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Dt25 Tube Upgrade: Quick Review Of Jj's Tube Upgrade


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#1 boyce89976

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Posted 04 January 2014 - 09:15 PM

I had always felt my DT could sound better.  The clean highs were a little thin, the midrange lacked body and punch and the lows were a little spongy.  So I swapped out the tubes on my DT25 last night with a set of matched JJ EL84s and a balanced ECC83S (a direct replacement for the 12AX7) and performed a re-bias on the new tubes.

 

(This is a very dangerous operation, and should not be done by anyone who lacks the knowledge and experience of re-tubing and re-biasing an amp.  If you choose to do this, you do so at your own risk).  

 

This is just an initial review as I have bronchitis and was running a 102* fever as I was putting the amp back together.  Not feeling good at all.  Anyhoo, below are my initial thoughts after about 15 minutes at moderate volume (just loud enough to get the tubes warm).  

 

First thing I noticed is that both tubes glowed with the same intensity.  With the stock tubes, V3 glowed significantly hotter than V2, which probably indicates their bias voltages were significantly different.  

 

Sounds on some of my clean patches (in Topology I, Pentode and Class A/B mode, but with various amps - PHD, Blackface, AC30):

  • Low end is definitely tighter with a more defined pick attack and better thump and compression
  • High end is cleaner, and meatier with better extension - definitely more of that "jangle" EL84s are known for
  • Mids have more body 
  • Sustain is clearly longer than before and there is more clean headroom... harder to drive the amp into breakup through aggressive picking, at least at moderate to moderately loud volume levels

On overdriven tones (same settings, but mainly with the PHD Motorway and an AC30 thrown in):

  • More "air" in split coil settings (a recent mod to the bridge pickup of my Charvel Pro Mod)
  • Fat, bluesy tones through the SH1-59 neck pickup, with plenty of pick attack and more sustain than before
  • Better low-end definition from the bridge humbucker, while also being meatier with a subtle, but crunchy, staccato pick attack (This is the closest I've gotten to the sound of Warren DiMartini's guitar sound on the opening riff of "Lay it down".  The low "D" was clear, tight and thumped like I always thought it should).  
  • Through my Strat with the N3 Noiseless pickups, in position 2 (without the S1 switch engaged) the cleans had the body I always wanted from this setting, while still retaining that high-end sparkle Strats are known for.

Overall, I am VERY pleased with the JJ's over the stock EH's and Chinese 12AX7.  Especially since they biased to within 1.4mV of each other - I definitely recommend the change, and recommend matched sets since there is only one bias adjustment pot.  I also recommend the balanced-triode ECC83S for the phase inverter position (in place of the 12AX7).    

 

Again, I do not recommend performing this kind of work if you have never done it before or aren't completely comfortable with partially taking your amp apart, replacing important parts and putting your hands into an amp that is pulling current... If you have any reservations at all, it's best to pay an amp tech the $100 bucks instead of risking your life.  If you do try this, you do so AT YOUR OWN RISK!  


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#2 Metalchef

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Posted 04 January 2014 - 09:26 PM

Thank u sir for guinea pigging this..... I appreciate it have replacements for my DT 50 on order... Good form....
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#3 boyce89976

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Posted 04 January 2014 - 09:40 PM

Seeing as the DT50 actually has preamp tubes, you may hear a bigger difference than I did in my DT25.  Another good description I thought about today is that the highs really opened up.  With the EH's I always felt like there was a ceiling in the frequency response, where things got brittle and a little harsh, whereas with the JJ's, in early testing, it sounds like the tube is capable of higher frequency response.

 

I'll be able to crank the volume a little bit tomorrow morning while the wife and kids are at Church, and will report back with any additional findings.     


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#4 TheRealZap

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Posted 05 January 2014 - 06:32 AM

the DT50 does not have preamp tubes...

it has an inverter 12ax7 like the dt25

and a boost 12ax7 which the dt25 does solid state...

all pre-amps are entirely modeled.

 

Seeing as the DT50 actually has preamp tubes, you may hear a bigger difference than I did in my DT25.  Another good description I thought about today is that the highs really opened up.  With the EH's I always felt like there was a ceiling in the frequency response, where things got brittle and a little harsh, whereas with the JJ's, in early testing, it sounds like the tube is capable of higher frequency response.

 

I'll be able to crank the volume a little bit tomorrow morning while the wife and kids are at Church, and will report back with any additional findings.     


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#5 boyce89976

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Posted 05 January 2014 - 08:25 AM

the DT50 does not have preamp tubes...

it has an inverter 12ax7 like the dt25

and a boost 12ax7 which the dt25 does solid state...

all pre-amps are entirely modeled.

 

Ah, I was under the impression the boost tube was a preamp tube, my mistake.  Thanks for clarifying!


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#6 boyce89976

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Posted 05 January 2014 - 09:07 AM

Okay, Just had a chance to crank it up and my review above holds true... better high-end, fuller mids, especially the lower mid-range/upper bass where you FEEL the punch.  The stock tubes never achieved that "feel it in your chest" kind of thump, but the JJ's do.  

 

My Strat sounded amazing at higher volumes.  The high-end is pristine, without any of the harshness I had before with the stock EH's.  I was running the DT master at 3 o'clock and the POD master at wide open, and my cleans remained clean.  The stock tubes would start to breakup, and get harsh, at those volume levels.  From a volume perspective, I would say it was plenty loud to play a rehearsal with an acoustic drum set.  I don't have my db meter handy, but I'll try to get an objective measurement.

 

Another interesting, and positive side effect, is a patch I built to mimic EVH's tone on 1984, particularly Panama, using the PHD Motorway.  I had to EQ in a lot of high frequency gain with the old tubes to get his warm, cleanish overdriven tone.  With the JJ's I may have to back the HF gain down a little, due to their better reproduction of the high frequencies.  It's not harsh, or overbearing, it just not as accurate an EQ setting as it was with the old tubes... again, that's a good thing! 

 

Oh, and sustain is definitely better... I would have to say it's at least 50% better.  I think this is attributed to the balanced and matched phase inverter better maintaining a consistent signal balance to the EL84's.  


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#7 boyce89976

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Posted 05 January 2014 - 09:15 AM

Just took a db meter reading from 4 feet, C Weighted, Slow Response (RadioShack Digital db meter):

  • Overdriven Channels:                     Avg 110db                     
  • Clean Channel:                               Avg 107db                     

 

I wouldn't want it any louder than that!  


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#8 Metalchef

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Posted 05 January 2014 - 05:00 PM

Awesome... Can't wait.... Yes Real Zap u are correct..... Now here in lies a question that both I and the service rep from where I ordered were stumped at..... How quickly will those 12AX7 wear out if they are not being used as preamp tubes.? I have ordered replacements but still knowing is half the battle... My AFD 100 uses up Pre's at about 1 set of pre's every 4-5 Power Amp Tube changes.
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#9 boyce89976

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Posted 05 January 2014 - 05:48 PM

Eurotubes told me the phase inverter tube should last between 2-4 times longer than the powertubes.  I ordered a matched quad of EL84s and one ECC83S, figuring I'd keep the ECC83S through the next EL84 change.   


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#10 Kcharriq

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Posted 05 January 2014 - 06:02 PM

I just replaced my original tubes with JJs. I took a reading before I pulled the original tubes V2=28.3 V3=32.8

 

I tested the tubes on my Orange VT1000 tester and the EL84s both showed good and matched. The 12AX7 showed worn, so I replaced it too.

 

This solved the problem with my amp crackling all the time.

 

I think the JJs sound much better, deeper bass and more clear.

 

Also a tip to get the amp out of the box..After removing the screws from the bottom take a paint spatula (putty knife)  and get between the case and the bottom of the amp. There is a metal plate covering the bottom of the amp and it sticks to the tolex. You can work it loose and the amp will slide out.


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#11 chrishlp

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Posted 13 February 2014 - 02:11 PM

Hi, I recently swapped out the 12AX7 in my DT25 on the basis that it was the 'weakest link' in the 3 valves.  As this is a phase inverter I spoke to HotRox Valves (uk) about what might be the best replacement and they recommended the TungSol Reissue valve, which has an option to select a valve for the PI position. 

 

After installing the new valve the most immediate benefit was that the amp hum I was getting (not speaker but amp) disappeared.  As to the impact on tone?  The bottom end tightened up, the tope end smoothed out, the dynamics increased and the overdrive sounded more natural.  These changes (except for the hum) were subtle rather than extreme which means the stock Chinese valve is not too bad; however the TunSol is a worthwhile upgrade.

 

As to changing the power valves, I'll think about this at some stage but for now I'm enjoying the DT25/HD500 combo as is!


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#12 edstar1960

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Posted 21 February 2014 - 09:08 AM

Hi, I recently swapped out the 12AX7 in my DT25 on the basis that it was the 'weakest link' in the 3 valves.  As this is a phase inverter I spoke to HotRox Valves (uk) about what might be the best replacement and they recommended the TungSol Reissue valve, which has an option to select a valve for the PI position. 

 

After installing the new valve the most immediate benefit was that the amp hum I was getting (not speaker but amp) disappeared.  As to the impact on tone?  The bottom end tightened up, the tope end smoothed out, the dynamics increased and the overdrive sounded more natural.  These changes (except for the hum) were subtle rather than extreme which means the stock Chinese valve is not too bad; however the TunSol is a worthwhile upgrade.

 

As to changing the power valves, I'll think about this at some stage but for now I'm enjoying the DT25/HD500 combo as is!

 

Is the replacement of the 12AX7 simple?  Or does it also need to be re-biased and carried out by someone who knows what they are doing?    Thanks.


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#13 innovine

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Posted 21 February 2014 - 09:22 AM

I just put a pair of jj el84s into my dt25. The hissy constant noise floor is totally gone. Otherwise sounds much the same, but I've yet to turn it up toeany appreciable volume
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#14 boyce89976

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Posted 22 February 2014 - 10:25 PM

Is the replacement of the 12AX7 simple?  Or does it also need to be re-biased and carried out by someone who knows what they are doing?    Thanks.

 

The 12AX7 is very easy to replace, and no biasing needed.  Just make sure you are VERY careful when doing it... there is current in the amp (even after it's been unplugged) that can kill you.  


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#15 innovine

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Posted 23 February 2014 - 07:01 AM

The jj el84s are supposed to make a bright flash when turned on, right?

Are they also much louder than the stock?
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#16 boyce89976

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Posted 23 February 2014 - 08:32 AM

The jj el84s are supposed to make a bright flash when turned on, right?
Are they also much louder than the stock?


They don't flash that I've noticed, but the do glow brighter than the stock tubes. I'm not sure they are louder if biased properly, but the are clearer and less harsh, so I can get more volume and still have definition.
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#17 innovine

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Posted 23 February 2014 - 02:48 PM

Ok, I turned the amp on just now, and when I went from standby to on, I immediately noticed something was up. Even with the master down there was a lot of noise and crackles. The tubes were also very bright. When I flipped the switch to Class A, the whole side of V3 started glowing cherry red. 

Bad tube? I put my stock tubes back in (1 year, lightly used) and they are MUCH quieter than the JJs, but luckily the amp is still working fine.

Should I return the JJs? I used them for a total of 20mins yesterday, 15 seconds today.


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#18 boyce89976

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Posted 23 February 2014 - 05:38 PM

Ok, I turned the amp on just now, and when I went from standby to on, I immediately noticed something was up. Even with the master down there was a lot of noise and crackles. The tubes were also very bright. When I flipped the switch to Class A, the whole side of V3 started glowing cherry red. 

Bad tube? I put my stock tubes back in (1 year, lightly used) and they are MUCH quieter than the JJs, but luckily the amp is still working fine.

Should I return the JJs? I used them for a total of 20mins yesterday, 15 seconds today.

 

Dude, that sucks!  Tubes usually have a 30 day warranty at least.  Hopefully just a bad tube.  I've use all 4 of the ones I got from Eurotubes (just to make sure they all worked and were bias matched) and haven't had any issues.  


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#19 chrishlp

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Posted 24 February 2014 - 04:11 AM

Hi edstar1960,

 

There is no need (or ability) to rebias the 12AX7 tube, so you only have to replace the tube.  Make sure your amp has been off for some time (I did mine in the morning after the amp had been off all night and the power tuned off at the mains). Make sure you don't touch anything other than the tube. 

 

Remove the stock and insert the new tube using a clean soft cloth so you don't get finger marks on the glass. 

 

Hope this helps.


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#20 Brazzy

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Posted 24 February 2014 - 10:25 AM

Sometimes you just get a bad tube sometimes you get'em and they "Last Long Time".


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