Noisy DC-2 - Need Help

[korngold]

Hey Guys,

I'm a forum noob, but I've had my DC-2 for about 6 months. I have a noise issue with the amp--i'm getting a humming/buzzing noise (kind of a combination) when I use the amp. When its on with no input, I just get the typical tube amp "hiss". But, if I increase the guitar level to 10, I get a rather loud buzzing noise through the speaker. The buzzing also varies with the amount of gain (more gain, louder buzz). The sound from the speaker is useable and I can still get by in a band situation, however the buzzing noise is constantly present through the effects loop and headphones. I'd really like to use this amp for recording, but this noise makes it impossible.

I had the amp put in a new cabinet a few months back, and it's been happening ever since. I do have new EL84 output tubes, but I haven't messed with/replaced the preamp tubes, primarily because I don't know what they're all for. There are 6 tubes (looks like they are all 12ax7)--I'm assuming 3 for the clean, 3 for the distortion channel, but I'm not sure which is which, and if I need to replace 1, some or all.

Any help is greatly appreciated. Be gentle, I'm kind of a tube noob. 8)


BTW--the sound out of the speaker is more than useable and still has the awesome DC-2 clean & tons of drive in the gain channel. So, it seems like its working fine. . . just that darn hum/buzzing I can't seem to get rid of.

Thanks!

 

[Monsta-Tone]

Does this happen in both channels?

Have you tried a different guitar?

Different cords?



It is possible that one of your Preamp Tubes is going bad. If you know when the amp makes this hum, then you can narrow it down to a source.

Here is a link to the Owner's Manual. It should tell you which tubes do what.

http://www.mesaboogie.com/manuals/Studio%20Cal%20DC-2.pdf


It is also possible that a Ground is missing or was damaged when transfering cabinets.



Lastly, your amp may need a tune-up, and changing all of the Electrolytic Capacitors.



Hum is usually caused by a bad Ground or a bad Capacitor, but tubes can do funny things when they start to go bad.

Hope this helps

 

[korngold]

Thanks for the help! I should have specified all that other stuff before--It does this with all 3 of my guitars and all of my cords. The "buzzing" noise goes away when I isolate the amp from my monitors/computer stuff (just found that out this afternoon), but the hum is ever-present.

i've looked through the manual a few times, and it doesn't seem to hold any info about which tubes do what. I'm not even sure which side of the 6 tubes is the gain/lead channel & which is the clean, to be honest.

Is there a way to tell if the tubes are going bad, or do you just have to start replacing them & see if it makes a difference? I had the amp cleaned/serviced by an authorized Mesa tech who moved the head from one cab to the other. The cab is from a Mark IV, so I would think the grounding, etc., should be the same or at least similar.

I just wanted to see if there was a possibility it was the tubes before I go and order a new set. Or, should I just order a few? I was planning to get a set of JJ 12ax7's for the clean side and a set of EH 12ax7's for the lead channel (but, again, I'm not sure which tubes go to which channel).

Thanks again for all your help!

 

[Monsta-Tone]

If the buzzing is caused by lights, computers, etc. You might try a ferrite bead on the Power Cord. Every Plasma & LCD TV comes with a ferrite bead on the Power Cord to eliminate any EMI (Electro-Magnetic-Interferance).


It is possible that the shielding in the top of the amp cabinet has been removed by a previous owner. This is there to prevent any unwanted interferance.

Also, maybe it is a wire dress problem. I don't know about Boogies, but Fenders & Marshalls are extremely prone to this problem. Sometimes just moving the wires around can fix really loud hums.

IF YOU DON'T KNOW MUCH ABOUT WORKING ON AMPS, DO NOT OPERATE ON THE ELECTRONICS YOURSELF. AMPS CAN KILL. I HAVE HAD THE **** SHOCKED OUT OF ME, IT DOESN'T FEEL GOOD. I CANNOT STRESS THIS ENOUGH.


As for Preamp Tubes: Try swapping them, 1 at a time, with known good ones. This will tell you if you have a bad Preamp Tube. This is a bit misleading though, I have had 2 or even 3 Preamp Tubes go bad in an amp due to age or fouled filament wiring.

I would swap some tubes around. If this doesn't work, you should take the amp back to the tech. Any self-respecting tech would fix it for free just to make things right.

I just sent the drawings.

If you are even remotely thinking about operating on the electronics yourself, you should buy some books and read them 1st. Amps can easily kill. Anything shiny inside is a potential hazard, and electricity can arc or jump to you very easily.

 

[korngold]

IF YOU DON'T KNOW MUCH ABOUT WORKING ON AMPS, DO NOT OPERATE ON THE ELECTRONICS YOURSELF. AMPS CAN KILL. I HAVE HAD THE sh!t SHOCKED OUT OF ME, IT DOESN'T FEEL GOOD. I CANNOT STRESS THIS ENOUGH.

Duly noted! Thanks for the warning--this is why I took it to the tech even for the cabinet swap. BTW, thank you for sending the schematics so fast! I wanted the schematics just to see which tubes went where, but I couldn't figure them out. :shock:

I'll print them & take them back to the tech. I seem to recall that he said this cab didn't have shielding in the top, but I could be wrong about that. Quick question before I blow $75 or so on tubes: Do ALL the preamp tubes control the tone? Or, is 1 of each set of 3 for the pre amp, another for the effects loop, etc.?

 

[Monsta-Tone]

I am not positive about the DC 2, but this is what the schematics show:

V1a 1st gain stage for both channels
V1b 1st gain stage for Lead channel
V2a 2nd gain stage for Lead channel
V2b 3rd gain stage for Lead channel
V3a 4th gain stage for Lead channel
V3b FX Return (FX Send is Solid State)
V4a 3rd gain stage for Clean channel
V4b 2nd gain stage for Clean channel (I know this is backwards, but it is probably logical for parts layout)
V5a Reverb Send
V5b Reverb Return (this is great, if the Reverb stops working, it is easy to find)
V6a & b Phase Inverter




If the hum affects both channels, you should try V1, V3, V5 & V6. If swapping these tubes (1 at a time) doesn't do anything, you will have to take it to the tech.


MAKE SURE THAT YOU TURN THE AMP OFF WHEN CHANGING TUBES. ALSO, MAKE SURE THAT YOU ALLOW THE NEW TUBES TO WARM UP BEFORE TAKING THE AMP OFF OF "STANDBY."

BE ABSOLUTELY CERTAIN NOT TO TOUCH ANY OF THE PINS WHEN REMOVING OR INSTALLING YOUR TUBES. YOU COULD GET ZAPPED WITH UP TO 300 VOLTS.

Hope this helps,
Andy