Replacement Tubes Recommendations

[STPLE]

I've had my DT25 for over a year now.  Most likely within the next few months to a year I'll need to replace the tubes.  While I have time to research and figure out what may be the best ones to try I thought I was ask other forum members.

 

I looked all over last night for recommendations for replacement EL84s but I can't find a thread.  I remember the old forums had a few (at least I thought).

 

Any recommendations for those who replaced their tubes would be greatly appreciated.  Even recommendation for the 12XA7 would be nice (I'm still not even sure what the 12XA7 does in the DT25).  My understanding is the 12XA7 will last a lot longer than the EL84s.

 

I play at church where we have a great variety of styles and tones.  Anywhere from very clean (chime) delays to overdriven humbucker distorted tones.  I use a lot of different models but the Blackface Lux Vib, J800 & PHD Motorway are my "go to" models.  I have used about 5 other models for various songs but those are the main ones.  I use all pre models.  I use the L6Link.  I run the master on the DT25 at 2 or 3 o'clock ever time I play (with an attenuator). 

 

I like the way it sounds now but just wondering if anyone has any thoughts or opinions on replacement tubes.  I'm not opposed to buying a new set of matched tubes Sovteks (what is in there now) but always open to change.  I know its all subjective and seasoned to personal taste but I would greatly appreciate the feedback.

thanks.

[boyce89976]

Hi STPLE, 

I did a ton of research on this over the past 30 days.  Most of the current production tubes are made by only a couple of companies, and many seem to be of questionable quality, with a few bright spots (Genelex - which are now made by Electro-harmonix).  

 

Based on my research, I just ordered 4 matched JJ EL84's and a balanced and matched JJ 12AX7.  According to Micheal at Eurotubes, the JJ's should give a cleaner more bell-like high end, a harmonically rich mid, and a tight, deep low end compared to the stock Electro-harmonix EL84's.  

 

I should have them later this week and will report the findings.

[STPLE]

That would be awesome.  From the little bit I read up on the various types of tubes - The JJ ones seemed to be the most intriguing.  Thanks.

[geppert]

FYI … I put a matched pair of Mesa EL84's in my DT25 and I quite like them better than the EH that came with the amp. I also checked and tweaked the bias. Very easy to do but requires removal of the Chassis, a multimeter, and a little know how. It can be dangerous if you totally don't know what you're doing in which case get a tech to do it.  JJ's also have a good reputation and you probably can't go wrong with them. I may give them a try next time? Keep your old tubes for emergency spares.

[STPLE]

Cool. In what ways do you like them better?? Did you also replace the 12AX7?

 

Has anyone tried the Preferred Series 7189 (Premium)??

[ice9mike]

Hi STPLE, 

I did a ton of research on this over the past 30 days.  Most of the current production tubes are made by only a couple of companies, and many seem to be of questionable quality, with a few bright spots (Genelex - which are now made by Electro-harmonix).  

 

Based on my research, I just ordered 4 matched JJ EL84's and a balanced and matched JJ 12AX7.  According to Micheal at Eurotubes, the JJ's should give a cleaner more bell-like high end, a harmonically rich mid, and a tight, deep low end compared to the stock Electro-harmonix EL84's.  

 

I should have them later this week and will report the findings.

 

I replaced both EL84's and 12AX7 just as you did and ordered from http://www.eurotubes.com/. They are extremely helpful and actually measure each tube to match them and ensure they are balanced. You pay a little extra for that, but it's worth it.

 

My opinion of the new JJ tubes is that they produce a cleaner sound when pushed hard. They give a little more headroom. The AC30 is more like you would expect from a real AC30. Very nice quality.

 

They also sell an EL844 that lowers that output volume of the power section and causes them to breakup earlier. I have not tried these, but they might be worth getting a pair. You'll have to rebias the tubes to account for the lower power output. I would love to hear any opinions from anyone who has tried these tubes in their DT25.

[STPLE]

Thanks.  Sounds like I may try those out and keep the stock tubes as a back up like geppert suggested. Did you change the preamp tube as well or just the power tubes?  Is your amp the combo or just the head??

 

I opened the back last night for the first time.  I noticed the preamp tube was a little loose - probably from lugging it back and forth to rehearsals and such.  I have the DT25 head (not the combo).  I've changed tubes before in amps but never have had to adjust the bias (I have a Peavey Classic 30 with a 112E cab).  Looking in at the DT25 head for the first time - it seems as though if you had a 9 pin bias probe (or two) and small Phillips head screw driver you would not have to remove the chassis.  It seems you could test the bias with the probes as long as the probes aren't too tall so as to bump into the top of the amp.  Seems safer if the chassis is not out on the table - still will probably take it to a tech.

 

Thanks again.

[boyce89976]

Well, it seems my tubes have taken a detour on their way from Eurotubes in Portland.  I was hoping to get them today, but it's likely they won't be here until Saturday, or maybe early next week.  

 

The JJ's from Eurotubes are pretty cheap... and you can buy matched pairs, quads or even in sets of eight.  I bought quads for a couple of reasons:

1.  I have a backup set that are the same plate voltage, so no re-biasing should I need to use one or both of the back ups

2.  I have a backup set for when the first set starts to age, and they'll sound just like the first two.  

 

The four EL84's and the ECC83S (12AX7) were just over $61 plus shipping.  Pretty cheap.  

[boyce89976]

Hooray!  Got the tubes this afternoon, but unfortunately, I'm under the weather, and running a fever so was only able to get them installed, bias the amp and play for about 15 minutes.  

 

*  First of all, even though I am a self proclaimed do-it-yourself-er, this is a technical process involving a lot of current/amps/voltage.  I describe the process as being fairly easy, but I WAS nervous.  Make no mistake, any time you reach into the back of an amp under power, you are AT RISK of injury or death, and I am not responsible for you if you decide to try this.  If you are not completely comfortable with the process DO NOT DO IT.  Your life is worth much more than the $100 or so a tech would charge to do it for you.  

 

A couple of observations: 

1. Should have checked the bias setting for the old tubes (just out of curiosity)
2. I wasted a bunch of time trying to remove the amp from the chassis, unsuccessfully.  In hindsight, this is not necessary.  
3. Wish I'd bought a few more of these JJ's tubes so I would have more than one matched backup set

Once I decided to try adjusting the bias with the amp still in the chassis, it was literally a 10 minute job.  The bias contacts and the bias pot are all on the underside of the amp next to the tube sockets, and, for the most part, easily reachable once the two tolex back panels are removed.  The bias pot is a little difficult to get to due to the speaker magnet, but a small phillips-head solved that, even though it kept getting stuck on the magnet.  

 

Below was my process:

 

I layed the amp over on it's face and unplugged everything.  Once I had the tolex back panels removed (and after giving up on removing the amp from the chassis) I used a latex glove to grip and pull the old tubes, and insert the new tubes, as I didn't want to get finger oils on the glass... not sure this is necessary, but...

 

Once the three new tubes were in (1 ECC83S in the phase inverter position, 2 EL84's in the power positions) I made sure they were fully seated a couple of times, and then plugged the speaker in, and a guitar and turned on the power.  I also made sure it was set to Class A/B and the volume was about halfway up on each of the gain stages (Drive, Channel Volume and Master Volume).

 

V3 lights up first (I guess because it's first in line from the power supplies), then V2 and V1.  

 

I sat there for a few minutes while it warmed up in standby, and used the opportunity to power up my multimeter and set it to DC Volts in the mV scale.  Then I sat there for a few more minutes with a flashlight eyeballing the bias contact points, thinking about the old game "Operation" wondering if I could get the spare rib, broken heart and brain freeze out without getting "buzzed"!  

 

After deciding there was really nothing underneath the chassis to be concerned about (not a good assumption as there is a massive power supply right next to V3).  I inserted the black probe in the hole labeled "ground" and the red probe in the hole labeled "bias" for V2..... Nothing.  No reading... must have missed the contact points... after several more attempts, and getting a little frustrated, I wiggled the black probe just right, and found an indention in the bias contact.  Found the same indention with the red probe, and viola... 16.7mV... wait, it's supposed to be 25mV +/- 3.  So I moved the red probe over to the V3 bias contact, and made the same measurement... 18.2mV...

 

Now you see why I was wondering what the old tubes were biased at... if they were that cold, it's great for tube life, but not for tone, and could explain why my highs sounded brittle, the lows were muddy and breakup was a little harsh, with the old tubes.  

 

I pulled the probes and inserted the phillips-head into the bias adjustment pot and gave it a VERY slight turn to the right... seriously, maybe only an 1/8 of a turn at most... re-checked the bias on V2... 34.2mV... whoops, too much... readjusted the bias and checked again... 24.8mV on V2... okay, that's good, checked V3 and it was 26.2mV... pulled the probes, wondering what I'd done wrong (ha ha), and then rechecked it again... same reading.    

 

Powered everything down, pulled the power cord and guitar cable, re-attached the two tolex pieces, sat the amp upright, and moved it back to it's usual resting place.  Reconnected everything and powered it back up with a guitar plugged into the front input (I don't usually use the amp this way, it was just a test to make sure I had sound).  I did!  Played for a few minutes on "American Clean" and "Brit Crunch".  Sounds good, but since I don't usually play the amp like that I couldn't compare to the way the amp sounded before.  

 

Plugged into my POD HD, and reconnected the L6 Link and played through some familiar patches.  Here are a few observations after 15 mins of playing:

 

On cleans (in Topology I, Pentode and Class A/B mode, but with various amps):

• 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):

• 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 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 - that's pretty good matching!  

 

Again, I do not recommend this if you have never done it before or aren't completely comfortable with partially taking your amp apart, and replacing parts and reaching your hands in... If you try this, you do so AT YOUR OWN RISK!  

[STPLE]

Hey that sounds good.  I am going to order a matched set along with a Balanced 12AX7.  I'm excited about trying them out.  Thanks for the review.  I'm going to bias my old tubes (from your advice) before I change them out just to see where it was running at now.

 

I'm making the assumption that the one should get the balanced 12ax7. 

 

I may try out the preferred series (premium el84) tubes someday as well.

 

thanks again.

[STPLE]

Ok not sure if anyone is still reading this discussion but I thought I would go ahead and share my experience/observations.  I will try not to repeat anything previously stated in this discussion - just be careful if you do this.

 

After a ton of research, talking with people and the forums (old forums as well) I decided to try to bias the amp myself.  I have a DT25 Head and Cab.  I probably would not have tried if I had the combo amp.  After taking the back off, I figured I can do it without taking the amp out of the chassis (if I had the right equipment).

 

After research I figured the best to bias this amp is with a dual bias probe & meter.  I bought one from www.asharpfretworks.com.  I'm very very pleased - it is a great piece of equipment (I bought the Quadstage BiasPro Q10 with the BiasPro 9C-S - 9 pin probes).  IMHO this is PERFECT for the DT25 head.  Briefly, it has a 1 ohm shunt resister (mA = mV) and the probes fit easily in the DT25 head (without taking it out).  Just pop the bias probes in and put the tubes in and you're all set (see attached picture).

 

I bought a matched set of EL84s (JJs) and a balanced 12AX7 (JJ) from www.eurotubes.com.  Thank you forum members.  Again very very good.

 

Since it was so easy to test I decided to have a bit of fun.  I tested the stock tubes, stock tubes with the new 12AX7 and then all new JJ tubes.

 

Stock Tubes - V3 = 24.4 mV while the V2 = 22.5 mV.  Note:  The reading was about 1mV lower right at first but I let the tubes cook and warm up for about 10 minutes.  Then they stayed pretty steady.  (I love the dual bias probe b/c I can instantaneously switch back and forth seeing where they are at with no problem.)

 

I put the new balanced JJ 12AX7 and kept the stock EH power tubes in.  I just wanted to know if the bias reading would change much.  They didn't - V3 = 24.3 & V2 = 21.7.  Again, both readings at first were about 1mV lower and then stabilized in about 10 minutes.

 

Finally, I put the JJs matched set in with the new 12AX7.  This is where things kinda got weird at first.  The original reading on both tubes were 20.0mV and 20.2mV - amazing.  I let that sit for about 2 or 3 minutes.  As you know the amp calls for 25mV +/- 3mV.  I turned it slightly so that one tube was at 25mV and the other ended up around 24.1mV.  I let the tubes cook there for about 10 inutes - one tube kept ramping up and moving up in mV while the other tube grew at a much slower rate or stayed steady.  After 15 minutes one tube stopped claiming at 27.9 mV while the other tube was at 24.1mV.  During this time I did turn the bias down a few times.

 

I switched bias probes as well as where the tubes sat just to make sure it isn't the bias meter/probes and the tube placement/amp.  The same thing happened.  It sat still for several minutes and then once I adjusted the bias one tube's bias level grew much faster.  I thought I had a bad tube at first.

 

I tried again the next day.  I remembered that both tubes would start and sit very steady until I adjusted the bias.  I figured I'm the weak link in this process.  Again, both tubes started at 23.4 and 23.5.  After ten minutes it didn't move.  I very very slightly made an adjustment to 25.1 and 25.0.  After 20-25 minutes of cooking they read 25. and 25.8.  Amazing.  Three things changed from the night before:  1.  It's not 1am (haha).  2.  I made the adjustment much much slower and didn't have to back it off.  3.  I cleaned the tube pins with contact cleaner (as per eurotubes advice from their website).  I believe the amp doesn't like the drastic changes in the bias screw from the night before (I may have moved the screw too fast, etc.).

 

Before I sat down to write this I hooked it up on last time:  Original reading was 24.8 & 25.7.  After 30 minutes they read 26.2 & 25.7.  Amazingly close for a matched pair.

 

If that got a little long - sorry.  Here is what I thought of how they sounded.  I didn't notice a change with just the balanced JJ 12AX7.  I thought when I put all new JJ tubes the amp sounded much better.  I loved the amp before and I like it even more now.  Not sure if it is the tubes being new as compared to old EH tubes or if the JJs just sound better (at least in my opinion).  I took the same approach as boyce89976.  I hooked it up like I do when I play with the Line6 Link and my tones.  I tried a few of main amp models (all pre).  I run the DT master steady at 2 o'clock (with an attenuator).  I tested an AC30 patch - I agree with ice9mike - wow really really clear and sound very nice.  The Blackface Lux Vib was more bright and clean - the breakup was nicer.  J800 sounded much more deep and full - very very nice.  I loved it.  The PHD and Treadplate weren't as profound as the other three but I still feel it was an improvement.  I wish I could have recorded new the EHs and then recorded the new JJs to switch back and forth.  I definitely recommend trying out the JJs.  I can't wait to try it out at rehearsal.

[Brazzy]

Very nice rundown on your bias job, thanks. When I do my DT50 Head I should do the same.

[boyce89976]

Nicely done STPLE.  We should exchange some of our church patches sometime.  

[STPLE]

Thanks guys.  Brazzy I'm not familiar with the DT50 if you'll have room to bias while it sits in the amp.  One word of caution is that my bias probes for the EL84s sit at just under 3/4 of an inch tall.  I did look a some very good bias probes/meter but the probes were almost 2 1/2 inches tall - too tall for what I wanted. Good luck - let me know how it goes.

[Brazzy]

Thanks guys.  Brazzy I'm not familiar with the DT50 if you'll have room to bias while it sits in the amp.  One word of caution is that my bias probes for the EL84s sit at just under 3/4 of an inch tall.  I did look a some very good bias probes/meter but the probes were almost 2 1/2 inches tall - too tall for what I wanted. Good luck - let me know how it goes.

 

I think I'm going to be taking the chassis out.

[boblaw]

anyone try el844s in der dt25? bias voltz? Tone?

i tried them and wouldn't know shizz from shinola!  Didn't blow up tho...

[boblaw]

no one but me dumb enough to try el844s in dt25

[radatats]

so really you got nothing out?   interesting, I would have liked to hear how they worked at lower volumes...

[boblaw]

Radatats, my experience was they indeed broke up at a lower volume and sounded tight and transparent.  Good clean tones too.  Which is what I was after, so I like them.  I could not pull them to normal el84 bias range and so I am not sure if that makes them a bad idea to run.  I did not test for the effect of bias setting on tone.  

[STPLE]

Over the past several months I bought and tried out quite a few different tubes.  I thought I would share my observations.  Please keep in mind I'm not a professional musician and this is not a scientific study.  I would change out tubes, play some songs with a few go to patches and listen.  I typically would put a new set (or new PI) in and keep them in for a week or so - bias them again - and then keep them in a week or two more. 

 

I hesitated writing this since tone is very subjective.  There are times when I love the sounds during rehearsal but then not like it the next day.  I won't describe what I heard because someone else with different pickups, guitar, band and music styles may come to a very different conclusion.  In fact I liked every tube I tried.  Some sounded better with different amp models.  This was good exercise in case I ever get two heads - I'll put in a different set of tubes in each.  I liked the way the amp sounded already but I was trying to warm it up just a touch and take a bit of harshness out and just a bit more headroom.  I didn't try any NOS tubes.

 

Here are the PIs that I tried:

1.  JJ ECC803 S

2.  JJ ECC83 S

3.  Mullard 12X7

4.  Tung-Sol 12AX7

5.  Tung-Sol 12ATW

6.  Sovtek 12AX7 LPS

 

I found (for what I wanted) I liked the longer plate PIs.  The Sovtek and the Mullard seemed very similar if not the same.  The order that liked them best to worse was:  Sovtek, Mullard, JJ ECC803 S.  Of the shorter plates PIs I preferred the Tung-Sol 12AX7 the best.  I don't know enough about how the amp works to be confident with the 12ATW.  It didn't wow me.  In fact I didn't notice that much of a change at all so I didn't keep it in that long.

 

The EL84 power tubes I tried were as follows:

1.  JJs (29 bias point)

2.  Genalex Gold Lion (25 bias point)

3.  TAD - STR (41 bias point)

4.  Tung-Sol (29 bias point)

 

I really liked the JJs.  If I went JJs I'd be happy.  Then I tried the Gold Lions.  Those were the tubes I really liked.  Very nice breakup and smooth.  The problem was after a couple weeks I went to bias them again and one read 19 and the other 31!  I had to send them back.  I think the new ones I got back have a lower bias point (can't confirm).  Anyway, it changed the way they sounded.  Not nearly as smooth and the breakup not as nice (more harsh again).  The Tung-Sols were my least favorite of the bunch.  To my surprise I ended up liking the TAD-STRs the best.  I like how they break up and it did warm the amp up a little bit.  FYI after playing about 15 hours on them the bias points were still dead on (24.8 and 24.1). 

 

I'm running now with the Sovtek LPS in the PI and the TAD-STRs.  I am liking the way those tubes sound together in this amp. 

 

Two tubes that I didn't try (maybe someday) are the Mullard EL84 and the new Telefunken Black Diamond EL84 (enhanced JJs???).  Anyone ever try these??

 

I hope this helps anyone looking for tube replacement ideas.  Again this is very humbly submitted.

[Melissiah]

no one but me dumb enough to try el844s in dt25

I have el844s since 6 months now. Strangely I can't bias over 9 but it's ok and won't break the amp from what i heard. So it's the easiest tubes to install. Install a matched pair and turn the screw to max clockwise and rock. I havent lost a lot of head room, I was expecting a quieter amp after that but it's still ok. My goal was to have a lower breaking point and it is.

 

My next tubes will be another set of 844

[boblaw]

+1  I have been running 844s 6 months and will continue to do so.  Very nice low break point.  Good touch response.

[innovine]

Ok so one of my new jj's just died in my dt25, after 4 months of very light use (an hour session a few times a week). I'm careful to wait a minute after powering on before leaving standby... I'm not playing very loudly. This is my second pair since march, am I doing something wrong or does this amp eat tubes?

[TheRealZap]

According to the L6 Service guys, the amp very specifically likes the EH tubes that come stock.

they've explicitly stated it a few times, but never stated what the consequences of alternate tubes would be....

 

Ok so one of my new jj's just died in my dt25, after 4 months of very light use (an hour session a few times a week). I'm careful to wait a minute after powering on before leaving standby... I'm not playing very loudly. This is my second pair since march, am I doing something wrong or does this amp eat tubes?

[stumblinman]

My JJ's died quickly and catastrophically too.  I learned my lesson and stick with EH in my DT25.  JJ's work great in every other amp I've owned.  Don't know why, but seems to happen that way.

[Melissiah]

My JJs el844 low power still rock after alomst a year.

 

Sometimes it happens to get faulty tubes... In my case the last faulty tubes I got were the EH shipped with my DT25.

 

But I'm not mad at all it wasn't the 1st time, I got faulty tubes from many other brands... $hit happens

[stumblinman]

I just replaced my PI in my DT25. First time in 3 years or so. Was starting to crackle and hiss. I have a crappy unlabeled 12ax7 in there now while I wait for my JJ's to arrive. Then I'll pop one in and feel more secure.

[innovine]

Can't say I'm feeling at all secure about buying more jj's. But the noise floor with the eh's was terrible. Guess maybe I'll try one last pair of jj's but I'll be keeping a backup nearby

[Melissiah]

Try 2 of theses

http://www.thetubestore.com/Tubes/JJ-Electronics/JJ-EL844-low-power-EL84

 

And a good phase inverter like that

http://www.thetubestore.com/Tubes/Preferred-Series/Preferred-Series-7025-12AX7

 

Don't mess up with bias just turn the screw to max and you'll never look back

 

It's not expensive for a test

[talwilkins]

I'm using Tung Sol at  the moment and they sound great.

I have a spare pair but I hope they hold out a while

[jegler]

I pulled the probes and inserted the phillips-head into the bias adjustment pot and gave it a VERY slight turn to the right... seriously, maybe only an 1/8 of a turn at most... re-checked the bias on V2... 

 

 

The bias adjust on my DT25 is tucked under the speaker magnet necessitating the chassis being pulled. Anyone have detailed instructions on how to pull that off? 

 

Also, what are other users experience with tube longevity? After about 80-90 hours of usage, I've had two tube sets go bad on me. (EH EL84)

[stumblinman]

If you haven't pulled the chassis out to bias the tubes when you changed them, then I'm not too surprised you've had the second set fail prematurely. It's not that difficult to pull the chassis out. There's a little glue that generally makes it a little tough to remove, but if all the screws are removed it should wiggle loose.

 

 

The bias adjust on my DT25 is tucked under the speaker magnet necessitating the chassis being pulled. Anyone have detailed instructions on how to pull that off?

 

Also, what are other users experience with tube longevity? After about 80-90 hours of usage, I've had two tube sets go bad on me. (EH EL84)

[jegler]

If you haven't pulled the chassis out to bias the tubes when you changed them, then I'm not too surprised you've had the second set fail prematurely. It's not that difficult to pull the chassis out. 

 

First failure was about 5 weeks after purchase. I sent it back to the factory for re-tube. (cost me $80 in shipping) Second failure was after about 30 hours of use after that change. Have to get another RA# for another servicing, and more shipping fees. I'm 3.5 hours away from the nearest service center. I would think they would check the bias at the factory. 

 

I'm poking around because I'm thinking of just changing and re-biasing the tubes myself to save shipping and I'm trying to work up the courage. I've never changed tubes in an amp and am scared as hell of re-biasing the DT

[wowza]

 

First failure was about 5 weeks after purchase. I sent it back to the factory for re-tube. (cost me $80 in shipping) Second failure was after about 30 hours of use after that change. Have to get another RA# for another servicing, and more shipping fees. I'm 3.5 hours away from the nearest service center. I would think they would check the bias at the factory.

 

I'm poking around because I'm thinking of just changing and re-biasing the tubes myself to save shipping and I'm trying to work up the courage. I've never changed tubes in an amp and am scared as hell of re-biasing the DT

[wowza]

Sorry meant to add a reply with that but I guess I don't know how to do it with a phone. Ive been somewhat hesitant to post anything because there is already enough going around about line 6 quality issues. I haven't had any issues with my dt50 whatsoever but I have been really wanted to seal the tubes for awhile and figured before I did so I needed to make sure I knew where the bias probe points where and that I could do this myself . I found the trim pot and the bias points which I found out are a little different on the heads than on the combos. In any case I measured the bias of the stock tubes at 10mv which is way under the 36mv I see is supposed to the correct bias voltage. I guess I'm glad it was running less than more but if mine was that over biased from factory one could be that under biased. I don't want to start another quality debate but it may be something to look into.

[Brazzy]

Sorry meant to add a reply with that but I guess I don't know how to do it with a phone. Ive been somewhat hesitant to post anything because there is already enough going around about line 6 quality issues. I haven't had any issues with my dt50 whatsoever but I have been really wanted to seal the tubes for awhile and figured before I did so I needed to make sure I knew where the bias probe points where and that I could do this myself . I found the trim pot and the bias points which I found out are a little different on the heads than on the combos. In any case I measured the bias of the stock tubes at 10mv which is way under the 36mv I see is supposed to the correct bias voltage. I guess I'm glad it was running less than more but if mine was that over biased from factory one could be that under biased. I don't want to start another quality debate but it may be something to look into.

 

I just fixed up my DT50 with new original tubes and biased them, the thing sounds great, just got done playing it steady for 2 hours. Here's my recent post about it, http://line6.com/support/topic/6699-dt50-head-output-just-went-silent-any-ideas-why/?p=61241

[jegler]

Don't mess up with bias just turn the screw to max and you'll never look back

not to sound like an ignoramus, but do you mean ClockWise or CCW. And does this then tend to make them run hotter or colder?

[spaceatl]

I am not sure that is correct as it's a balance type pot that affects both test points...The idea is the get each tube as close to match as possible using the test point...Getting strange mv readings could mean a resistor out of tolerance...It's not the same as injecting and reading real current, but it is good enough...

 

Of course, the bias pot has no effect when the amp is running Class A...The bias only affects Class AB and it needs to be in that mode when adjusting...If you never run Class AB, then there is no need to adjust the bias.

[Brazzy]

I am not sure that is correct as it's a balance type pot that affects both test points...The idea is the get each tube as close to match as possible using the test point...Getting strange mv readings could mean a resistor out of tolerance...It's not the same as injecting and reading real current, but it is good enough...

 

Of course, the bias pot has no effect when the amp is running Class A...The bias only affects Class AB and it needs to be in that mode when adjusting...If you never run Class AB, then there is no need to adjust the bias.

Sounds like your saying I have a resistor in my amp (DT50) that is off spec enough to give me a bias reading mismatch of 4 mv with an ultra matched set of tubes. Although every 4 hours I check my bias as I'm curious to see what gives with time and it looks like they're creeping closer to being even, so far by only 2mv. I should add, I plug the amp into a power strip not a device that ensures exactly 120v is flowing. Almost forgot to mention the amp is working great.

[Melissiah]

not to sound like an ignoramus, but do you mean ClockWise or CCW. And does this then tend to make them run hotter or colder?

Full clockwise. I get someting like 9 on bias that's the max I can reach. Spec says the amp should be biased at 25 +-3 but el844 are not regular tubes. Full clockwise mean hotter.