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DT25HD/DT25112 speaker hum
by Smart1960 on 2012-03-09 11:06:17.7260


Could someone please help with the following problem? I need to know if this is serious enough to return the head.

I’ve had my DT25HD and DT25112 cab for just over a week. All has been well and the amp has performed brilliantly until I powered it up this morning and there was a very noticeable hum coming from the speaker.

The amp was set up as follows: Channel A – Triode – Voice Switching I – Class A/B – Master and Volume at hallway position.

I spent an hour changing the setup in various ways but came up with the following problem combinations:

  • Whenever the amp is in Triode mode there is a very noticeable hum. Changing to Pentode kills the hum completely.

  • With Class A and Triode set the hum gets much much louder.  Again changing back to Pentode kills the hum completely.

I’ve owned tube amps for twenty years and recognise the sounds that you can expect and this is definitely not normal. Closest experience I can come up with was a problem with the power tube on a Fender amp I owned. The noise I’m experiencing on the DT25 is similar but louder.

Any help would be appreciated.

Thanks in advance.


Extra note -changing volume or master does not change the level of hum and it is present with or without a guitar.

Re: DT25HD/DT25112 speaker hum
by gtrman100 on 2012-03-09 12:08:55.6090

Hum in Class A mode is totally normal- all the DT amps have some hum in that mode. It's just a function of the circuit design. But it sounds like you might have a problem with the DT head if the hum has increased. You might have a bad output tube EL84. Since you have two of the same amps, why not try taking the tubes out of the combo and put them into the head and see if the hum is less?

Chinese tubes are notoriously unreliable and tend to fail in the first hours of use. That may have happened to you. If there is a bad tube, it will be covered under warranty. Good luck!

Re: DT25HD/DT25112 speaker hum
by STPLE on 2012-11-16 21:26:29.0940

Hey Smart1960 did changing the tubes work??  I have the exact same issue.  I just bought a DT25 head and cabinet (used).  It is great condition and plays very nicely.  I noticed the exact same hum in triode and worse in class a.  It goes away in pentode - except for normal buzz/hum of a tube amp.  The hum in triode is not normal or shouldn't be.  How did you fix yours??  Changing the tubes and rebias work??  Thanks.

Re: DT25HD/DT25112 speaker hum
by geppert on 2012-11-16 23:06:47.9540

Smart1960 and STPLE ... see my related recent reply to this topic on the thread below


The information above may not be current, and you should direct questions to the current forum or review the manual.