven't pulled my DC apart yet to see how things are situated but I'd be real suprised if the CKs could be replaced without pulling the board.

If the hum is ONLY on one channel by the way, thats not the filter caps. Its more likely a bad pre-amp tube or a bad blocking cap. If its really there on both (crank the clean to see) it could be filters, or it could be a bad tube.

 

[Monsta-Tone]

I always replace the bias and footswitch supply caps too, but have gotten out of the habit of changing cathode bypass caps on Mesa amps. They are really high quality, and have extremely low current passing through them. I've never seen one fail. Now, if it were a Fender or Marshall, that would be an entirely different story.

 

[eeviac]

AT LAST !

After a few hours of trying different things I found the main culprit to be a bad power supply to my noise suppressor pedal . Swapped it out for another and the loud hum went away. I think i need a decent power supply for my 3-4 pedals - any recommendations?

During the process of finding out the problem i noticed the that there is still a quieter hum coming from the amp. I usually run the output volume at around 7.5 and control volume with the channel masters. This works well as i prefer the warmer more thicker sound when pushing the power amp. When i turned down the output volume the hum reduced and went away completely at around 1.5 (where the power amp just becomes audible). When increasing the channel volume to compensate the level of hum did not increase and seems to be only affected by increasing the output volume.

Is this a sign of worn or bad power tubes? Ive had the amp a few years and ive never changed the stock mesa tubes i have and i have not noticed any degrading in the sound quality or power levels?