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Is It A Tube Problem?

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The most important thing is not to die when you are messing with a tube amp...

 

Personally, I prefer to stay far away and isolated from the 500 or so volts of plate voltage in the DT50 that can kill you dead...

 

I should have also mentioned that the rear bias mod offers an increased margin of safety when you put the test points and adjustment on the back of the chassis vs an open chassis. That's actually a big part of my own motivation to do the mod...

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Personally, I prefer to stay far away and isolated from the 500 or so volts of plate voltage in the DT50 that can kill you dead...

 

I feel this was directed toward my last post. I realize this is so true and I did not mention it. Although anyone reading and researching how to do anything on a tube amp that requires opening it up should be well aware of how to safely do it.

 

There are so many ways to get the correct education on this that there is really is no excuse to be unaware and uneducated when come to doing anything one takes deep interest in.

 

Patience is the key. It gives one the ability to be sure of his/her direction and the abilty to concentrate on any subject. I've witnessed many mistakes made by people who do not practice patience in learning.

 

Even with this said I suppose there is no real way to prevent someone from doing the wrong things in life.

 

Still a tech that is well educated could zap themselves to death just so someone can play a guiar or other instrument and amplify it. I say they are great people risking their lives to make some money and people happy.

 

The definition of patience I'm using in this is as follows.

 

pa·tience

ˈpÄSHÉ™ns/
noun
noun: patience
  1. 1.
    the capacity to accept or tolerate delay, trouble, or suffering without getting angry or upset.
    "you can find bargains if you have the patience to sift through the dross"
    synonyms: forbearance, tolerance, restraint, self-restraint, stoicism; More
    calmness, composure, equanimity, imperturbability, phlegm, understanding, indulgence
    "she tried everyone's patience"
    "a task requiring patience"
  2. 2.
    chiefly British term for solitaire (sense 1).
Origin
This is an excellent idea:

I should have also mentioned that the rear bias mod offers an increased margin of safety when you put the test points and adjustment on the back of the chassis vs an open chassis. That's actually a big part of my own motivation to do the mod...

Edited by Brazzy

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What I did was just go with the AMP Heads Bias probe tester which makes it possible to see what both tubes are doing while your turning the bias pot. It's so easy to take the chassis out of the DT50 Head it's pointless for me to modify it.

 

The most important thing is to put "Line 6 Approved Tubes" in it, IMO. This morning I've been playing with the head on Low Volume Mode to check that it works to my expectations as I've spent 4 hours on the High Volume Mode and am happy with it thus far. So far the LVM is working great and as expected. I've been using the my noisey strat going right into the front Low and High inputs. I don't even need a noise gate.I've tried all of my other guitars with the good results too.

 

4 hours?  that's all?  Did you JUST get your amp back?

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4 hours?  that's all?  Did you JUST get your amp back?

 

 I now have 8 hours on it, Yes I just got it back. I try to post things while it fresh.

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right, just moves the stock test points to the back of the amp...

 

here is a thread I did in the old forum regarding my MK1 mods....I don't think the pics are there...I will see if I can dig those up...The hardest part is desolding the pots...But I guess it's a little easier in the DT50 as there is only 1...but this might translated into two pots as the DT's are a balance type single pot...

 

http://line6.com/supportarchivenew/thread/24828/

 

So then checking bias with a voltmeter is perfectly acceptable, even if you're not running the EH tubes?

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I should have mentioned safety in general earlier in the thread...It's easy to forget when you work on these things all the time....

 

the provided test points are just a ballpark reading regardless of the tube type...bias probe is always a better set of measurements...

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right, just moves the stock test points to the back of the amp...

 

here is a thread I did in the old forum regarding my MK1 mods....I don't think the pics are there...I will see if I can dig those up...The hardest part is desolding the pots...But I guess it's a little easier in the DT50 as there is only 1...but this might translated into two pots as the DT's are a balance type single pot...

 

http://line6.com/supportarchivenew/thread/24828/

 

 

I've ordered new 12AX7s and new EL34's which will arrive today, but I'd like to try just the 12AX7s to see if that gets rid of my noise floor.  Thus I don't need to pull the amp down of its lofty perch, pull the chassis out, or pull the cords or any of that...  Except that I ordered a GT matched inverter like you used once and a Mullard for the boost, so I need to know which slot is which.  Looking at the back, can you please tell me, is the boost on the left and the inverter on the right?

 

THANKS!

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Don't mess with it yourself, have an authorized Line 6 service center do it.

They will have access to service info and the know-how to use it. This is not

our grandfather's Marshal stack, it's a hybrid. If you don't know where the

trim pots go, then don't go there. Leave to the experienced tech who's worked

on these. I see too many failed attempts come across my bench.

 

And using info from a Spider Valve link won't apply to the DT series. Different

beast, different gain structure,... not to be taken lightly. Nor is its supply voltage.

There are high voltage supply lines that should not be tampered with. Leave it

to the service center with the knowledge and the service info access.

 

Besides, if you get zapped, it hurts.

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CORRECTION: it's the one closest to the power jack....to the left if you are facing the back...

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Because of the way Bogner designs the tube portions of these amps,

and that this is a hybrid and not the usual bill-of-fare tube amp, you should

re-tube using the specified EH from Electro-Harmonix tubes.

 

You can't use the EH bias spec on other tubes. The spec is dialed in for

the EH's.

 

All too often these come across our bench for no other reason than having

non-specified tube inside (JJ's, Mesa, Groove Tube), using the DT amp

specified bias. That does not work.

 

They (JJ's, Mesa, Groove Tube) would have to be bias slightly different,

because the characteristic curves from one to the other are not exact. That

little of difference, makes all the difference. I've seen too many blown amps

come across my bench just because of that. It's a hybrid and a hot rod,...

best don't mess with it.

 

Stick with the specified EH's. 

Psarkissian, could you please expound a bit about what goes wrong when other tubes are installed to the EH spec?  I follow you when you say that they wind up on your bench, but what is it that specifically goes wrong?  Do the tubes get too hot and cause the board to melt?  If not that, what?

 

THANKS!

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-Red Plating, for one. Too much current going through the plate element.

 

-Biasing other brand tubes to the same spec as the EH's would not be a

good idea.

 

-Circuit is designed to operate in the linear region of the "characteristic

curves" of the tube. Other brand's linear regions don't always match curve-to-curve.

If you go non-linear too soon, distortions and other related problems occur,...

stuff starts to get damaged or blown. Bias and gain stages play into this.

 

-And with a hybrid amp like this, one of the things that typically goes, are a

couple components on the processor board.

 

-There are multiple gain structures on the amp board, using tubes with afore

mentioned aspects also play a part in what can get damaged.

 

 

Other aspects I won't go into because it's a tube engineering thing that most

people won't get (except tube geeks like me),... and because there are also

aspects I can't get into because it's proprietary.

 

Pushing a hot rod to the edge is not a good idea here. It's already a hot rod,

and a hybrid one at that.

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:wacko: --- And the plate voltage is?

 

Without revealing anything...Can you tell me your bias percentage target? Is it typical 70%...That's really all I would like to know...

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There's a bias voltage specified by Bogner and our engineering dept.

 

Can't get into Plate voltage on a DT-50,... sorry. It's a lot, so don't mess

with it. It hurts when you get zapped.

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Sorry, I did NOT ask for the bias or the plate voltage values....just what you think the bias % target is for Class AB Mode...

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-Red Plating, for one. Too much current going through the plate element.

 

-Biasing other brand tubes to the same spec as the EH's would not be a

good idea.

 

-Circuit is designed to operate in the linear region of the "characteristic

curves" of the tube. Other brand's linear regions don't always match curve-to-curve.

If you go non-linear too soon, distortions and other related problems occur,...

stuff starts to get damaged or blown. Bias and gain stages play into this.

 

-And with a hybrid amp like this, one of the things that typically goes, are a

couple components on the processor board.

 

-There are multiple gain structures on the amp board, using tubes with afore

mentioned aspects also play a part in what can get damaged.

 

 

Other aspects I won't go into because it's a tube engineering thing that most

people won't get (except tube geeks like me),... and because there are also

aspects I can't get into because it's proprietary.

 

Pushing a hot rod to the edge is not a good idea here. It's already a hot rod,

and a hybrid one at that.

THANKS FOR THE INFO!!  

 

Just to be sure I understand your answer... Can damage to the printed circuit boards and/or their components occur without the tubes red-plating?  

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Yes. Red plating isn't the only way to damage the unit.

 

To explain that further gets  into aspects of the circuit

that I can't get into. It all goes to the characteristic curves

of the tubes.

 

JJ's have a tendency to damage certain parts of the processor board, 

Mesa's tend to "red plate", and Groove tubes tend to do a host of other

things.

 

Part of it is that it's a Bogner hot rod, part of it is that there are multiple

gain topologies that are on this amp that can be engaged, and so messing

with it is not a good thing to do.

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Yes. Red plating isn't the only way to damage the unit.

 

To explain that further gets  into aspects of the circuit

that I can't get into. It all goes to the characteristic curves

of the tubes.

 

JJ's have a tendency to damage certain parts of the processor board, 

Mesa's tend to "red plate", and Groove tubes tend to do a host of other

things.

 

Part of it is that it's a Bogner hot rod, part of it is that there are multiple

gain topologies that are on this amp that can be engaged, and so messing

with it is not a good thing to do.

 

Forum moderators: This summary of the tube issues, and the prior info recommending using only Electro Harmonix brand tubes in the DT amps should be a STICKY POST at the top of the DT forums; with some kind of title like:

 

Read this before you change the tubes in your DT amplifiers!

 

The key points:

 

·  For DT50 amps: use matched pair of Electro Harmonix EL34 power amp tubes with a pair of Electro Harmonix 12AX7 tubes.

·  For DT25 amps: use matched pair of Electro Harmonix EL84 power amp tubes with a single Electro Harmonix 12AX7 tube.

 

Use ONLY Electro Harmonix brand tubes in your DT amplifiers!

·         JJ's have a tendency to damage certain parts of the processor board

·         Mesa's tend to "red plate"

·         Groove tubes tend to do a host of other damages

 

Because of the way Bogner designs the tube portions of these amps, and that this is a hybrid and not the usual bill-of-fare tube amp, you should re-tube using the specified EH from Electro-Harmonix tubes. You can't use the EH bias spec on other tubes. The spec is dialed in for the EH's. All too often these come across our bench for no other reason than having non-specified tube inside (JJ's, Mesa, Groove Tube), using the specified bias. That does not work.

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Yes. Red plating isn't the only way to damage the unit.

 

To explain that further gets  into aspects of the circuit

that I can't get into. It all goes to the characteristic curves

of the tubes.

 

JJ's have a tendency to damage certain parts of the processor board, 

Mesa's tend to "red plate", and Groove tubes tend to do a host of other

things.

 

Part of it is that it's a Bogner hot rod, part of it is that there are multiple

gain topologies that are on this amp that can be engaged, and so messing

with it is not a good thing to do.

 

This is really helpful information--THANKS!

 

Looking at the other side of the equation, what can go wrong if only EH tubes are used?  Folks report that EH tubes can expire anywhere from 3 months to 2 years, some have actually melted glass, and I believe I read that some have red plated as well.  Information like this suggests that it would be folly to assume that EH tubes are rock-solid, that nothing can go wrong as long as these are being used.  Rather, it would appear that as tubes break in after initial biasing, and as they age, the potential for resulting damage to a DT from what would seem to most guitar players as normal usage is very real.  Can you tell us, how safe are our amps if we only use EH tubes biased to the Line 6 voltage spec?  What is at risk during normal operation and ownership?  What are the warning signs that a problem might be developing?  What precautions can we take to ensure that our amps don't wind up on your bench?  

 

THANKS!

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Melted glass is usually a result of 'red plating'. It can get so hot,

that the glass will melt and pucker until it cracks and loses vacuum.

Seen that more times than I would like to. 

 

Depending on how much gigging you do, tubes ought to be changed

out every 12-18 months. If you're a recreational player and don't gig

or record much, you can get away with 2 to 2-1/2 years.

 

But, yes,... I've seen tubes in amps in general (at other places I've worked)

last two weeks in one amp and 5-years in another. Partly the amp,

partly the tube.

 

 

"What precautions can we take to ensure that our amps don't wind up on your bench?"--- Don't try to modify

it, use specified tubes, change out tubes on a regular basis. It's your friend, your tools, take care of your tools.

And used Line 6 authorized service centers, as they will have access to proper service info,... at least while your

warranty is good.

 

And by all means,.... please DON"T go inside these amps yourself, the shock hazard is too great. Don't want read

about how someone got zapped messing with a tube amp. This is dangerous,... I'm a tube geek, I was trained to

do this, been dealing with tube amps and solid state amps since the 1970's. And been zapped enough to know better.

 

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Melted glass is usually a result of 'red plating'. It can get so hot,

that the glass will melt and pucker until it cracks and loses vacuum.

Seen that more times than I would like to. 

 

Depending on how much gigging you do, tubes ought to be changed

out every 12-18 months. If you're a recreational player and don't gig

or record much, you can get away with 2 to 2-1/2 years.

 

But, yes,... I've seen tubes in amps in general (at other places I've worked)

last two weeks in one amp and 5-years in another. Partly the amp,

partly the tube.

 

 

"What precautions can we take to ensure that our amps don't wind up on your bench?"--- Don't try to modify

it, use specified tubes, change out tubes on a regular basis. It's your friend, your tools, take care of your tools.

And used Line 6 authorized service centers, as they will have access to proper service info,... at least while your

warranty is good.

 

And by all means,.... please DON"T go inside these amps yourself, the shock hazard is too great. Don't want read

about how someone got zapped messing with a tube amp. This is dangerous,... I'm a tube geek, I was trained to

do this, been dealing with tube amps and solid state amps since the 1970's. And been zapped enough to know better.

 

Thanks again!  We really appreciate your info!  You're our only source for this kind of knowledge, it seems.  Being that tube life seems to vary so dramatically, the advice to change them out "regularly" seems too open to interpretation for my comfort level.  I hope it's okay to ask just a few more questions!

 

1)  How safe are our amps if we only use EH tubes biased to the Line 6 voltage spec?  

 

2)  What is at risk during normal operation and ownership (i.e. what can still go wrong related to tube failure--using EH  tubes, of course)?  

 

3)  What are the warning signs that a problem might be developing (i.e. how do we know when our tubes MUST be replaced or bad things will happen)?  

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1. Good.

 

2. Low risk.

 

3. Volume cut-outs, crackling noises,.. stuff like that.

 

 

Just play the amp, don't mod it or do anything strange to hot rod the hot rod.

 

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1. Good.

 

2. Low risk.

 

3. Volume cut-outs, crackling noises,.. stuff like that.

 

 

Just play the amp, don't mod it or do anything strange to hot rod the hot rod.

 

Well that seems simple enough.  So I'll play my heart until until such time as I hear volume cut-outs or crackling noises, and when that happens I'll have the tubes swapped with only EH and make sure they bias a DT50 at 36mv or a DT25 at 25mv.  I will not worry if I get white noise, high noise floor, or a ground buzz with no guitar cord plugged in.

 

That takes a load off  my mind.  THANKS!

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Well that seems simple enough.  So I'll play my heart until until such time as I hear volume cut-outs or crackling noises, and when that happens I'll have the tubes swapped with only EH and make sure they bias a DT50 at 36mv or a DT25 at 25mv.  I will not worry if I get white noise, high noise floor, or a ground buzz with no guitar cord plugged in.

 

That takes a load off  my mind.  THANKS!

 

It sounds like either there are major power issues in the space where you are using the DT, or the amp needs to get serviced by Line6...

 

I had crazy weird problems with my gear in our last apartment, made playing guitar at home basically impossible, in terms of using the POD, or amps, or doing recordings, or anything. Moved to a different place, all those problems vanished.

 

My apologies if you mentioned this previously, but have you had a chance to take your gear to a different location, and try it on different power sources?

 

Like, take the DT to a Guitar Center and make up a story, tell them you want to try some guitar they have through your amp to see if you like how they sound together before you consider buying.. Do a test, and note if the symptoms stay with the amp, change, or go away.

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It sounds like either there are major power issues in the space where you are using the DT, or the amp needs to get serviced by Line6...

 

I had crazy weird problems with my gear in our last apartment, made playing guitar at home basically impossible, in terms of using the POD, or amps, or doing recordings, or anything. Moved to a different place, all those problems vanished.

 

My apologies if you mentioned this previously, but have you had a chance to take your gear to a different location, and try it on different power sources?

 

Like, take the DT to a Guitar Center and make up a story, tell them you want to try some guitar they have through your amp to see if you like how they sound together before you consider buying.. Do a test, and note if the symptoms stay with the amp, change, or go away.

I filed a support ticket with Line 6.  I was asked for and provided a recording of the excessive noise floor and was told it "might be a little high."  I called and asked about the ground buzz and was told it was probably tubes.  My power is CLEAN.  I bought a power conditioner anyway, out of desperation, and this actually produced power that was dirtier than what's at the outlet because the buzz got louder.  I took it back and the amp got quieter again.  

 

Perhaps you don't realize this, but these symptoms are present in all amps, it's just a question of degree, and with the DT we are told it's high gain so it will be noisey.  (which mine definitely is/was)  My amp is now in the shop awaiting service due to the above and the fact that it's in the series where it may have the bad transformers.  I believe I forgot to mention excessive heat here, but it's on the list to be looked at as well.

 

The reason I wrote what I wrote was to give Psarkissian a chance to correct me if I was wrong.  If those symptoms do not lead to amp breakdowns--and I hope they don't--then we don't have to chase ghosts, wonder where to draw the line between normal and abnormal, or wonder if we're damaging our amps by just using them.

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Regarding the 12AX7 "stock" tube from the DT25 amps, the 12AX7 tubes they took out of my DT25's say "12AX7B", and the word "CHINA"

 

I thought these were not EH brand, but then I just found the same exact thing on Sweetwater:

 

http://www.sweetwater.com/store/detail/EHX12AX7C

 

Interestingly, under the brand EH, Sweetwater offer this tube in four options:

(which could be an error on their website..)

 

- 12AX7 Chinese

- 12AT7 Tesla/Slovak (JJ)

- ECC83 Tesla/Slovak (JJ)

- 12AX7 Mullard (made in Russia)

 

I ordered a different pair of EH 12AX7 tubes; these

http://www.thetubestore.com/Tubes/12AX7-ECC83-Tube-Types/Electro-Harmonix-12AX7-EH

 

"These are not relabelled Sovtek 12AX7-LPS tubes. There is a marked difference in construction and performance. The 12AX7 EH has a nice balanced sound, fairly low noise floor and excellent performance in terms of microphonics. The lack of microphonics may be in part from the return of the shorter plate structure or materials. '

 

I am still wondering whether I should have stuck with the balanced triodes option, or if that even matters for the function of the 12AX7 tube in the DT25 amps. Also wondering if the "shorter plate" structure matters. 

 

I am confused as to why Sweetwater is using the Electro Harmonix brand for tubes made in China, and then used as stock tubes in the DT25 amplifiers, but not the DT50's. Especially after such a focus on EH brand being the only acceptable option. If it's made in China, it's not Electro Harmonix, or at the very least, it's not comparable to the EH brand tubes made in Russia. For the DT50 amps, they use a pair of "actual" Electro-Harmonix BRANDED, Russian made 12AX7 tubes, not the China labeled version.

 

Near as I can tell, the stock 12AX7 that was in both of my DT25's is this brand, Shuguang - are they also owned by EH?

 

http://www.thetubestore.com/Tubes/12AX7-ECC83-Tube-Types/Shuguang-12AX7-B

 

"The Chinese 12AX7B tube is a good choice for high gain guitar amps. The 12AX7B tube has a little more overdrive than the standard 12AX7A tube and lower noise."

 

"Let's talk Chinese 12ax7's"

http://www.thegearpage.net/board/showthread.php?t=797149

 

"Audio Tubes Valves, 12AX7 ECC83 EH, JJ Tesla, Shuguang"

http://www.ebay.com/gds/Audio-Tubes-Valves-12AX7-ECC83-EH-JJ-Tesla-Shuguang-/10000000000932958/g.html

 

"Shuguang (Chinese).  Third and last tube to be tested is the Chinese 12AX7B.  These tubes have undergone many design changes over the last few years and the Chinese plant has invested heavily in new tooling and materials.  The 12AX7B is one of the latest generation tubes to come from the chinese Shuguang factory and we feel is one of the best produced by them.  Despite Chinese tubes having a bad reputation over the years we feel this latest tube is excellent.  We found that the tube was very well balanced right across the frequency spectrum with little colouration noted in any part of the frequency curve.  These tubes worked the best in our Leak 20/20 Hi Fi amplifier where a very articulate and well balanced sound stage was noted.  In our guitar amp we were surprised yet again.  Clean the tube sounded neutral and the true tone of our Strat and Gibson 355 was faithful.  Driven into crunch these valves sounded really bluesy with a massive bottom end.  At high gain the tube was not as quiet as the other two tubes tested but this did not cause problems and was only apparent at extremely high gain settings.  The overdrive was smooth and detailed producing pleasing harmonics and overtones.  We liked these tubes and was surprised at how well they actually performed. particularly clean in Hi Fi and Crunchy blues."

 

http://line6.com/support/topic/930-no-more-hiss-w-new-12ax7-tubes/

 

So, the ACTUAL stock 12AX7B tube that Line6 / Bogner use in the DT25 is NOT ELECTRO HARMONIX. Not unless EH also owns Shuguang brand too... Hence, made in China, not Russia. Feeling thankful that I saved the two stock "China" 12AX7B tubes that came out of the amps; just in case...

 

Will be having my amp shop put the EH branded Russian 12AX7's in them, wish me luck.

 

http://www.harmonycentral.com/forum/forum/guitar/acapella-28/1481653-

 

For what it's worth, the China 12AX7B is the "9th generation" and is supposedly a better tube than the 8th generation 12AX7A China. Go figure, that's going full circle all the way back to the contested DT25 FAQ that states the DT25's get Chinese 12ax7, the DT50's get EH Russian 12ax7.

 

Chinese Shuguang 12ax7B's.. No wonder Freidman, Splawn, Bogner.. use as stock tubes.

http://www.thegearpage.net/board/showthread.php?t=1452243

 

http://www.rig-talk.com/forum/viewtopic.php?f=3&t=76359

 

""As per Charley at Bogner HQ, their latest releases in the XTC and Uberschall are ALL Chinese 12AX7B."

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I have been told by Line 6 from the beginning to use short plate 12AX7s in the Bogner amps....SVs too...In the DT25 there is only one 12AX7 and it is an inverter...part of the power amp....

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I have been told by Line 6 from the beginning to use short plate 12AX7s in the Bogner amps....SVs too...In the DT25 there is only one 12AX7 and it is an inverter...part of the power amp....

 

Got my fingers crossed. I was hearing some unpleasant general hum and buzz from both the DT25's after I had the shop put the JJ's in all tubes on both. That scared me; because shortly after I was reading all these posts and threads about the Bogner design requiring EH brand only just spooked me too much to use them until I get them retubed with EH. I played some when I got them home; sounded good, just some noise floor / hum / buzz that probably shouldn't be there, or should be lower in volume. 

 

I just got the two sets of EH EL84 and EH12AX7 tubes today, all Russian made / Electro-Harmonix brand, so I am hopeful that settles it. Not sure how long I have to wait to find out; dropping them off Friday morning, and will probably have them back this time next week, hopefully sooner..

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JJ's have a tendency to damage certain parts of the processor board, 

Mesa's tend to "red plate", and Groove tubes tend to do a host of other

things.

 

What do you know about Tung Sol, do they have any negative tendency?

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This is not as mystical as it is being made out to be...Unfortunately, getting a bad tube happens more often now than it did in the past. As the inventories drop, the will continue to be the case. I think these days, the Russian tubes tends to be reliable...I tend to stick with Russian on power....12AX7s you can do what you like...just stay away from the long plates...

 

I am about to order new power tubes for my DT-25...I really want to try a nice pair of 7189s in this amp....but those things are $$...hard to find real ones...

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https://www.tubesandmore.com/tech_corner/12ax7_comparison_of_current_made_tubes

 

Anyone care to speculate on how the pair of Electro-Harmonix Russian 12AX7 tubes function differently in the DT50 than the single Chinese 12AX7 in the DT25? Never mind; just answered my own question I guess; the DT50 has the variable 12AX7 boost, which the DT25 does not.

 

If it was a preamp tube, it would be a different logic behind why/which/how.

Leaves me wondering / curious, why they used two different 12AX7 options between the DT25 and the DT50.

I think as spaceatl mentioned, it's not a huge mystery - it seems to be rooted in supply and demand; limited new production tubes that fit within a certain quality range I suppose. Not surprised to find China making strides and improvements in factory production quality - I mean, the DT50, DT25, the HD500x - those are all assembled in China too....

 

So, Oddly, my first impulse reaction to it being a Chinese tube meaning it was lower quality, was apparently not accurate.

Lots of reports saying the newest round of 12AX7B from China are actually being used in a lot of gear these days, and Bogner in particular actually calls them out as being such in the specs of their other amps, based on the slot.

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What's a good bias for jj's in a dt25? I find it hard to understand why this is difficult, its not like there's magic and voodoo involved. The different properties of eh's and jj's should be well known, so a suggestion for a bias shouldn't be too hard. What say you, tube geeks?

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What's a good bias for jj's in a dt25? I find it hard to understand why this is difficult, its not like there's magic and voodoo involved. The different properties of eh's and jj's should be well known, so a suggestion for a bias shouldn't be too hard. What say you, tube geeks?

innovine, you got your amp up and running yet?

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JJ's have a tendency to damage certain parts of the processor board,

 

Which parts, exactly, and why? Citations needed.

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innovine, you got your amp up and running yet?

Yep, thanks for asking. The tube protection fuse went. I'v a 315mA in there now until I find a 375mA. Seems ok so far. Works using the eh's which I put in when troubleshooting, and not the jj's which were in it at the time the fuse went. I'll give them a try soon and see what happens, but wouldn't be surprised if one died and took the fuse with it. I'm gonna try a colder bias (20mv?) with the next jj's. I'm hoping that some of the tube gurus will suggest a reasonable bias value rathe than more scaremongering about needing to use eh's because of magic characteristic curves and bogners hot rod design. That's all just guff.

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innonovine: "JJ's have a tendency to damage certain parts of the processor board--- Which parts, exactly, and why? Citations needed"---

 

Unless you're an authorized service tech and signed an NDA (non-disclosure form),... I can't get into that

any deeper than I already have. Sorry.

 

Don't use JJ's, use the specified EH's,... the JJ's will go outside the linear region of the characteristic curve too easily,

and that will create problems further downstream, and stress the circuits.

 

And I would have to have give an online course on tube amps for the uninitiated.

 

Right, there is no voodoo,...  just knowing what the technology is and where the limits are. 

Don't go beyond the limits,... physics is unforgiving in that way.

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Load of lollipop.

 

Innovine, did you find that the JJs had a better noise floor than the EH power tubes?  Have you ever tried a different tube in the inverter slot, and if so, did that help with the noise?

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If you were to take transparencies of the characteristic curves of the the different

brands of EL34's and EL84's, and overlay them, the linear regions of the transfer

may line up, but it's the where and how non-linear regions of the curves differ that

will create the problems.

 

I've built my share of tube amps, I've been servicing tube amps for decades. No voodoo,

no lollipop,... it's just plain physics of electronics.

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If you were to take transparencies of the characteristic curves of the the different

brands of EL34's and EL84's, and overlay them, the linear regions of the transfer

may line up, but it's the where and how non-linear regions of the curves differ that

will create the problems.

 

I've built my share of tube amps, I've been servicing tube amps for decades. No voodoo,

no lollipop,... it's just plain physics of electronics.

so Bogner got these amp's mojo from the non-linear regions?

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For guitar amps, Bogner knows how to deal with it, he's good at it.

Audiophile tube amps are another beast and specially all their own.

And there are techs like me who deal with both,... and radio station transmitters as well.

 

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