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DT25 Volume drop
by Elexorien on 2013-01-23 03:10:12

Hi All,

I was playing at rehersal last night and all was going well. Then out of the blue, the volume of my amp dropped slightly. Not massively, but enough to notice. I'm running my guitar into a POD HD500 via L6 link into the DT25.

At first I thought i'd caught the volume pedal, but this wasn't the case.

Any help is appreciated, and more info can be given if needed!



Re: DT25 Volume drop
by tdownerd on 2013-01-23 03:17:13

I had the same problem....it got worse....packing it up today for warranty repair. 



Re: DT25 Volume drop
by Elexorien on 2013-01-23 04:28:45

How old was your one?    



Re: DT25 Volume drop
by tdownerd on 2013-01-23 04:43:42

Maybe 6 months old!  needless to say, I'm thrilled.



Re: DT25 Volume drop
by sparkyERTW on 2013-01-24 06:12:23

Just a thought before packing it up (you too, tdownerd), but have you checked the tubes?

FYI: http://www.geofex.com/ampdbug/lowpower.htm">http://www.geofex.com/ampdbug/lowpower.htm">http://www.geofex.com/ampdbug/lowpower.htm



Re: DT25 Volume drop
by geppert on 2013-01-24 09:39:36

I second Sparky's motion. First thing I would do before sending any amp back is to replace the power tubes and check bias (very easy to check/adjust bias if you have a voltmeter). I like the Mesa Tubes and they are reasonable. To be 100% sure on the tubes, get a new 12AX7 as well. You always need a spare set of tubes anyways so it is not lost money and it could save you a painful wait for repair, This is all you can do without digging into the guts of the thing which would void your warranty. I had some "crackling" issues that were totally fixed by tube replacement. If new tubes don't fix the problem, take them out, put the old ones back in, and ship er' off.



Re: DT25 Volume drop
by Krontab on 2013-01-24 15:33:53

Agreed the stock tubes = meh. I had the crackle issue as well. I replaced and biased with Mesa EL84's. Pay attention to the color as when it comes time to replace the Mesa tubes you wont need to bias if you get the same color again. I tested the amp with several tubes GT, Sovtek, JJ (Tesla) EL844, and Mesa. Sovtek were okay but a little hard clipping and the bias variance was just in tolerance. The EL844 were hard to bias as they operate at a much lower level and they sounded like mush. The GT (Medium) and Mesa were of similar high quality and start to get warm at about 1 o'clock on the master. I went with the Mesa's due to the color coded bias feature. Biasing the combo requires a little disassembly and time. I would prefer to avoid that in the future. I replaced the the phase inverter with a Mesa SPAX7 (short plate 12AX7) as well. That does not require a bias to replace.



Re: DT25 Volume drop
by sparkyERTW on 2013-01-24 17:49:45

Seems like a common problem, not just with the DT25/50s but with shipped amps in general.  The stock tubes on most sub-$1000 amps are perfectly good tone wise but not the most rugged; that combined with the less-than-tender care they get when being transported tend to shorten their lives.  If I had a nickel for each time I've walked into a music store and tried a perfectly new amp whose tubes were already shot, I'd be a rich man.

I must be a shut-in; I had no idea Mesa put out their own tubes (or rather selected & branded).  I guess they don't get carried at the places I hunt for tubes.



Re: DT25 Volume drop
by Krontab on 2013-01-24 22:51:33

Yeah its one of the selling points on Mesa amps that you do not have to rebias as long as you use their tubes. In fact you cannot rebias them and have to use their tubes.



Re: DT25 Volume drop
by tdownerd on 2013-01-25 19:35:03

I wish I would have read this prior to sending back. They will retube it for me while there should I request a Mesa SPAX7 for the (1) 12AX7, and two Mesa EL-84/6BQ5 Russian Matched pair for the (2) EL84 tubes?

thanks!




Re: DT25 Volume drop
by spaceatl on 2013-01-25 21:13:20

Never hurts to ask...



Re: DT25 Volume drop
by Krontab on 2013-01-26 11:23:03

I should note: The phase inverter is less important to the tone stack. Any quality 12AX7 will do (eg. GT, Mesa, or JJ). The SPAX7 is on the pricey side.



Re: DT25 Volume drop
by spaceatl on 2013-01-26 11:36:12

+1...However, an AX7 inverter tube runs at much higher current than it would as a preamp tube. Typically, it is a good approach to change your inverter at the same time you change your power tubes...

I am a long time Mesa tube fan..However, .a little while back I got several GTSAGAX7 balanced inverters...In comparing those directly to the SPAX7, can't tell much difference those two...But the price is a wash between those two...both are around $20...But I can tell you that I have SPAX7s in my ADA MP-1 that might be 20 years old and are still going strong...



Re: DT25 Volume drop
by Krontab on 2013-01-26 11:53:07

I recently AB'd a JJ vs the SPAX7 and it sounded the same to my ear. I kept the JJ to replace the SPAX7 when it gets crunchy.



Re: DT25 Volume drop
by JTSC777 on 2013-01-28 17:43:59

Mesa tubes are just rebranded by Mesa and manufactured by JJ or Sovtek etc.. Go online and buy tubes from the tubestore or another online company. Always take out and save the stock tubes as spares. They are not good for anything else. Line 6 /Fender/Bogner/Egnater etc....are not going to give you good tubes with a mass produced amp. JJs' are good and very tough in these circuits.Remember EL84s' are designed to run at 100% dissipation.Larger octal based types (6L6-EL34-6550 etc...) run at around 70% when properly biased. EL84s' are designed to have a short life so keep several sets of spares if you are taking these amps out for any higher volume work. I am hoping to grab a used DT25/50 in the near future but when I do I will change the tubes before I use it. Good Luck and Have Fun!



Re: DT25 Volume drop
by spaceatl on 2013-01-28 18:55:25

".Remember EL84s' are designed to run at 100% dissipation."

That one made me throw up in my mouth...



Re: DT25 Volume drop
by sparkyERTW on 2013-01-29 06:14:55

Seconded. I've can't remember the last time I read a post where every sentence made me shake my head.



Re: DT25 Volume drop
by tdownerd on 2013-01-30 02:51:29

Update on mine:  Turns out it was a "worn out tubes" issue.  I guess I didn't think they would go that fast....(but I did purchase a demo model, so who knows how many hours were on it prior to me getting it).

So I've had tube amps before but never had much experience needing to change tubes but any input on how often I should expect to change them in the future.  Amp is played once or twice a week at band volume.

Todd



Re: DT25 Volume drop
by Krontab on 2013-01-30 05:02:34

Mesa may rebrand but they pair them with like biases so if you use a particular color you dont have to rebias on the next swap. The additional testing adds just a few dollars but it is worth not having to pull the amp out of the chassie to rebias.



Re: DT25 Volume drop
by spaceatl on 2013-01-30 05:32:24

That's true of any set of trace matched tubes by any company...Not just Mesa...Trace matching is nothing new...Mesa is using a very old and reliable approach for fixed bias amplifiers....Use fixed resistors...

The problem with skipping a bias check with these amps in thier stock form is that there is a bias pot...bias pots drift over time from gshock and degradation. Especially the crappy little blue pcb types...It's a good idea to check those...

If you really want to make your amp a true fixed bias, measure the value on the pot when it is adjuted proper and replace it with a fixed resistor...



Re: DT25 Volume drop
by sparkyERTW on 2013-01-30 06:25:18

If I recall suggestions say about a year for power tubes (and the recommendation to change the phase inverter while you're at it), but I think that's for 'daily' use, so you might be able to get a few years out of them.




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