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DT25 Hum issue ... question
by geppert on 2012-07-15 08:34:19

Scenario ... DT25, Standby off (switch up) both Channel Volumes off, Master Volume off.

1) If I switch to Triode Mode I get a noticeable low level hum

2) If I switch to Class A Mode (Triode + Class A) I get a more noticeable hum. Very annonying at home (would get totally masked at band practice). Can't use the amp in that mode at home due to this noticeable (and irritating) hum.


Is this normal for this amp?

What to check??? Reseating Tubes? Bias ??

Re: DT25 Hum issue ... question
by spaceatl on 2012-07-15 11:49:46

Always a good idea to reseat tubes on a new amp...especially if it was shipped...checking bias is not bad idea, but that doesn't affect class A operation...

Do you get hum when you have no guitar connected? Have you tried a different power circuit?

Re: DT25 Hum issue ... question
by geppert on 2012-07-15 13:29:38


Hum happens whether gtr is connected or not (same amount). Happens the same on all 4 Topologies.

Just reseated all 3 tubes and no change. Hum still there and same amount. Tubes are not microphonic.

Cannot tolerate this hum when amp is sitting beside me at home. Like I said, it does get masked with the band. Dead quiet in Pentode / Class AB.

For my take, it shouldn't be huming like this.

Re: DT25 Hum issue ... question
by STPLE on 2012-11-16 21:29:25

Did you figure out how to fix the hum??  I have the same problem.  I just bought a DT25 head and cabinet (used).  I have experienced the same thing in triode and more pronounced in class a triode.  Just wondering did you fix it - if so, how?  thanks.

Re: DT25 Hum issue ... question
by geppert on 2012-11-16 22:58:47


Nope I do not have a resolution on this yet and I am thinking seriously about sending the amp in for service under warranty. It must be a hit or miss thing as I don't see a lot of people complaining about it. Below is what has happened to me so far ....

Note: this is with EVERYTHING turned off (all knobs at "0"), no guitar plugged in ... Basically just the outbput power amp to the speaker.

I will also tell you that I returned the first DT25 head that I bought and got a 2nd one .... same problem.

1) On ALL 4 Topologies (NFL I, II, III, IV) Pentode and Class AB is pretty much dead quiet

2) On ALL 4 Topologies, Triode mode produces hum most noticeable at Bedroom Levels. At gig levels not so much because the higher volumes masks out the hum, but it is there.

3) On ALL 4 Topologies, Triode plus Class A produces slightly more hum. Definitely noticeable (and annoying) at bedroom levels.

4) NFL III produces the loudest of the 4.

I replaced all the stock tubes with Vintage Amperex. No difference. Hum still there. I also tweeked the bias ... same problem.

I also noticead that it "appeared" to increase slightly with the Firmware 2.0 update, although I think this is an output power problem and I don't think the firmware update does anything to the power section ??? (not 100% sure on that). So this in itself doesn't explain why it appears to be more noticeable now?? Maybe it's just my imagination?

I had some correspondence with Line 6 Tech Support (through a support ticket), but in the end they didn't offer much if any explanation. I sent them several audio files of the hum issue during the discourse.

Some here on the forums say "this is normal" but I don't buy it .... or I should say, I still think my amp is humming more than it should, especailly for a $1000 piece of gear.

Since I have SV212 as a backup I am thinking I should send the head in and see if they can fix it, or best case scenario, replace it yet again with one that doesn't hum although I doubt they will do that?

I think it may be a bad batch? .... but again, no recalls, and no heavy complaints on this forum about the hum, so I am scratching my head a bit?

In my office, working on my computer, I could NOT have this amp sitting beside me, turned on, set to Triode and Class A. The hum is totally, noticeable and annoying. I just can't wrap my head around the suggestion that this is "normal".

All that being said, I do like the sound of this amp. I am glad I got the head and cab as I prefer it to an open back combo.

I generally have the amp set for Pentode and Class AB (actually Pentode and Class A is ok, very little hum but still there). At band levels I generally use NFL III in Pentode mode and Class A.

Unfortunately I live a bit remote and my local L6 dealer doesn't stock these amps, so I have no way to compare my DT25 head with others in the store.

Lastly, I have had a few direct online chats with other DT25 owners who have said they have no hum in the same conditions that I get it in. So agan, scratching my head.

The big downside with sending it in is the likelyhood of a long (or very long) wait to get it back.

Sorry the news couldn't have been more positive.

Re: DT25 Hum issue ... question
by STPLE on 2012-11-17 08:35:52

I  bought my amp used (in great condition - like new).  I don't therefore have a warranty.  The seller said I can return it.  Do you think I should and then buy another one or new one??  I would use it in a church setting so unlike playing out there will be moments of very low volume (talking volume).  I was going to mess with tubes, bias, etc. but maybe it is worth moving on.  Tell me what you think. 

Re: DT25 Hum issue ... question
by geppert on 2012-11-17 10:12:40


I can't honestly say for sure whether this is "normal" or not since I have no way to compare my amp with other DT25's. Like I said, in Pentode and Class AB it is steller. I would recommend that you go to any local L6 retailer who has them in stock and check the hum deal. You know exactly what to do. It doesn't even require that an instrument be plugged in. If you have no way to do this (like me), then it is a decision you have to make about returning it. If you got a really good deal on the amp then it might be worth fixing if in fact it needs to be fixed? If this hum level is "normal" (which I have a hard time believing it is) then you know what you have and whether you can work around it? Unfortunately, that has yet to be determined for me. I have no idea what it would cost to fix out of waranty but you can be assured the repair probably won't happen quickly unless the repair person is on site at the store you take it to and they don't have a huge backlog of work. In my case, this amp will be going on a world tour to get repaired. At least I have a workable backup solution. Again, as I said before, there are very few complaints about this specific issue so I am leaning towards the possibility that something may be askew with our particualr amps. Only a trip to the repair center (or comparison with other DT25's) will tell however. Curious, what is your serial number? I will reiterate that I do really like this amp, especially with the closed back cabinet. IF however, you need to play quiet in church, and if you want to use the Triode/Class A settings, the hum will surely bug you. This is not the case for me. I am always in a battle to get over the drummer's volume .... lol !~!

Re: DT25 Hum issue ... question
by STPLE on 2012-11-17 11:00:55

I don't think it is normal.  It might be but I doubt it.  I don't live close to local L6 dealer either.  I can't compare it.  Oh-well, I might send it back and try again.  I'll let you know if that same buzz is there or not.  I can't imagine it would be.  I'm familiar with the "tube amp buzz" and ok with the normal fuzz buzz, etc.  This one is different.  I did call Line 6 yesterday and they said probably was bad tubes and bias (from shipping) - that is why I asked you if it worked when you replaced that stuff.  I have a POD HD 500 and love the amp.  I love the warm tones (that is why I bought it).  I haven't played with the POD HD 500 with it yet just straight guitar.  If I don't keep it I'll probably get another one. I think this one is defective.  It is a great amp.  If you get it fixed let me know if it buzzes still or if I get a another one I'll let you know.  Thanks!!

Re: DT25 Hum issue ... question
by geppert on 2012-11-17 11:15:27

I am going to mess with the bias today to see if it alters the hum level? I will also re-try the original electro-harmonix tubes for comparison. I always test the bias and tweek for the recommended values but it doesn't hurt to retry. Other than that, I can't see what else I can do about it? If the hum level can't be reduced, I will most likely send it in.

PS: I also use an HD500 with this amp.

I'll be curious to hear if you send the current amp back and get a new one if it solves the problem?

I asked about your serial number to see it it was somewhat close to mine? This might indicate a bad "batch run".  Mine is (21)A921M6206000215 .... which is somewhat close to the original head I purchased.

I'll post later on the outcome of the bias tweeking.

Re: DT25 Hum issue ... question
by geppert on 2012-11-17 15:51:35


OK …. here's the update

I have 4 x EL84's, 2 x Electro-Harmonics that came with the amp and 2 x Amperex Vintage I bought used off eBay.

First of all, adjusting Bias up and down had NO noticeable effect on the HUM issue in Triode mode and/or Triode plus Class A mode. So we eliminate that.

1) With the Amperex in, I noticed that one tube's bias was quite low. Lower than it was when I first put it in by a fair bit. Probably a 10mv difference between the two? Definite and noticeable hum in Triode and Triode plus Class A modes.

2) With the two EH tubes in, bias between them was quite close. About 4mv between them. Still, noticeable hum in Triode and Triode plus Class A modes but NOT as much as with the two Amperex.

3) I then tried the higher voltage Amperex and the closest (in voltage) EH together … BINGO … relatively NO HUM in Triode and only a slight (but acceptable) hum in Triode Plus Class A. This was a significant difference than I had been experiencing since the beginning with this issue.

This was a great discovery. It means (through my empirical testing at least) that this hum issue is directly related to the power tubes. Not sure why I am getting some hum between the two EH tubes and vitrtualy none with one Amperex and one EH but I have managed to get this amp more than acceptably quiet.

I did check the low voltage Amperex against both EH's and still there was noticeable hum. I am probably going to have to write off that low voltage Amperex. The good news is, I now have a quiet amp in Triode and Triode plus Class A modes and I do not see any need to send it anywhere for service at this point. A little time is needed to give the evaluation more credence.

So STPLE, before you return your amp, my suggestion is that it might be worth a try to experiment with a new pair of closely MATCHED EL84's from a reputable vendor. I do know now, that this amp can be "hum quiet". One trusted member on these forums likes the Mesa tubes. I'd probably give them a try? Maybe inquire first from the vendor if the tubes can be returned or exchanged if you are not satisfied?

Let us now how you make out with yours.

Re: DT25 Hum issue ... question
by STPLE on 2012-11-18 14:31:25

That is awesome.  I am glad that the amp is not defective.  I'll report back to you what I end up doing.  I am not as sophisticated with the tubes and biasing as you.  I have only put tubes in an amp that didn't require biasing.  Not sure if I'll take it to a guitar tech or not.  Not sure how much that will cost.  Again, thanks for your help.  Good luck.  I love that amp. 

Re: DT25 Hum issue ... question
by geppert on 2012-11-18 16:26:53

Well there's always a back door possibility that your problem is caused in a different way than mine but I suspect not. I wouldn't worry so much about the biasing. I would just swap a new pair of matched EL84's in and see if the problem goes away. However that being said, any decent tech could replace and bias new tubes in 1/2h, so if that option is open to you easily, I might consider going that way, however, anybody can replace a couple tubes Be careful, those EL84's can get real hot. Good luck and I hope it works to the point where you are satisfied with the amp?

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