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#41 psarkissian

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Posted 07 August 2015 - 07:36 AM

Bottom line,... use the specified tubes. Yes RealZap, there are reasons 

for that. I could get into a long dissertation on that, and a list of examples

of what has come across our benches. The consequences are that these

amps end up on our benches.

 

-It has to do with characteristic curves of the tube specs.

-The way Bogner designs the amp sections.

-Spider Valve, Alchemist and the DT series have a digital section for processing,

so these are basically hybrid amps.

-Since these are hybrid amps, you can't get away with the same kind of tube

swapping like we used to do back in the 1960's and 1970's.

-You can't use the same bias on other tubes as you do with the EH's.

-EH's go through more scrutiny.

 

Consequences are red plating which damages tubes and surrounding circuitry,

blown digital boards and other such failures. Bottom line,... use the specified tubes.

 

And RealZap,... keep up the good work.


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#42 jegler

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Posted 07 August 2015 - 12:23 PM

I finally plucked up the courage to change and re-bias the tubes on my DT25 combo. I didn't get all scientific about it just changed the tubes and adjusted the bias with a multimeter. Used a pair of EH's from a matched quartet. I posted a video of it on youtube:

 

 

As a post script to this, I can say that the EH's have sounded great for over a year now...


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#43 ice9mike

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Posted 16 August 2015 - 10:08 AM

Is there a recommendation for hard, medium, or soft tubes? What does the Dt25 come with?


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#44 psarkissian

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Posted 17 August 2015 - 07:24 AM

I don't have a spec on that. I'll have to check with someone

and get back to you.


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#45 ice9mike

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Posted 17 August 2015 - 02:14 PM

I don't have a spec on that. I'll have to check with someone

and get back to you.

 

Please do, I want to do what is right for my amp and the big box stores make it difficult to tell which EL84EH is the right version. I would think "Medium" is probably the one to get, but I want to make sure.


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#46 psarkissian

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Posted 17 August 2015 - 03:16 PM

Oh, and BTW,...

it's a good idea to use a variable AC (or VariAC) when biasing any tube

amplifier in general,... including the DT series amps, and maybe our other

tube amps as well. Wall mains can vary a little bit from region to region.

Just generally a good practice.


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#47 psarkissian

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Posted 19 August 2015 - 08:52 AM

Been in touch with a guy at EH, just to confirm. We have a mix here

in repair of the EL84EH, for the DT25's. The plate currents (Ip) are less than 30.

 

Upper 20's (25 and up) to 30 are Hard, we have some between 20-24 which

are Medium, and we have a few at less than 20 which would be Soft.

 

DT50 series uses EL34EH, so the ranges will be a little different.


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#48 mtreehugger

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Posted 19 August 2015 - 09:02 AM

Been in touch with a guy at EH, just to confirm. We have a mix here

in repair of the EL84EH, for the DT25's. The plate currents (Ip) are less than 30.

 

Upper 20's (25 and up) to 30 are Hard, we have some between 20-24 which

are Medium, and we have a few at less than 20 which would be Soft.

 

DT50 series uses EL34EH, so the ranges will be a little different.

 

Which is OEM?  (soft/medium/hard)


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#49 psarkissian

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Posted 19 August 2015 - 09:45 AM

It's more physics in general industry than OEM, though

the OEM may have their own augmentations and preferences

as an adjunct to that standard.

 

It's a sliding curve range, proportional to the plate current (Ip)

and transconductance (Gm), within that variety of tube, that

determines hard, medium or soft.  

 

I'm preparing a posting for the Knowledge Base on the

subject of vacuum tubes as it relates to our product.


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#50 ice9mike

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Posted 19 August 2015 - 10:09 AM

It's more physics in general industry than OEM, though

the OEM may have their own augmentations and preferences

as an adjunct to that standard.

 

It's a sliding curve range, proportional to the plate current (Ip)

and transconductance (Gm), within that variety of tube, that

determines hard, medium or soft.  

 

I'm preparing a posting for the Knowledge Base on the

subject of vacuum tubes as it relates to our product.

My question is now, does Line 6 recommend one of these varieties? And what if any tone differences can be expected from each?

 

Thanks for your quick answers.


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#51 psarkissian

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Posted 19 August 2015 - 11:03 AM

Part of it also has to do with the proportion of vacuum vs gas.

 

Soft--- more gas (a gassy tube), less vacuum, saturates more easily, so it distorts more easily.

 

Hard--- less gas, more vacuum, higher saturation point, harder to distort and so more headroom.

 

Medium--- in between, has good headroom, but will distort if you push it.

 

 

That and the proportion of plate current (Ip) verses transconductance (Gm) are determining factors.


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#52 CipherHost

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Posted 19 August 2015 - 01:57 PM

Does this mean DTs were manufactured and sold with a variety of hard, soft, and medium EL84EH tubes?


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#53 psarkissian

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Posted 19 August 2015 - 02:17 PM

Not sure, possible,... the tubes are acquired in batches,

and there will probably be minor variation from batch to batch. 

One batch may be harder or softer than the another, note "er".

And yet, remain within a region that the amp likes.

 

There are periodical reviews of the tubes, and I chat with Mike

Matthews (owner of EH) about tube stuff every year at NAMM,

to get a heads up on his tube stuff. 


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#54 kawasami

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Posted 04 June 2016 - 06:36 AM

Thankyou very much jegler for the instruction video. By following it tube replacement for DT25 head was really easy. I replaced all tubes to new EH tubes and the sound improvement was huge.

 

I bought my amp used. After few weeks I noticed some problems with the sound. Voicing 2 played like guitar was out of tune. Voicing 4 had a lot of low end eaven when bass knob was all the way down. So I figured it is time for tube replacement and it was. Old tube bias were 21 and 32. New tubes are now 24,8 and 25,3. The quitar shop didn't sell these as matched pair but they told me that they choose me a good pair. I hope they are close enough. At least the sound is now great :)


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#55 psarkissian

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Posted 06 June 2016 - 08:56 AM

For the power stage, they have to be a matched pair.

The differences can cause "non-linear distortions" in the the gain.

And that's bad.

 

Might sound fine now, but if you bring the gain too high for too long,

non-linear distortions will stress it. Tube life can gets shortened.

Un-matched pairs are problems, "close",... is more than likely not close

enough. Or tubes wouldn't need to be sold in matched pairs to begin with.

 

These are not the tube amps of old like in the 1960's and 70's. These

amps are not as forgiving. If it's under warranty, get it done by an Line 6

authorized service center, and a matched pair of EH tubes.

 

Been inside tube gear most of my my life.


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#56 kawasami

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Posted 08 June 2016 - 08:52 AM

Thanks psarkissian. According to your advise I will order matched EH tubes just to be safe. Couldn't find them on my local shops so need to find online. Any advise where to order on Europe? I live in Finland.

 

I don't believe my amp is under warranty any more and replacing tubes is easy enough to do my self so don't see any reason to take it to service center. 

 

Just wondering has anyone measured bias after installing new matched tubes? How close are they? As I mentioned above my non matched new tubes are now 24,8 and 25,3. They work creat now but will replace them when I find matced set. 


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#57 psarkissian

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Posted 08 June 2016 - 09:19 AM

The video on biasing is pretty good. Check for other video tutorials.
Bias is on the Knowledge Base,... 25mV +/-3mV.

http://line6.com/sup...0dt25-faq-r144

Never power up a tube amp without the correct output load on it.


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#58 Brazzy

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Posted 08 June 2016 - 10:45 AM

The video on biasing is pretty good. Check for other video tutorials.

Bias is on the Knowledge Base,... 25mV +/-3mV.

 

http://line6.com/sup...0dt25-faq-r144 

 

Never power up a tube amp with the correct output load on it.

 

Do you mean to say "without the correct output load"?

 

I'm not busting stones but sometimes words can throw people off.

 

BTW, Any info you share I appreciate.


Edited by Brazzy, 08 June 2016 - 10:48 AM.

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#59 psarkissian

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Posted 08 June 2016 - 11:08 AM

Thank you,... thought I caught that.

Done, corrected. Thanks.


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#60 psarkissian

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Posted 08 June 2016 - 11:14 AM

Also, it's a good idea to use a variable AC, set it for your area AC Mains volt level,

when biasing your tubes. This applies to any tube amp, not just ours.


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#61 Brazzy

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Posted 08 June 2016 - 12:05 PM

Thank you,... thought I caught that.
Done, corrected. Thanks.


Glad to help. I'll be checking the knowledge base for updates as I think you said you'll be adding to it at a later date.
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#62 psarkissian

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Posted 08 June 2016 - 12:18 PM

Yes, the brand of tube for the power stage, was listed as Sovtek,
but the one we approved was EH.

EH owns the Sovtek factory, but the EH labeled ones get extra testing
and QC for the likes of us and other picky clientele.

Updated that early today (see the link a couple posts up).

http://line6.com/sup...r-reminder-r811

... and I'll be adding to this every now and again as the necessity and opportunities arrive.
The tube amp primer is an informational for general edification.

 

So please don't go into the insides of a tube (valve) amp, unless you have had some serious

training on this stuff (like I have). I don't want to read anyone's obits prematurely.

 


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#63 jegarcia

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Posted 11 June 2016 - 02:20 PM

@ Just Startin'

 

I also have a DT25 which I love. Given all the comments on this forum about replacement tubes I wanted to get a clear and concise response so I opened a support ticket with Line 6 and here is what they said.

 

1 - For the 12AX7 preamp tube you can buy Electro Harmonix, generic chinese, Ruby Tube, or Sovtek. 

2 - For the two EL84 power tubes you must use Electro Harmonix or Sovtek (Electro Harmonix owns Sovtek) 

 

While I have read about other brands that people have tried...my recommendation is that you stick with what Line 6 recommends. Also if you don't have training on amp repair and components installation....stay away from doing it. You can damage the unit or get hurt.

 

Peace out

 

JG


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#64 psarkissian

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Posted 14 June 2016 - 07:28 AM

Use the EH version of the EL84's when possible.

With power stage tubes, they have to be matched pairs, that's crucial.


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#65 jegler

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Posted 21 June 2016 - 07:36 PM

Use the EH version of the EL84's when possible.

With power stage tubes, they have to be matched pairs, that's crucial.

just hoping for some clarification on what kind of tube voltage differences are "allowable" when biasing the DT-25. I've bought my EH el84's in matched quartets, and then used a pair at a time. After my third tube change, I've noticed that the voltage difference between V2 and V3 can be up to 5mV. This latest swap left me at 22.4mV for V2 and 27.8mV for V3. Is this acceptable? DT-25 is rated 25mV+-3. does that mean one can be 22 and the other 28?   


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#66 psarkissian

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Posted 22 June 2016 - 07:56 AM

"I've bought my EH el84's in matched quartets, and then used a pair at a time"---

That works, that's fine.

 

The differences you're seeing is normal and expected, that's what dialing-in the bias is all about.

 

You can try taking the tubes and switching them to opposite sockets between them and see if it brings

the differences a little closer. It's an old analog tube amp trick used since the 1960's, used with push-pull

class tube amps back in the day. Sometimes,... sometimes it's useful.


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#67 Brazzy

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Posted 22 June 2016 - 08:10 AM

"I've bought my EH el84's in matched quartets, and then used a pair at a time"---

That works, that's fine.

 

The differences you're seeing is normal and expected, that's what dialing-in the bias is all about.

 

You can try taking the tubes and switching them to opposite sockets between them and see if it brings

the differences a little closer. It's an old analog tube amp trick used since the 1960's, used with push-pull

class tube amps back in the day. Sometimes,... sometimes it's useful.

 

I tried to switch the tubes and I still had the same 4mv difference. In my case it didn't change anything. Also, it was the DT50.


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#68 psarkissian

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Posted 22 June 2016 - 09:42 AM

Okay then, that fine.

 

When that's the case, it's how you know that, between that and bias,...

... that's as close as it gets.


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#69 Brazzy

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Posted 22 June 2016 - 10:01 AM

Okay then, that fine.

When that's the case, it's how you know that, between that and bias,...
... that's as close as it gets.

 

Ok Cool, thanks.


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#70 jegler

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Posted 22 June 2016 - 09:34 PM

"I've bought my EH el84's in matched quartets, and then used a pair at a time"---

That works, that's fine.

 

The differences you're seeing is normal and expected, that's what dialing-in the bias is all about.

 

You can try taking the tubes and switching them to opposite sockets between them and see if it brings

the differences a little closer. It's an old analog tube amp trick used since the 1960's, used with push-pull

class tube amps back in the day. Sometimes,... sometimes it's useful.

yes, thanks much for the info. psarkissian is on it. So can I say as long as one power tube is at least 22mV, and the other goes no higher than 28mV, I'm cool? Like, more so than normal? :)  If that's so, then I'm assuming that if I can't get to those tolerances, I should probably bail and  not use those tubes? Thanks again-


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#71 n9nu

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Posted 06 September 2016 - 02:38 PM

Hello

 

If you do decide to mess around with your amp while it's on, do yourself a favor and invest (if possible) in an isolation transformer. It could save your life should you accidentally 'light up' your body.

 

They can be had for under $80.00 or you can make your own.

 

 

 

An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons. Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings.

Isolation transformers block transmission of the DC component in signals from one circuit to the other, but allow AC components in signals to pass. Transformers that have a ratio of 1 to 1 between the primary and secondary windings are often used to protect secondary circuits and individuals from electrical shocks between energized conductors and earth ground. Suitably designed isolation transformers block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers, medical devices, or laboratory instruments.

 

 

Tim Dickerson

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ARS N9NU

 


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