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Curing the DT25 noise problem - 12AX7 or EL84s?
by Pezza on 2013-02-12 05:42:14

I have had the DT25 head for just over a year now and have started to get the infamous 'wind blowing across a mic' noise. I have a few questions:

1. Have people found that it was the 12AX7 phase inverter causing the problem or was it the EL84 power tubes?

2. If it is the 12AX7, can I just swap it without any adjustment/biasing etc?

3. Which 12AX7s do you recommend? I gather that it won't have a huge impact on tone as it's not a preamp/poweramp tube.

I'm not familiar with replacing tubes so I'm hoping that a simple swap of the 12AX7 will cure the problem!

Thanks



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by tormod1 on 2013-02-12 21:22:08

I have the exact same problem, so I really hope someone has got the answer to this..



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by MGblade on 2013-02-13 05:40:48

`12ax7 can be replaced without biassing,

i would replace it with the same model/brand

you can check the power tubes with the pencil method, tap the tubes with a graphite pencil,

if they make noise while doing this through the speakers it,s a sign that the tube is worn, and need to be replaced.

the power tubes need to be biased and hat is fairly easy on the dt25 head.

grts Marcel



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by Krontab on 2013-02-13 16:59:11

Had the windy mic issue as well. Not only did I have to replace the tubes I had to install a power conditioner. Line 6 support helped me identify the power supply issue.



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by tormod1 on 2013-02-15 04:29:24

Well, tried to swap the 12AX7. Did nothing to solve the problem at all...

Will try to swap the EL84 power tubes as next step..



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by theoptimizers on 2013-02-15 07:40:16

Also, you might want to try an AC power line conditioner.



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by geppert on 2013-02-15 09:02:37

I am willing to bet it is the power tubes and I also 2nd the motion for a power conditioner. Ever since Spaceati mentioned it, I bought the "Triplite LC1200" (same as he uses). It runs all my computer stuff (and gtr amps) at home and I take it out for practice and gigs. Nada had a problem with the amp (DT25 Head/Cab). Have changed all the tubes though. Did have some noise at one point. Definitely was due to the original E.H. EL84's that were the culprit. Also had "noticeable hum" in Class A & Triode mode, again positively attributed it to the EL84's. Replaced with Mesa. Very happy and quiet now. Don't forget to adjust bias if you replace tubes. Very easy to do with a multimeter but requires removal of chassis to get at test points. Caution is always advised however. If nervous about doing it yourself it is a 30 minute job for a tech. Maybe less if he s really fast.



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by spaceatl on 2013-02-15 12:07:36

+1...I like the SPAX7 or GT-SAGAX7 for inverters...

I cannot recomend the Tripp Lite AVR enough...price is really reasonable I think...

One thing a lot of folks do not realize is that using an AVR (transformer type) eliminates ground loops...why? It's isolated power...the transformer removes the potential of any voltage on the ground from the house side...Basically, my rigs have never had hum of any kind in any venue I have been in...I think you have to be gentle with these hybid amps...They are basically embedded computers with high voltage tube power amps...the less voltage swings and no voltage potential on ground is a good thing I think...



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by geppert on 2013-02-16 08:58:58

I agree with Space on the isolation thing. It is the hidden (bonus) factor when you use an AVR. I have no hum or ground loops either. For the $ invested into these amps, I think the AVR is very worth while insurance. The concept of "embedded computers with high voltage power amps" is a good perspective to keep in mind and brings relevence to treating them accordingly. As I mentioned before, I use the same AVR on all my puder and amps when at home as I get a lot of power outages which of course includes spikes on and off. I also see line voltage swings quite often which the AVR corrects of course. With these amps especially, I wouldn't be without one. I got mine for a decent price with some surfing on the internet.



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by tormod1 on 2013-02-20 01:17:39

Two new EL84's did the trick. Dead quiet!!



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by bmoyer on 2013-02-21 08:01:37

tormod1, I'm curious.  Did you re-bias after the new EL84's, and what brand did you buy?  Nothing like solving your own problems



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by tormod1 on 2013-02-21 10:02:19

I bought Electro Harmonix, both the EL84's and the 12AV7. Did not re-bias, mostly because I don't know how to:-).



Re: Curing the DT25 noise problem - 12AX7 or EL84s?
by bmoyer on 2013-02-21 10:37:16

Hey, this is good to know.  I had my DT25 112 go noisy on me.  After ordering the same tubes in an effort to solve the problem I finally finagled my way into getting the amp replaced.  So far so good, but I'll keep the tubes for spares.




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