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No tube breakup from the power amp?
by Martijndrift on 2012-06-19 02:52:41

Hello everyone,

I bought a dt25 to go with my pod hd a few months ago, I thought it sounded pretty good.
Lately though I start to dislike the tones more and more. For some reason it seams like all the power amp is doing is making things louder and harsher. I especially notice this in topology II. When you turn the master volume up on other marshall style amps they get this really smooth distortion and sag. My dt 25 though just sounds more in your face and harsh when I turn it up (comparable to a solid state amp), I also don't notice any more distortion than at lower volumes. I was also getting popping noises every now and then, and I was receiving russian radio in topology 4 (I live in holland)

Hoping this was a tube problem, I replaced the tubes with a matched set of sovtek el84 (the same as standard). I also changed the phase inverter tube just to be sure (EH 12ax7).

Although the popping went away, and it started to pickup less noise, it still sounds incredibly harsh and boxy at higher volumes, there just doesn't seem to be any warm power tube distortion.

When I take topology I for example and I turn both the volume and master up to 10, shouldn't there be a bit of distortion or should it still be as clean as running the master volume on 2?

When I run it in class A it does seem to get a slight bit of power tube distortion at higher volume, although it still sounds harsh and undefined, it can be nowhere as warm and still sparkly as for example a real vox ac30. This kind of rules out the tube bias being off, (although I swear the tubes in other tube amps send out alot more light)

All in all I just have the feeling like the power tubes just aren't doing anything except making things louder and brighter, even at lower volumes my ears start to fatigue in 20mins. Does anyone have an Idea what this problem could be, or is it just me not liking the sound of the amp anymore?

Thanks for your help!

Martijn



Re: No tube breakup from the power amp?
by Crusty_Old_Rocker on 2012-06-19 05:29:44

Okay, you need to check the bias on the power amp valves.  The amp when run in class A mode is cathode biased, so you won't experience the bad or brittle tone that can come from a bias that's too hot or too cold.  You can't rule out the bias without checking it.  The light coming from the valves comes from the heaters, the heaters aren't affected by the bias, they run on 6.3 volts AC straight from the power transformer.

If you want the power amp valves to break up more, I would suggest you get in touch with Bob from Eurotubes.  When ordering EL84s you can specify an early break up on the valves and Bob will deliver.  I still have my stock EL84s in my DT25s but in my home brew class A amps, I am running JJs that I got from Eurotubes that are graded for early break up and I love them.  My 30 Watt class A gets some real power amp goodness happening at lower volumes, which is great.  Without those valves, the amp would be deafening before the EL84s would break up.

So, get yourself some early breakup graded JJs from Eurotubes and don't just put them into the amp, you need to have the bias adjusted otherwise when in class A/B the amp isn't going to sound good at all.

Cheers,


Crusty



Re: No tube breakup from the power amp?
by Martijndrift on 2012-06-19 05:49:31

Thank you! I'll check the bias when I get home this weekend, if it's correctly biased, I"ll check out some eurotubes!



Re: No tube breakup from the power amp?
by Crusty_Old_Rocker on 2012-06-19 05:55:22

Let us know how it goes.

Cheers,

Crusty



Re: No tube breakup from the power amp?
by MGblade on 2012-06-19 11:29:01

Ha Martijn

ik heb ook de dt25 met hd500, mijn ervaring is precies het tegenovergestelde, hij klinkt zowiezo erg goed, maar hoe harder hij gaat des te dikker, beter hij gaat klinken.

maar dat blijft subjectief natuurlijk

ik krijg ook vrij snel al eindbuis oversturing met crunchy presets, ik heb er JJ,s in zitten.

het zou idd iets met je bias afstelling kunnen zijn, lijkt mij dat ze iets te koud afgesteld zijn.

groet Marcel



Re: No tube breakup from the power amp?
by edstar1960 on 2012-06-22 01:16:38

Martijndrift wrote:

Thank you! I'll check the bias when I get home this weekend, if it's correctly biased, I"ll check out some eurotubes!

How do you check the bias for the tubes on a DT25?  And how can you adjust if necessary?  Thanks.



Re: No tube breakup from the power amp?
by Crusty_Old_Rocker on 2012-06-22 04:48:47

Hi Edstar,

If you don't know how to check the bias and how to adjust it, then it's probably best to have it done by a professional.

If you do know then the bias spec for the DT25 EL84s is 25 mV +/- 3 mV, just remember that the amp has to be in class A/B mode when adjusting the bias.

Cheers,

Crusty



Re: No tube breakup from the power amp?
by edstar1960 on 2012-06-22 07:22:54

Thank you Crusty! A professional it is then.  So I will have to find someone, if and when the tubes need to be replaced.  My DT25 is only 1 day old so hopefully it should be good for a while yet!



Re: No tube breakup from the power amp?
by daferalo on 2012-06-22 07:40:16

Hi Edstar,

This thread might be helpful for you http://line6.com/support/message/374450#374450. It is not hard to do. However, you must be carefull as there is a real risk for human healthy, and for the amp itself.

Best Regards,

Daf



Re: No tube breakup from the power amp?
by edstar1960 on 2012-06-22 07:55:01

Thaks Daf! 



Re: No tube breakup from the power amp?
by Martijndrift on 2012-06-22 10:17:37

Ok guys, I just checked the bias, it was at 22 volts which is a bit on the low side, so I changed it to 26, this already gave quite abit more warmth.
However I also ordered some jj el84, put those in, checked bias (which happened to be correct) and gave those a go.

I must say it's a pretty big difference especially when you crank them. The highs sound softer, the bass is less flubby and the overal tone is just warmer!

Thanks for your advice!

Martijn




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