HELP! DC-3 Problem for any of you tech types!!

[GregN]

At my gig tonight, my DC-3 had some issues. Turns out it's only on the clean channel and it's a hum that sounds like when you turn your amp up and touch the tip of your guitar cable w/ your hand. Dirty channel is fine. Of course, the first thing I thought of was that it was a tube issue so I started subbing 12AX7's. Took a spare and replaced V1. No change. So, figuring that the one I pulled wasn't the culprit I left that sub tube in there and put the one I took out of V1 and replaced the one in V2. No change. Replaced the one in V3 w/ the one I pulled from V2. No change. Went all the way down the line and got the same result. I've got to assume the power tubes are fine since the dirty channel works as it should. Does anybody have any ideas on what this might be? Obviously I'll check the solder joints on the tube sockets but could this be a resistor going open or something along those lines? I'd really appreciate any input from some of you that have troubleshooted Boogies.
Thanks,
Greg

 

[Blueracer]

Hi Greg,

I have an idea where your problem may be. First set up in the rhythm channel so that you get the hum. Now turn down the reverb on each channel and turn them all the way down. My guess is that the hum is still there but we want to eliminate the reverb as the source. Next turn down the master on the lead channel. Again turn it down all the way. My guess is that this will either decrease or eliminate the hum. Now to futher isolate the source turn the lead master up again. The hum should return. Now turn the lead gain down all the way. Hopefully this again will either decrease or eliminate the hum. If this works you can stop here.

If the above doesn't work next turn down the master on the rhythm channel all the way. This will hopefully reduce or eliminate the hum. If it does turn the rhythm master back up then turn the rhythm gain down all the way. If this reduces or eliminates the hum you have further isolated the cause. BTW if the hum is in the rhythm channel my original idea of what the cause is is incorrect.

Either the above procedure will work or not. Either way get back and post what the results were and we can continue. Hope this helps.

Gary

 

[GregN]

Thanks for the reply! Well, like I said, it's in the clean channel. The dirty channel works fine so none of the things you mentioned for the dirty channel made any difference. On the clean side, turning that channel's master down, of course, got rid of the hum. Turning that back up and turning down the gain, did not eliminate the hum. It only changed it's frequency. BTW, this is a LOUD hum. Hum's probably the wrong word for it. Loud buzz might be more appropriate.
I guess I'll run all the tubes through my tester just to double check that maybe more than one tube went down on me but these are JJ's that are less than 8 mos old.
Any other ideas? I really appreciate the help!
Greg

 

[GregN]

Well, here's an update. I tested all the tubes and they all test fine. So I pulled the chassis and reheated every tube socket solder joint that I could get to and it seems to be working okay now. I'll test it later on this afternoon again and leave it on for a bit to get further confirmation. Did this problem sound like it could've been caused by a cold or cracked solder joint? Would that cause a loud buzz like that?
Thanks,
Greg

 

[Blueracer]

Hi Greg,

This is good. You have eliminated much of what could be causing this. This would include your lead preamp, the reverb, the eq and the power amp. I doubt that it is a tube but go ahead and check them. When you turned the rhythm gain down did you turn it all the way down and if so what happened. You mentioned the frequency changed but was there any change in the volume of the buzz.

If you did turn it all the way down and it eliminates or decreases the buzz then it is likely something to do with V1a. If it only changed the frequency then it may have something to do with V4a or b. A further test would be to pull V1 out and see if the buzz is still there. If it is then the involvement is something to do with V4. Also try turning the rhythm treble, bass and middle all the way down and see what happens. Try these things, test your tubes and post back. Hope this is helping.

Gary

 

[Blueracer]

Hi Greg,

You posted while I was writing my reply. Glad to hear that your amp is back in working order. To answer your question yes a cold solder joint definatley could cause your problem. As you pointed out in your first post it sounded like when you put your finger on your guitar cord. It sounded like you were picking up 60 cycle hum in one of your stages most like in either V1a or V4. Reheating all the tube joints is something of a shot gun approach and I am glad that it worked out for you. If things revert post back and I will try to help.

Gary

 

[GregN]

Thanks for the help Blueracer. You know, when I think about it, what happened in the first set last night was the amp was working fine and then on one song I turned on one of my boost pedals (RC Booster or DOD 250) and that's what would kick the amp into buzzing like that. So I'm thinking the boost in volume would cause a little more rattling to the whole amp and probably cause the faulty solder joint to open. I'm thinking this because as I'm testing the amp today, I'm really cranking the volume to try and get it to do it and I can hear how the components in the amp get a little livelier as the volume goes up. I hope I'm right and that it was, in fact, a cracked solder joint. Gotta love PCB mounted tube sockets or jacks, etc. Learned that from fixing some old pedals w/ pcb mounted jacks. Always one of the first things to look for is cracked solder joints from all the putting in and pulling out of 1/4" plugs.
Anway, I don't feel secure enough to take the amp to my gig tonight but I'll definitely be putting it through it's paces here at home over the coming days to see if I can (and hopefully NOT) make the symptoms come back. I'll be sure to post here again if it acts up!
Greg

 

[Blueracer]

Hi Greg,

I was thinking the same about playing a venue and the high vibrations causing a cold joint to open. Hopefully you got the offending joint and that there aren't any more lurking around. Good luck with your amp and your gigs.

Gary