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Dt25 Hum - What Is An Acceptable Level?

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#1 PANT666666

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Posted 13 March 2014 - 07:54 AM

The DT25 112 is my first tube amp.   The other amp I own is a fairly quiet solid state amp by Behringer (KFX3000). 

 

The DT25 seems quiet  (almost silent) when in its default Class A configurations, and hums (at a reasonably low level ) when in the Class AB mode.

 

Particularly noisy are the III and IV bias settings.

 

Now I am not saying the hum is really a problem at all, but I bought this amp used and I want to be sure that my newbie ears are not witnessing a problem that the vendor needs to address.

 

So... my question is - Should I experience some lower level humming when the amp is in particular voicings because that's the way that voicing is supposed to be - or should all voicings be silent when they are at 50% Drive / Volume.

 

Thanks in advance for your help


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#2 Brazzy

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Posted 13 March 2014 - 08:06 AM

I see a lot of people say the stock tubes are noisy. I have the DT50 Head and it has a bit of noise on those channels also. I would say it's normal. When I play that idle noise simply gets drowned out. It doesn't bother me so I'm going to use these tubes for a long time or till they crap out, lol.


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#3 stumblinman

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Posted 13 March 2014 - 09:04 AM

Expect some hum. I have a DT25 and there's a small amount of hum. Hum and noise I categorize differently.

Noise is a background hiss that may have crackles or pops as well. That is generally a tube issue or high gain noise that you hear while no notes are being played.

Hum is a by-product of analog electrical circuits and in small but noticeable amounts is typical.

It doesn't seem like what you are experiencing is out of the ordinary.
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#4 geppert

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Posted 13 March 2014 - 11:17 AM

I had what I considered an unacceptable hum in my DT25 with the stock tubes (purchased new). Mine was seriously evident in Pentode mode. It was almost completely alleviated by a set of new tubes and bias adjustment. The new tubes were not the stock EH's. I keep the original EH's as spares, however, if I was buying new again, I'd get a set of 4 matched so that I would have 2 new spares ready to go. Since one of the big factors of these amps is the the "sound" of bending the output tubes, I feel that it is a totally justified expenditure to get tubes that are really good. It is not like you have to replace them every month. There are a lot of replacement tube options (manufacturers) and this has been discussed quite a bit on this forum. One thing I would do is to check the bias and see how close the readings are between the two output tubes (matched pairs should be quite close). If you can't do this yourself then get the vendor to check it for you and re-tweek if necessary to see if this solves the problem?

 

I also want to say that since you are buying this new, you have every right to complain to the vendor if you feel there is too much residual hum coming from the amp. I did with mine and got a replacement head, but, it still provided more hum than I thought it should have. Rather than wait for yet another replacement I just changed the %$# tubes, problem solved.

 

As Stumblinman mentioned, there is a "certain amount" of hum and hiss too be expected from these amps. Where you draw the line is a bit of a grey area. All I know is, that when I am sitting next to my computer at home, I don't want to hear any hum from the amp. That's where I drew my line.


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#5 innovine

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Posted 13 March 2014 - 11:34 PM

I had a lot of hiss with mine, even with low gain amps and the master fully down. I replaced the tubes with jj's and now its deathly quiet... can't hear that it is on until I touch the guitar, and then it kicks in way louder than my acoustic. I've actually left it on by mistake, its so quiet now. Its almost like having a noise gate on, compared to the stock tubes. Never had any hum though, just hiss.
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#6 nikoniablue

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Posted 16 March 2014 - 03:39 AM

There's virtually no hum on my DT25 - I've been pleasantly surprised. That was with the original tubes and a like for like replacement set. Other noise seems no worse than the comparable original amps.  Some of it could be the effect of high gain amps on digitised signals (I think the DtoA in my JTV-59 adds digital hiss) but I note others have had trouble even on low gain where mine is effectively silent.  


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#7 Brazzy

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Posted 16 March 2014 - 06:37 AM

There's virtually no hum on my DT25 - I've been pleasantly surprised. That was with the original tubes and a like for like replacement set. Other noise seems no worse than the comparable original amps.  Some of it could be the effect of high gain amps on digitised signals (I think the DtoA in my JTV-59 adds digital hiss) but I note others have had trouble even on low gain where mine is effectively silent.  

 

Like your DT25 my DT50 has no hum at all. Some low level noise is evident as you go into the higher gain channels which IMO is normal. Im still on the original tubes so I don't know what I'm going to hear until I change them out and rebias.


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#8 innovine

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Posted 16 March 2014 - 07:48 AM

Here is the results of a test. I'm using very new JJ el84s:

 

Blackface Dbl Nrm, class a/b, pentode, Drive 10'o'clock, channel volume at max, master volume at max, nothing connected to the input, no line6 link. I need to put my ear to the grille cloth on the front of the amp just to hear anything at all, and even then its the faintest hiss. With the drive at 12 o clock I can start to hear the hiss, but stepping away from the amp this gets lost in the background noise from computer fan and traffic outside. 

 

Treadplate, class a/b, pentode, master volume maxed, channel volume 50%, drive 9 o clock, nothing connected to the amp. There is noticible hiss from a few feet away. Notice the reduction in channel volume AND drive, and it is still much noisier than the blackface.

 

With just a 1m guitar cord inserted, I can hear a lot more noise on both amp models.


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