Tube problem?

[madmax]

I just got my Mk IV back from repairs. Turned it in because there was a HUGE humming noise in the background. The sound I had from before in it was awesome though, no problem there. Just the humming. (could this be caused by a faulty connection or something?) The previous owner said the tubes were about a year old or something, I dont remember excactly but he said there was no need to change them (the preamp tubes). Anyways the tech didnt even listen to the hum, and just went right on and switched the 5 preamp tubes. The hum decreased (I think?) but the amp sounded awful when I got it back. Really way too bright, buzzy gain and hard. Didnt even sound like the same amp imo. I was wondering what you guys thought on this? I paid about 300 dollars for nothing here so... The tubes that were originally in were some Fender Groove Tubes, and the new ones were "TAD 12AX7A-C Selected". This guy said that on modern tubes you cant really tell the difference in sound, just a tad but this was a whole CHANGE. Well, I dont know, but Im not going back there in any case. Read something about Tung Sol or something around here? People seem to recommend these? Like so many others hehe, Im after a Petrucci-ish tone. Now Im off to put the old tubes back in

 

[madmax]

Okay, some days pass, and Im a little wiser hehe. Theres nothing wrong with the new tubes, but the humming is still there so it must be the amp. I have my Lead Drive all the way up on 10, gain: ca. 8, treble: ca. 7. So I understand that there would naturally be some noise, but is it supposed to be ALOT of humming with these settings?

Sorry for double postings, but I hope someone here could help me out.

 

[thunda1216]

Is your guitar equipped with single coils? If so, move away from the amp. Are you using the amp under flouresent lighting? If so, turn them off. Turn off the TV. Turn off the computer. Also make sure everything is plugged into the same power source (such as the same power strip for both the amp and your effects) so as to eliminate ground loops...

also, turn the lead drive down a bit and see if it clears up

ty

 

[madmax]

That's the thing. Okay. This hum is only in lead channel, it's there even if the guitar isnt plugged in. If I pull down on the gain, drive or treble its reduced to the point where its gone when you remove all gain. But these are pretty common settings right? is it normal to have alot of hum / noise on those settings? Its just so overwhelmingly much

 

[fretout]

I'm not sure if you have a combo or a head, but did your amp ever have any problems with the speaker outputs/speaker cable/speaker connections? If so, you could have been causing stress to your transformer (due to the lack of load) , and this can cause unwanted humming in the amp, even after a tube change. You may want to try replacing the power tubes if they haven't been yet (I think you said only the preamp tubes were replaced).

Also, you may want to verify that a fuse with the proper value is being used.

 

[madmax]

Its a combo, I havent had any problems with connections and such. Maybe previous owner did. I replaced the power tubes some time ago. Havent really got a clue when it comes to wiring and fuses there. But shouldnt a tech go through all this when he checks the amp? After all I told him to fix the humming in my amp, which is something he did not do.

 

[fretout]

Its a combo, I havent had any problems with connections and such. Maybe previous owner did. I replaced the power tubes some time ago. Havent really got a clue when it comes to wiring and fuses there. But shouldnt a tech go through all this when he checks the amp? After all I told him to fix the humming in my amp, which is something he did not do.

Absolutely! A tech should have inspected the entire amp for all possible problems, but in my experience, it is difficult to find a good repair tech that will take the time to inspect all the intricacies of a high gain tube amp. If the tech did not fix the problem, I would only pay him for the parts that he used, but not his time, because the problem was not corrected.

Maybe some of the forumites here can help you identify where the humming is coming from. Here is my advice FWIW...

1. Turn off all electronics in the room that you are playing in (this includes flourescent lighting!).

2. plug your power cable into the cleanest power source available to you (sometimes you can pick up dirty power which can contribute to "60 cycle hum"). If you don'y have a power conditioner, try plugging straight into the wall, and also, try plugging into a "power bar". Determine if there is a noticable difference in the humming. (just like thunda said)

3. If the humming persists, try lightly tapping on the preamp tubes (with a chopstick or a pencil) and see if they make any noise. If they do, you will want to replace the offending tubes.

4. If the humming persists, you may want to pull the chassis out of the combo. Turn the amp off, unscrew the 4 screws on the top of the combo, pull the chassis out toward the rear of the amp (be careful not to touch the capacitors!). Inspect the lead channel preamp section of the board. Determine first if there are any burns on the PCB board, and then try inspection all the solder points.

If you have done all this, and the amp still hums, I would send it back to Mesa for repairs. Since the humming is only on the lead channel, I doubt that there is a problem with the power tubes, or the transformers (that's a good thing because they are the most expensive to repair), but it sounds like the problem is in the preamp for your lead channel.

Just my advice FWIW. Hopefully this helps.

 

[madmax]

Hey thanks alot! I talked to the tech today on the phone. He said it was pretty hard to figure out the humming unless you have something to compare it to (another amp). But I'll try your steps ! Thanks ; )