mtreehugger

I really appreciate your help! I'm in the process of searching for a service center now. I suppose that if for some reason my buzz is normal to the amp and the only way I can get rid of the buzz is with external power conditioning, the tone of this amp is so awesome that I'd bend the rules and do whatever it takes. However, I doubt it's normal as I'm not reading about this issue with other DTs and also because the buzz is new rather than something that was always there. I'm thinking its some kind of solder or PCB trace issue. Hopefully the tech will find it soon!!!

i believe you can use any 12ax7. for my dt50 i went with a mullard for the boost and a groove tube sag-ax7-mpi (matched phase inverter). i can't say it helped as i'm now getting a ground buzz which i hate even more than the noise floor. I put the eh's back in and the noise seems less but the buzz is still there. maybe the buzz is sucking the noise? much more info toward the end of this thread: http://line6.com/support/topic/8112-which-12ax7-socket-is-the-phase-inverter-on-a-dt50/

My DT50-112 buzzes in all voices, classes, modes, and channels--everything except standby mode. I've tested each and every combination as you prescribed, and here are the breakdowns/patterns it follows: It buzzes worse in class A than AB. It buzzes worse on pentode than triode. The buzz follows the noise floor pattern through the voices, i.e. I is lowest,, louder in II, louder still in III, and in IV it gets quieter and changes in tone, which I believe to be due to harmonics but can't vouch. I checked my line voltage, and ground on one leg is about 4V different than the cold side. For the other leg it's only .7V so I plugged into that outlet. No difference. No difference with ground lift either, as was the case yesterday, so this entire paragraph isn't surprising. I do not believe power conditioners should be necessary in buildings built after 1975 because electronic devices are supposed to come with adequate filtration and surge protection. I have numerous amplifiers and other devices in my house, and only one has a ground buzz issue. I believe this to be a defect in the amp and I'm crushed.

Good stuff, thanks! Will report back , hopefully you won't mind...

Same as always, I'm plugged into the same power strip plugged into the same outlet. I tried the ground lift switch and that made absolutely no difference. I did have some buzz issues before when I was running HD500 studio direct (no amp), though intermittently. The source of that was a surface mount failure inside the HD. This bizzare component the size of a caramel square eventually fell right off the board, enabling me to isolate and correct the problem. I think I'll try disconnecting the POD and XLR cable and see if that changes anything. Any other ideas? The timing of my noticing this seems more than coincidental to me...

Bummer, man, the ground buzz is harshing my buzz, man.

Disappointingly, I've been hearing a slight 60 cycle hum beginning with test 2, and even going back to test 1 config does not abate. (even with no guitar cord plugged in) The question now is: Was the hum always there, but I just couldn't hear it before through the whoosh of the noise floor? (which is now much improved) -- I believe I'll swap back the original tubes tomorrow--for tonight enough of that has taken place already. Sorry I don't totally follow you... What is SPAX7? Is that equivalent to a SAG-AX7? So you're going to try a matched PI tube in the boost socket (which is also called a buffer socket)?

well test 2 might be slightly more noise than test 1, but still less than what i was getting. also, it seems that switching voices is much smoother now, without that loud krack. onto final config...

i just tried the mullard in the pi socket and noise floor seems to be cut in half. next up: mullard in the boost socket with original eh back in its pi socket. then on to my final (i hope) test with both new tubes in place.

You are a saint, spaceatl! What would happen if someone stuck the balanced pi tube in the boost socket? My guess is not much, but kind of a waste...

Did you find this to be correct? V4 is the inverter? I did the exact same thing with my dt50-112, buying the balanced pi and then realizing i don't know which socket...

I've ordered new 12AX7s and new EL34's which will arrive today, but I'd like to try just the 12AX7s to see if that gets rid of my noise floor. Thus I don't need to pull the amp down of its lofty perch, pull the chassis out, or pull the cords or any of that... Except that I ordered a GT matched inverter like you used once and a Mullard for the boost, so I need to know which slot is which. Looking at the back, can you please tell me, is the boost on the left and the inverter on the right? THANKS!

Maybe, maybe not. There's only 3-4 tubes in these things. They could probably get similar results with looser tolerances only if they went with more gain stages of lesser gain per stage (i.e. more tubes). Did you see the stuff in the other thread, in particular the article by the guy from Mesa-Boogie who says only buy Mesa tubes for Mesa amps due to exacting tolerances? (same deal, right?) I can't wait for my new 12AX7s to get here so I can see if my noise floor will drop off. It worked for spaceatl so it should work for me--and maybe for you too? Should be any day now. I will definitely report my findings!

So then checking bias with a voltmeter is perfectly acceptable, even if you're not running the EH tubes?

4 hours? that's all? Did you JUST get your amp back?

in my mind your #1 and #2 are the same problem as what I reported, though I could be assuming too much. from what i can tell, it doesn't make much difference which class or pentode settign you use, it's the voices ii & iii (and even iv to some extent) with the excessive white noise. for your problem #3 i have the same thing or very similar in low power mode with voice iv and maybe some other voices too, except i don't need to use a full 1/3 of the pot to get sound. i always use lpm, so i can't comment on the issue without lpm. one thing i did notice was that it was worse the 1st time i switched into the voice (and heard nothing but the faintest whisper), and after fiddling around, the next time i switched into that voice i could hear it. i don't consider mine to be a problem, being that computer modeling being integrated with traditional tube amplifier design is what's happening here. i just move the dial. yours may be more severe than mine, however.

I don't have a problem with noise -- except the noise floor. I get that noise with no volume, no gain, no guitar cord plugged in...

What will you check the bias with--a voltmeter?

I bought mine used 10 days ago and it acts the same way. Worst noise in voice III, and I get it with volumes all the way down and gain all the way down and no guitar cord even plugged in. I have initiated a support ticket in as well.

I actually started a support ticket due to the white noise roar. They replied that as a high gain amp there is a higher noise floor (picture "I cannot change the laws of physics, Captain!"), and asked me to record a clip, which I did. Now I'm waiting. In the meantime I'm also waiting for 4 new tubes, and I'm playing loud enough that they can hear me outside the house to drown out the noise. I can still talk loudly over my playing, but it's now audible outside the house to the point where the sound carries, which means that the teenagers who party with my permissive neighbor's children all night every night can now hear me play and might be inspired to explore just how lucrative selling hot musical gear can be. (thanks to imports, not as lucrative as it was years ago) Assuming the noise I'm hearing is "normal" (and it sounds like it might not be based on Brazzy's reports), if I were playing with live drums I wouldn't mind much. It would be like "Dude, turn off your Big Muff when we're talking, 'kay?" and I'd have to go to standby for any period of silence longer than 5 seconds. Yeah, actually, I would mind. Scratch that earlier sentence. I don't recall this white noise from auditioning a DT50 at GC, or on the day I bought it 2nd hand, or the next day either, so I think it's a new development, possibly something that grows worse over time, but I can't be 100% sure of that being that my radar is up more now than earlier. So did you get your amp running again?

"I don't like Celestions for my tone...I totally enjoy listening to other players that use them..." I often wonder why what sounds great with other players doesn't work the same for me. Is it because we expect a certain type or velocity of response based on what we heard earlier, and then when we're actually playing we experience something a bit removed...? Or am I just a bit eccentric as a player? Hmmm... more on that later, perhaps in another life. With different speakers one might expect different tubes too. You're a tweaker, that's for sure! So this mod you speak of, I saw some posting on it in other thread(s). Once completed, are you simply connecting a voltmeter to the test points? And then there's the question of why: Do you test and adjust the bias regularly during the life of the power tubes? THANKS!

Thanks for the post! You know that when I "called <you> out" that I did it in a deferential, admiring way, right? Okay, so this guy and psarkissian are both saying to stick with the original tubes and don't mess with the bias (ceptin ya kaint bias a boogie). And they're both saying that messing with these things can put your amp on a tech's bench. But you aren't following that advice. Care to divulge the bias voltage in your DT50?

Thank you so much for unraveling this mystery for us!!!

i think some have had success with the jj's. i spose the operative insight here from l6 psarkissian is "You can't use the EH bias spec on other tubes. The spec is dialed in for the EH's. All too often these come across our bench for no other reason than having non-specified tube inside (JJ's, Mesa, Groove Tube), using the DT amp specified bias. That does not work." notice that he doesn't say "never," he says the EH spec doesn't work for other tubes. (he strictly non-recommends other tubes in general too, as a general rule...) From another post in an other thread, spaceatl said that the proper way to bias is with a signal generator and an ocilloscope, adjusting the bias to remove the "notch" you see on the scope. if i were you, i'd verify that your tech did this, and ask him what the bias voltage he arrived at was. if he didn't do this, i.e. if he merely set the bias to the spec given by line 6 for eh tubes only, and all he used was a voltmeter, then he didn't do your tube install properly, and you could very well just give him back the tubes and refuse to pay. if you supplied the tubes, refuse to pay and take it somewhere else. well, that's my 2 cents after taking all of this in. okay, i'm not a guitar amp service tech, but i did graduate top of my class at electronics school and learned gobs of junk that i still kinda remember.

really??? oh man, that s_cks!!! yeah, i do think it was your lipping off that brought it down on you. (not really) how long have you had the amp? did you buy it used like i did? did you try putting the old eh's back in?