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spaceatl

My DT-25 Bias Info

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We have not been able to get much in terms of answers about target bias % on a DT-25. So I pulled some of my notes from 2-3 years ago the last time I measured my DT-25 and I have gone ahead and done some measurements on my DT-25 head tonight...Just in case this was bothering anyone else as much as it was bothering me.

 

Keep in mind that I am not am employee of Line 6. I wanted to know what the Line 6's bias target % for my amp so I can at least have it operate in a safe range if I have to change tubes at a gig quickly. Needless to say, my queries went un answered.

 

EL84 tubes are pretty notorious for thermal runaway (red plating)...In a typical amp that experiences a run away tube, this would means checking the filter caps, grid resistors (AB) and/or the cathode resistor (A). But these amps have some digital stuff and heat and extra current draw is not good thing around that stuff anyway.

 

So what's the deal?

 

Transformers for power means everything is relative to the input voltage and could vary by 5% in multiple spots along the way. So your amp might slightly different...

 

house voltage:120.8 volts AC

 

Class A plate voltage: 404 volts DC

Class AB plate voltage: 414 volts DC

 

So this is that basic information we need to make an "informed" bias calculation for AB Mode. No Need to worry about Class A, that's a Cathode resistor and basically the mode is safer than fixed bias where the grid resistors run much hotter.

 

What Line 6 states:

 

25 mv (+- 3 mv)

 

Well, my amp has never been biased that high. However, if I were to set my bias to 25mv, with my plate voltage being 414 volts, then I am looking at 83%...If you are up around 420, then 25 mv puts you at 90%...Why does an AC30 keep flashing thru my brain?...Why even change modes? anyhoo...

 

What I am doing:

I have been running this amp at 70% bias since it arrived... 20.29 mv would be perfect...I am right around there...I have had this DT-25 since before they were released and I have been using it regularly (once a week for 2-3 hours). Much heavier the first year...One tube change right around the time they were released (amp was 3 or 4 months old at that point). Stock tubes are fine and are in the gig bag for spares...

 

Anyway, this is what I know about the DT-25....DT-50 is way overdue for a tube change and I am doing a bias mod to that amp...I will post my findings on that amp at a later time...hope this helps at a few of us that know this stuff isn't as much voodoo as has been made out.

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Thanks bro... good info to have.  I don't understand why if the tubes are so critical, Line 6 doesn't sell the exact ones we need from their website.  and I call total BS on the liability excuse...  not really convinced on the can't use other tubes stuff either but...

 

will be interested to hear about your bias mod...

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You always have good informative posts "S". Very good info. I was reading up one of your other posts about understanding ground loops. Good Stuff as usual.

 

My DT50HD measured 499 on the plate at 36mV. I got this reading using a dual probe. I was replacing the original tubes with new original tubes I bought from AMP HEADS. They were advertised as ultra matched. Using the dual probe I checked both tubes for bias volts and got 32 on one and 36 on the other with plate voltage of 499. Both tubes measured 499 while the bias volts were different.

 

The difference between the tubes concerning bias volts concerned me but I'm not the one who made this amp or tubes. So far I'm thinking the DT50 I purchased new is not reliable, yet. I shouldn't need to do more than adjust the tubes to 36 mV bias reading on the tube that reads the highest value, and I should be on my way to plugging in my guitar or HD500 and playing.

 

BTW, I like to know about the fine details of the amp, which is why I love following post like yours. On the other hand as a consumer I shouldn't have the issues I'm having with a new amp such as the DT50, although I'm all about innovation, building new things and improvement so I expect some problems which is why a warranty is very important. :)

 

Rock On! Carry On!

Edited by Brazzy

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Spaceatl, thank you so much!!!

 

Question:  Line 6 warranty info states that an amp that has been repaired or has had a repair attempted by anyone other than a Line 6 authorized service center will not be covered under the warranty.  So if a person wants to put in their own tubes, did they just void their warranty?  If a person biases their power tubes, is the warranty voided?  If a person measures their bias voltage, is that an "attempted repair," and thus their warranty is void?

 

I have come very close to doing these things but do not want to void my warranty.

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We have not been able to get much in terms of answers about target bias % on a DT-25. So I pulled some of my notes from 2-3 years ago the last time I measured my DT-25 and I have gone ahead and done some measurements on my DT-25 head tonight...Just in case this was bothering anyone else as much as it was bothering me.

 

Keep in mind that I am not am employee of Line 6. I wanted to know what the Line 6's bias target % for my amp so I can at least have it operate in a safe range if I have to change tubes at a gig quickly. Needless to say, my queries went un answered.

 

EL84 tubes are pretty notorious for thermal runaway (red plating)...In a typical amp that experiences a run away tube, this would means checking the filter caps, grid resistors (AB) and/or the cathode resistor (A). But these amps have some digital stuff and heat and extra current draw is not good thing around that stuff anyway.

 

So what's the deal?

 

Transformers for power means everything is relative to the input voltage and could vary by 5% in multiple spots along the way. So your amp might slightly different...

 

house voltage:120.8 volts AC

 

Class A plate voltage: 404 volts DC

Class AB plate voltage: 414 volts DC

 

So this is that basic information we need to make an "informed" bias calculation for AB Mode. No Need to worry about Class A, that's a Cathode resistor and basically the mode is safer than fixed bias where the grid resistors run much hotter.

 

What Line 6 states:

 

25 mv (+- 3 mv)

 

Well, my amp has never been biased that high. However, if I were to set my bias to 25mv, with my plate voltage being 414 volts, then I am looking at 83%...If you are up around 420, then 25 mv puts you at 90%...Why does an AC30 keep flashing thru my brain?...Why even change modes? anyhoo...

 

What I am doing:

I have been running this amp at 70% bias since it arrived... 20.29 mv would be perfect...I am right around there...I have had this DT-25 since before they were released and I have been using it regularly (once a week for 2-3 hours). Much heavier the first year...One tube change right around the time they were released (amp was 3 or 4 months old at that point). Stock tubes are fine and are in the gig bag for spares...

 

Anyway, this is what I know about the DT-25....DT-50 is way overdue for a tube change and I am doing a bias mod to that amp...I will post my findings on that amp at a later time...hope this helps at a few of us that know this stuff isn't as much voodoo as has been made out.

 

Spaceatl, I thought I read that Class A amps burn through tubes very quickly compared to Class AB.   Is that true?  

 

Sorry, but I don't follow your percentage calculations--was this info meant only for people who are well versed in tube amplifier theory?

 

You've most likely seen the strong advocacy here in this forum for using only the spec bias and EH tubes and such.  However, you've been running yours much cooler (@ 20.29 mv?).  If all a person has is a voltmeter, will the tubes last longer at a lower bias voltage?  Are these hybrid amps, with ICs and PCBs, more or less likely to fail and need repair if they bias their tubes to a lower voltage than spec?  

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Yes, tubes last a little longer in any AB tube amp running 70% vs 83%...But tone can get cold also....So there is an ear component to this as well...Mine sounds great to me, so I leave it at the typical 70%...Always a compromise between stability and finding the magic happy place....70% is very typical...and economical...

 

Tubes are tubes....Not many factories left, so It's all about the curve trace matching and the traces themselves...A nice matched set at 10 grid bias intervals works fine for me...There is a difference in terms of the curve response the designer wants in terms of obtaining the aural vision and the technically tolerances that will function just fine.

 

The notion of designing a tube amplifier to a very specific tube curve trace with extremely tight tolerances is a throwback to 40 years ago....There simply is not enough tube stock to select from to build amps like that anymore....Back then, fixed bias meant fixed bias...you did not put pots on it, you used resistors...and you have the curves for your amp and you replaced the tubes with a close a match as you could find...If this were really true, certainly RB would have insisted on the curves being in the manual...

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Question:  Line 6 warranty info states that an amp that has been repaired or has had a repair attempted by anyone other than a Line 6 authorized service center will not be covered under the warranty.  So if a person wants to put in their own tubes, did they just void their warranty?  If a person biases their power tubes, is the warranty voided?  If a person measures their bias voltage, is that an "attempted repair," and thus their warranty is void?

 

I'll let you know.  They have my DT25 right now and it's got JJ's that were installed and biased by me.

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I'll let you know.  They have my DT25 right now and it's got JJ's that were installed and biased by me.

 

Also make note of which 12AX7 tube they put in your DT25.. The DT50's get EH brand Russian 12AX7's, the DT25's get Chinese 12AX7; or at least, the stock 12AX7 tubes in both of my DT25's, one from Zzounds and one from Sweetwater both had the Chinese non-branded 12AX7B tube in each, not the Electro-Harmonix labeled, Russian made 12AX7.

 

Wondering if there is some reason for why they didn't stay consistent with the 12AX7 tube choice, and how it matters when retubing the amps. I also just (very recently, less than two weeks ago) had an amp repair show, who are a licensed Line6 repair center, put JJ's tubes in my DT25's, but I am going to have them swapped back out for all EH tubes this Friday - including the EH branded 12AX7, as opposed to the Chinese "which brand?" tube that Line6 chose for the stock 12AX7.

 

Especially given the primary issue and complaint is noise floor, and it would appear the 12AX7 does play some type of role in how much buzz and hum the noise floor of these amps emit.

 

All the focus being placed on the EH brand, Russian made tubes, and they use non-branded Chinese 12AX7 tubes in the DT25 but not the DT50?? Weird.

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Also make note of which 12AX7 tube they put in your DT25.. The DT50's get EH brand Russian 12AX7's, the DT25's get Chinese 12AX7; or at least, the stock 12AX7 tubes in both of my DT25's, one from Zzounds and one from Sweetwater both had the Chinese non-branded 12AX7B tube in each, not the Electro-Harmonix labeled, Russian made 12AX7.

 

Wondering if there is some reason for why they didn't stay consistent with the 12AX7 tube choice, and how it matters when retubing the amps. I also just (very recently, less than two weeks ago) had an amp repair show, who are a licensed Line6 repair center, put JJ's tubes in my DT25's, but I am going to have them swapped back out for all EH tubes this Friday - including the EH branded 12AX7, as opposed to the Chinese "which brand?" tube that Line6 chose for the stock 12AX7.

 

Especially given the primary issue and complaint is noise floor, and it would appear the 12AX7 does play some type of role in how much buzz and hum the noise floor of these amps emit.

 

All the focus being placed on the EH brand, Russian made tubes, and they use non-branded Chinese 12AX7 tubes in the DT25 but not the DT50?? Weird.

 

I had been wondering that myself, until I read your post in the other thread, where you printed that Bogner amps are now using Chinese 12AX7's, and also the piece about their characteristics differing from other tubes.  After reading that, my guess now is that due to the slight differences in the amps (EL34s vs EL84s for one), the chinese tube either sounded better or was a better fit in the DT25 architecture than EH or others.  The DT50 is an older design, so it could also be that Chinese 12AX7s were not reliable enough at that time for serious consideration, so they tweaked it around the EHs.  O'course I could be wrong on both theories...

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As I recall, the DT25 is using something like a FET buffer to hit the inverter tube...Certainly it I would think it was a somewhat evolved design...But it is also a different design with different concerns in terms of the power tubes...EL34s are a different animal...a whole lot of power there...

 

I think dropping one tube has everything to do with heat and insuring the EL84s stay stable...ALL EL84 amplifers run very hot and designing this one to run plates at over 400 volts, stay stable and switch modes in a small form factor might have been just enough for them to go for Class A solid-state buffer to feed the inverter over a 4th tube...EL84 amps are hotrods...I think this one is extremely innovative design that looks to me like it might be built to last...

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I think this one is extremely innovative design that looks to me like it might be built to last...

 

You're saying this about the DT25 and not the DT50?

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both

 

Hi spaceatl. Would you be so kind as to explain what position the controls should be set at when setting the bias or just checking the bias on the DT50. Such as should the voicing switch be set on Voicing "I"? Should the class switch be set on Class AB? Should it be on pentode or triode? Should the master be at 12 o'clock? Off standby? Or iit being set on Class AB the most important, etc.

 

It would be nice to reveiw the particulars for basic maintenance. Maybe in a different thread, here or in a PM.

 

I'm not trying to make you responsible for the accuracy of any description you may give. Just trying to pick your experienced brain.

 

Whatever happens have a good day!

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Sure. Happy to share what I do, but keep in mind that I do NOT speak for Line 6 and if you want to learn about taking care of a tube amp, there is plenty of good information on the web...Lots of good how to's about setting the bias on a tube amp...I would never discourage someone from wanting to learn this stuff...Just keep in mind that the voltages are dangerous...THEY CAN KILL YOU...However, there are safe procedures anyone can follow and learn how to do it safely.

 

the bias adjustments only apply to the Class AB Mode...You would actually figure this out pretty quickly when you meter read zero on the test points in Class A.

 

Master Vol all the way down....After all, adjusting bias is about the "idle" state....If you put the amp on standby, you will have no readings...It becomes pretty clear once you are into doing it...You are just taking measurements...

 

One note is that you do not need to let the amp warm up to measure the plate voltage....You can simple take out the tubes, power up and measure it fro mthe tube socket...This can give you decent starting place for calculating safe bias level (in my DT25 case 414 volt plate; 70% of 12 watts = 20.4mv bias target)

 

My own procedure for adjustment with new tubes is to:

  • remove the old tubes with the power UNPLUGGED FROM THE WALL.
  • turn the bias adjustment(s) fully counter clockwise. (All the way down)
  • power on for 5 minutes on standby
  • Set as CLASS AB
  • power up off standby and take measurements immediately....bias should be very low...
  • Switch to CLASS A
  • let them cook for 5 or 10 minutes
  • Switch to CLASS AB
  • start adjust the pot(s) up slowly until I hit the target.
  • Let it cook for another 15 minutes
  • check again and adjust
  • play on it for a couple of days
  • check and adjust as needed

 

The voicings and pentode/triode do not change the plate voltage at all. I measured the plate voltage in each state in case an average would be needed....

 

I have my amp set Class AB/Pentode with Master down...NFL made no difference to the bias that I could measure...

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uh, when do you put the tubes back in in that procedure?

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It might not be clear, but I have the DT-25 head....So there isn't an easy way to get at the back of the tube sockets to measure the plate voltage with the tubes in...It's a logistics thing....Easier for me to yank the tubes, measure the plate voltage, power off and put the tubes back in....The tubes have nothing to do with the plate voltage reading....So certainly the tube are in when you are adjusting bias...On the combo, you can more likely get to the plate socket pin with the tube in and do it all at once...

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Sure. Happy to share what I do, but keep in mind that I do NOT speak for Line 6 and if you want to learn about taking care of a tube amp, there is plenty of good information on the web...Lots of good how to's about setting the bias on a tube amp...I would never discourage someone from wanting to learn this stuff...Just keep in mind that the voltages are dangerous...THEY CAN KILL YOU...However, there are safe procedures anyone can follow and learn how to do it safely.

 

Great post spaceatl and thank you very much. How explained it is the way I did mine the first time except I used a tool which probes both tubes in the DT50 and I can switch between the 2 tubes and measure plate voltage or bias voltage.

 

It also came with good instructions.

 

GDual_BiasTester_MPD.jpg

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so replace the tubes goes right before let them cook for 5 minutes...

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so replace the tubes goes right before let them cook for 5 minutes...

 

If this confusing to you, then you might need to read up a little more and perhaps get some assistance from someone who knows how to do it safely....tubes are ALWAYS removed when the amp is powered OFF and the tubes are COOL....Removing the tubes is an easier way to measure the plate voltage in the head...does not apply to the combo...

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It's very confusing.  And I still don't follow the math:  "414 volt plate; 70% of 12 watts = 20.4mv bias target"  Okay, you measured 414V, and 70% is a target.  I get that.  But where does the 12 watts come from, and how can you take 70% of watts and come up in millivolts?  Isn't that like saying Jethro dated his grandma so his neighbor gave birth to a pig?  It doesn't compute.

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The watts (in this case 12) is the maximum dissipation of the tube itself.

 

Max Dissipation of an EL84 is 12 watts (This is on the datasheet)

 

((Max Dissipation / Plate Voltage) * 0.7) * 1000 = bias mv

 

So you take any tube, find the max dissipation on the datasheet..measure your plate voltage on the amp...plug in the numbers and that gives you the safe bias value to start with for a Class AB tube amp

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The watts (in this case 12) is the maximum dissipation of the tube itself.

 

Max Dissipation of an EL84 is 12 watts (This is on the datasheet)

 

((Max Dissipation / Plate Voltage) * 0.7) * 1000 = bias mv

 

So you take any tube, find the max dissipation on the datasheet..measure your plate voltage on the amp...plug in the numbers and that gives you the safe bias value to start with for a Class AB tube amp

Ahhhh...  Thanks!

 

It's unlikely that I'll want to be that technical in my approach.  Although... I do have an oscilloscope that I "stole" from the auction back when I was in electronics school that seldom gets used...  So after you arrive at your starting point--which could also be taken from the Line 6 specs (25mv & 36 mv, respectively), then do you go to the scope to dial out the notch?  Or could a person just kinda tweak the bias to their own liking based on their own ears, providing that they ensure that the bias voltage never exceeds the Line 6 spec?  Can a bias that's too low hurt anything other than your tone?

 

I guess I'm also asking, as I have no knowledge, what's the deal with the notch?  When I first read about it, getting rid of it seemed to be the primary thing, but most of what I'm reading in these forums lately is absent any discussion of that.  I would think that if the bias were on the low side your tone would be cleaner with more headroom, and the amp would run cooler.  If the bias was dead-on, you'd have exactly what Bogner wants you to have.  If the bias is high, now you're melting tubes, boards, etc.  Is any part of that true, or am I way out in left field?

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could a person just kinda tweak the bias to their own liking based on their own ears, providing that they ensure that the bias voltage never exceeds the Line 6 spec?  Can a bias that's too low hurt anything other than your tone?

 

 

I'm waiting on this question too

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people have been tweaking bias to their liking for 50 years or more....This is nothing new...sure you can do it...With a conventional amp you run the risk of burning some grid resistors...with a hybrid, you run the risk of damaging other things too...But a tube can fail at anytime and that risk is always there....running hot bias just increases this perceived risk...A tube could blow running cold bias...you never get rid of this risk...

 

but then this amp runs in the "non-linear" regions.... :rolleyes:

 

low bias on an AB tube amp makes your power tubes last longer because they idle cooler....technically, better for any class AB amp because you generate less idle heat...This is true for any class AB tube amplifier...

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people have been tweaking bias to their liking for 50 years or more...

 

I remember seeing something about these new power tubes made with a computer like circuit inside an anodized aluminum housing. Anyone remember what company is making them? It was just secently in this forum somewhere. Anyway, wouldn't it be great if these worked in the Dt's and increase longevity? Man that would be great. I realize it wouldn't be a tube amp anymore but if the reliability factor increases it would be just the ticket.

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 Man that would be great. I realize it wouldn't be a tube amp anymore but if the reliability factor increases it would be just the ticket.

 

They have that already (computer-like circuits that emulate tubes).  They call it POD HD, and it's fully compatible with your DT!

 

(sorry, couldn't resist)

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Yeah!! That's them, Thanks Man :) I think these might be the ticket to reliability or at least I hope for my wallets sake, lol.

 

They have that already (computer-like circuits that emulate tubes).  They call it POD HD, and it's fully compatible with your DT!

 

(sorry, couldn't resist)

 

LOL Good One Bro!!!! :)

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people have been tweaking bias to their liking for 50 years or more....This is nothing new...sure you can do it...With a conventional amp you run the risk of burning some grid resistors...with a hybrid, you run the risk of damaging other things too...But a tube can fail at anytime and that risk is always there....running hot bias just increases this perceived risk...A tube could blow running cold bias...you never get rid of this risk...

 

but then this amp runs in the "non-linear" regions.... :rolleyes:

 

low bias on an AB tube amp makes your power tubes last longer because they idle cooler....technically, better for any class AB amp because you generate less idle heat...This is true for any class AB tube amplifier...

 

Whatever happened to the idea of adjusting bias using an O-scope to dial out the "notch?"  Actually, spaceatl, you're the only source I've run across that specifically mentioned the notch (although my research has been anything but thorough).  Is having a notch in your output sine wave maybe not so bad or even desirable to some?  Please elaborate on this if possible.  THANKS!

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Whatever happened to the idea of adjusting bias using an O-scope to dial out the "notch?"

 

Something I found might be useful.

 

The Link to the page >> http://www.aikenamps.com/index.php/crossover-notch-biasing-why-it-should-be-avoided

 

"Crossover Notch" Biasing - Why It Should Be Avoided

 

The "crossover notch" method of biasing is touted by some as the only "correct" method of biasing a guitar amplifier.  This couldn't be further from the truth, and this method should be avoided, as it has numerous flaws that make it not only unreliable, but in some cases, even hazardous to the operation of the amplifier.  At best, particularly with lower plate voltages, it biases the amplifier to a very cold point of operation, just above class B.  At worst, particularly at high plate voltages, it can bias the tubes too hot, because you cannot remove all the crossover distortion in some amplifiers without exceeding the plate dissipation, either at idle or at full power.

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Something I found might be useful.

 

Wow!  That certainly is useful!  Thanks!  Great job googling that tidbit!!!  So then the relatively simple method involving a voltmeter would seem to be the only way to go.  It's strange that actual Line 6 employees have stated that O'scopes must be used by Line 6 authorized service centers whenever tubes are to be replaced.

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Wow!  That certainly is useful!  Thanks!  Great job googling that tidbit!!!  So then the relatively simple method involving a voltmeter would seem to be the only way to go.  It's strange that actual Line 6 employees have stated that O'scopes must be used by Line 6 authorized service centers whenever tubes are to be replaced.

 

Glad you found it useful. I've been reading from that site on occasion. It's good stuff and they keep it updated.

 

About Line 6's procedures. The DT Line is so special (complicated) inside that it makes it a nightmare for someone who just wants to use it to make music to even think about being able to diagnose and fix problems that might develop. I have experience with prototyping and some of the things I had to make and work on weren't the most complicated things but to someone who's not fresh on the particulars would be lost without my notes which of course was the beginning of the service manual or instuctions for the project. The DT fits this senario IMO.

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So I got my DT25 Head back, from the Calabassa, CA service center, today.  It was exactly 31 days from shipped to returned.  I wish they would have been thoughtful enough to include an explanation of what they did to it.

 

I can tell for sure that they replaced the matched pair of JJ's EL84 tubes with Electro Harmonix EL84H tubes (Made in Russia   14 03).  They left the JJ ECC83S though.

 

I guess I'll have to call them to see if they replaced anything else.  I doubt that the tubes were solely responsible for blowing both fuses.

 

Now to go plug it in and bask in the glorious sounds I've been missing for a month.

 

UPDATE:  I just called Line 6 and the tech told me that, along with the EL84H tubes, they replaced the Power Amp board and the Main Board (that the tubes plug into.

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So I got my DT25 Head back, from the Calabassa, CA service center, today.  It was exactly 31 days from shipped to returned.  I wish they would have been thoughtful enough to include an explanation of what they did to it.

 

I can tell for sure that they replaced the matched pair of JJ's EL84 tubes with Electro Harmonix EL84H tubes (Made in Russia   14 03).  They left the JJ ECC83S though.

 

I guess I'll have to call them to see if they replaced anything else.  I doubt that the tubes were solely responsible for blowing both fuses.

 

Now to go plug it in and bask in the glorious sounds I've been missing for a month.

 

UPDATE:  I just called Line 6 and the tech told me that, along with the EL84H tubes, they replaced the Power Amp board and the Main Board (that the tubes plug into.

 

Thanks for the update, DarrellM5!  

 

So both fuses blew and you sent it in for warranty service. I'm curious, did you call and they suggested this, or was it your idea to get it checked out?  

 

Weird that they didn't send a note along with, I guess they want customers to call in.  They have a good policy about that, even extending beyond the warranty period (although it takes time to get through).

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It blew the line fuse so I replaced it.  Then, within minutes of operating, it blew the tube protection fuse.  The tubes looked fine visually (no signs of damage and a healthy glow prior to blowing the other fuse) so I opened a support ticket.  Then they suggested I send it in for a diagnosis.

 

Since they left one of the JJ tubes in, I'm assuming that the tube in that position doesn't matter too much.  I think I'll be sticking with the EH tubes in the other 2 slots though, even though I don't think the JJ's had anything to do with the amp failure.

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It blew the line fuse so I replaced it. Then, within minutes of operating, it blew the tube protection fuse. The tubes looked fine visually (no signs of damage and a healthy glow prior to blowing the other fuse) so I opened a support ticket. Then they suggested I send it in for a diagnosis.

 

Since they left one of the JJ tubes in, I'm assuming that the tube in that position doesn't matter too much. I think I'll be sticking with the EH tubes in the other 2 slots though, even though I don't think the JJ's had anything to do with the amp failure.

Wow. How much time did you get on it? Didn't you just get it back?

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It only went in the one time.  I've had it back for a few days now and it sounds great.  I've spent about 12 hours playing it since Friday morning.

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