DT25 LOUD HUM USING III, CLASS A, PENTODE

[edstar1960]

My setup:

 

JTV59 > VDI > HD500 > L6 LINK > DT25 combo

 

I have had all components for some time now but not always using all together so the DT25 has not been heavily used or gigged much at all.

I have not noticed this before - so thought I would ask on the forum just for a sanity check - it may be normal and I have just not noticed it before.

 

Now - whenever I pick an HD500 pre amp model that uses the DT25 in mode III, CLASS A, PENTODE, such as the AC15 or AC30 models - the unit produces a much louder hum and background hiss than in any of the other combinations or with models that you may think would be noisier such as the Marshall models.  This is with the DRIVE level set well below 50% for a clean sound.

 

I tried placing a noise gate at the beginning and end of my chain with aggressive settings on the HD500 and it made no difference. Even tried turning the MASTER down on the HD500 and no difference.  Lovely loud hum and background hiss.  Switch to other configurations and it's much much quieter.

 

So - is this normal?  Or is my DT25 or my dream rig set up playing up?   Thanks in advance for responses to my sanity check!    :)

[Brazzy]

I have the DT50 and topo III has more noise (not sure about hum) than the others and I think in both A and A/B and this is when I go into the front with my strat. I think it's the same when I L6 the HD500 into it. Now I'm going to have to pay closer attention next time I fire it up, lol.

[edstar1960]

Thanks Brazzy. 

 

My DT25 has a nice re-assuring mains hum - just very slightly there in the background - as many tube (valve) amps have.  With config III, CLASS A and PENTODE, that gets noticeably louder - even when the DRIVE is low for a clean sound - but I hadn't really noticed it before, so I am just trying to get some re-assurance that it's not a problem.  

 

I played an outside gig at the weekend and unknown to us, the generator was playing up, it finally failed on the band immediately after us just after they started their second song, it caused a power surge and took out the main PA, the monitors and then fried their amps and keyboards!  Not pretty - the keyboards were literally smoking!  I found out later that apparently the neutral wire had come loose. Things were odd during our set too - so I wondered whether the generator was already failing and maybe had already had a few power spikes during our set and that it may have caused slight damage to my DT25 resulting in this more noticeable hum in that configuration.  I just want to check whether it is actually normal and that I am just being over sensitive to any noise because of our near miss with disaster at the weekend.

[moogary]

I have a DT50 and the hiss is noticeably louder both with the HD500 and without (even with the master volume dialed down to zero).

[Brazzy]

Thanks Brazzy. 

 

My DT25 has a nice re-assuring mains hum - just very slightly there in the background - as many tube (valve) amps have.  With config III, CLASS A and PENTODE, that gets noticeably louder - even when the DRIVE is low for a clean sound - but I hadn't really noticed it before, so I am just trying to get some re-assurance that it's not a problem.  

 

That generator failure must've been scary man, smoking keyboards and all WOW!!. I'm not an expert and my DT50 Head has never left the house. I've had to replace the power tubes (originals) in this one after approx. 100 or more hours (around 5 months). It has always had a tiny noise floor if you sit right in front of the cabnet, if I'm 10 feet from it I hardly hear it and when I'm playing I don't hear it. I tink it's normal for this amp just my opinion of course. I only have expereince with this amp, a spider Jam, Spider III 75w, some smaller amps, and amp modelers. When this amp was new and I switched to Topo III, I heard an increase in noise but quickley ignored it and started playing and tweaking. While playing it in Topo III, it sounded good so I just chalked it off as normal for this amp configuation.

[edstar1960]

That generator failure must've been scary man, smoking keyboards and all WOW!!. 

 

Sure was! Strong smell of electrical burning too!  It was in a marquee and I was at the front just about 10 feet from the stage - fortunately nothing actually ignited - but the bands gear was toast!    :(

[edstar1960]

Thanks for the replies - seems like Topology III, CLASS A, PENTODE, is just the noisiest setting.

[geppert]

I had a hum issue in my DT25 in Class A  "Triode" Mode. To make a long story short, replacing the tubes and re-biasing reduced the problem by about 80% which was more than tolerable for me. The new tubes were also matched closer than the originals so that might also have had something to do with it?

[Brazzy]

Come to think of it I may have improved the noise floor a bit when I replaced and biased the power tubes. Maybe the tubes got rattled a bunch during the 3000 mile trek cross country. I read somewhere the tubes are vibration sensitive which is one of the reasons I wanted the head.