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spaceatl

My DT-25 Bias Info

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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.

What I find interesting is that after I got mine back and played it for about 12 hrs. It too quit. It's still at Line 6 getting scrutinized.

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Hopefully that won't happen to mine and I hope yours gets fixed quickly.

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Here’s a video detailing replacing and biasing tubes in the DT-25 combo amp. 



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Recently while playing my amp suddenly dropped volume drastically. I smelled a slight burning smell.  These were tubes installed by Line6 (I sent the amp in last year for another problem).

I figured it was the tubes, so I replaced them, but when I went to bias them, the weirdest thing happened.  It was real low (7-9mA), but when I raised it, it rose out of control to 40-50mA, then suddenly shot down again.  Happened every time I tried turning it up even slightly - I couldn't get it past 14mA before it would repeat the cycle.

Thoughts?

(please leave out replies of "send it in")

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Depending on how much you play and how hard you drive the amp,...

... tubes should be swapped out every 12-18 months if you're a gigging musician,

if you're living room rock star who plays at home on the weekends, then

every 20-24 months should suffice. 

 

I've seen new tubes last two weeks, I've seen new tubes last 5-years.

No matter how well the tubes are scrutinized and checked at the factory,

there is always the element of chance. It's like that with all electronics.

 

If it's been a year and you're a gigging musician, then it's probably time

to change the tubes out. Use the specified tubes, matched pair power tubes.

 

Sorry, but I'm going to say it, take it to Line 6 authorized tech to check and

make sure that if the tubes did go down, that it didn't take anything else down

with it. Basic good idea for any piece of electronics gear, tube or solid state.

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Recently while playing my amp suddenly dropped volume drastically. I smelled a slight burning smell.  These were tubes installed by Line6 (I sent the amp in last year for another problem).
I figured it was the tubes, so I replaced them, but when I went to bias them, the weirdest thing happened.  It was real low (7-9mA), but when I raised it, it rose out of control to 40-50mA, then suddenly shot down again.  Happened every time I tried turning it up even slightly - I couldn't get it past 14mA before it would repeat the cycle.
Thoughts?
(please leave out replies of "send it in")

 

 

you reported in another thread that you put in JJs, and psarkissian answered there to only use EH tubes, correct?

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Hi everyone, I've replaced tubes and set bias for the second time now and noticed that when my class amp switch is set to Class A my V3 voltage goes up to 10 volts.  My V2 stays at 22.7 mv.  When on Class A/B both are within 2 mv of each other.

 

V2 is 25.3 and V3 is 22.7

 

Anyone else noticed this and is this normal?

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Hi everyone, I've replaced tubes and set bias for the second time now and noticed that when my class amp switch is set to Class A my V3 voltage goes up to 10 volts.  My V2 stays at 22.7 mv.  When on Class A/B both are within 2 mv of each other.

 

V2 is 25.3 and V3 is 22.7

 

Anyone else noticed this and is this normal?

 

I'm not sure about the actual spec but I think that may be the nature of a "hot rodded" tube amp which these are considered.

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The tube with a 10 volt bias has a problem. The amp needs to be taken in nd checked out. 

Maybe it's nothing more than a failed tube, maybe it's something else. Needs to be checked 

out by a tech who knows the product, preferably an authorized tech. 

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This is happening on brand new tubes.  Anyone know of a good amp tech in the St. Louis area?    I only found one called ATD Guitar Repair.

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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

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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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?

 

This is interesting to me. I have a DT50HD that petered out early in it's life. So I ordered matched EH tubes, purchased a tool so I could bias it and I had a similar 4mv difference in the power tubes. So if the tubes were supposed to be matched there must've been something else wrong or the tubes just weren't matched very well. In the end for me the amp petered out again with the new tubes which didn't have any real time on them at all and it went back to Line 6 to get sorted out.

 

When I got the amp back it worked and sounded great so I didn't touch anything. I was curious to check the bias and see what it was when it came home but I bit the bullet and left it alone so I don't know how matched they were or what the difference was in mv's between the tubes.

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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?

 

My guess is you are probably just fine since you are using matched quartets and have been for a little while...

 

You could use a couple of bias probes (Weber) to check it to remove things like the test point resistors being +-5% tolerance...or to just give yourself some piece of mind as it is a superior way to check bias vs the test points.

 

I do stand by my first post in this thread...Measure the plate voltage and adjust to 70% dissipation...These are hybirds and running them at 80% just makes them run hotter...and that isn't good for the DSP board. At least, I feel that way about the DT-25 head...

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Hi everyone, I've replaced tubes and set bias for the second time now and noticed that when my class amp switch is set to Class A my V3 voltage goes up to 10 volts.  My V2 stays at 22.7 mv.  When on Class A/B both are within 2 mv of each other.

 

V2 is 25.3 and V3 is 22.7

 

Anyone else noticed this and is this normal?

I can tell you that upon my latest tube change, I started with V2 about 22mV and V3 around 27mV. when I stepped through my HD500 presets, any patch that used Voice III on the DT (which is the only voice where I use Class A) they were lower in volume than before the tube change. I swapped V2 and V3, hoping to bring the voltages closer (didn't), but with the hotter tube in V2, my patches were all leveled again. I had created the patches using a pair of tubes where V2 was hotter. This leads me to believe that, at least with class A/Voice III, one needs to take into account tube voltage along with everything else when leveling patches. One more thing to keep track of...

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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)

 

 

Can I do this with just a multimeter? If so, could you give some details as to exactly how to proceed with measuring my plate voltage? Y'know without electrocuting myself? Don't want to go probing dangerous holes. I am interested in implementing your 70% philosophy with my next tube change. Thanks for spreading the knowledge.

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jet city have had something like this for years: http://www.jetcityamplification.com/#!retrovalves/ctjr

Has anyone any experience using  SStubes with the DT25 in the V1 socket? 

 

From the mnfctr site: 

Before you buy, make sure that the anode (plate) voltage in your device at is  least 30V.

the DT is ~ 400V right?

 

Still no  SStube equivalent for power tubes.

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Can I do this with just a multimeter? If so, could you give some details as to exactly how to proceed with measuring my plate voltage? Y'know without electrocuting myself? Don't want to go probing dangerous holes. I am interested in implementing your 70% philosophy with my next tube change. Thanks for spreading the knowledge.

 

Yes, and you can do it 100% hands free also...But if you do not already know this, I would suggest you get some assistance in taking the measurement. The risk is simply too high and there are much better safety resources than some dude on some forum.

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1) I would suggest you get some assistance in taking the measurement.

2) The risk is simply too high and there are much better safety resources than some dude on some forum.

1) I'm trying to get assistance from you!   2) some other dude from some other forum?

 

c'mon, it can't be too awful bad. look at it this way: if I succeed, I can't sue you, if I fail, I'll be dead and can't sue you. You are clearly an expert and have experience with this specific task and this specific amp. Pretty Please?

 

the stuff I found on other forums is specific to other amps. This is from the Marshall forums: 

 

close-Enough Plate Voltage can be Taken with Power tubes in place, If you want dead-nuts-on remove the tubes. Set Your Meter to DC Volts in the range that applies (+500VDC) Both Power switches ON, Ground the Black lead to Chassis, Red Lead to Pin 3. BE EXTREMELY CAREFUL, You can DIE. Best to Clip the Black Lead on and use the red probe with one hand, Keep the other hand in your pocket.

 

it seems intuitive that I would be measuring with the tubes removed from my DT25 combo. Do you have a speaker load connected? Are you measuring from pin 3? Pin 7? Can you confirm if this pin numbering diagram for EL84's is accurate? And if this is wiring side of socket shown, do I have to horizontally flip the image to get an accurate picture of what I'm seeing as I look at my amp sockets from the plugin side?

 

 

 

power_tube_pin_diagram_EL84.jpg

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With the DT25 there are ports for you to put your meter probes into

for metering the voltages for biasing. DO NOT go directly to the sockets

to take measurements. This is High Voltage stuff,... leave it to the pros.

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With the DT25 there are ports for you to put your meter probes into

for metering the voltages for biasing. DO NOT go directly to the sockets

to take measurements. This is High Voltage stuff,... leave it to the pros.

Im trying to measure the plate voltage, not the bias voltage. If I can do it with a multimeter, the only difference between me and "the pros" is the correct knowledge. You guys are the pros, so school me. I already did some research, I just need you guys to help me take it to the hoop.

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There is no user access through the ports to the plate voltage for good

reason, it a High Voltage Shock Hazard. And there's no reason to be going

to the plate voltage.

 

In school, they teach us us to stay away from the plate voltage,

it hurts when you get zapped. It's High Voltage, stay safe,... leave it alone.

 

Been doing this this more than four decades, and was trained by top

people in this profession. And if I slip, I still get zapped every now and then.

It hurts, leave it alone. Don't do that.

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I used the Dual Bias Tester from AMP-HEAD once to check plate voltage and bias of my DT50HD. I thought it was worth the bucks to keep me safer.

 

 

I checked their web site and they don't offer the PV switch for the EL 84 probe anymore.

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Don't recall us ever posting plate voltage info, since it's not something

for end users to be messing with in the first place.

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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...

I understand that there is no way to get inside the chassis to measure the plate voltage with the tubes in. But it sounds like it can be done with the tubes removed from the socket. I'm pretty sure I can clip my DMM black probe to the chassis and red probe goes to pin 3. I think at this point I just need to know if I need a speaker load connected when the tubes are out and I power on. Also does it matter if I'm measuring from V2 or V3. Hopefully spaceatl will weigh in with something useful. 

Tube_Bias_DT25_Line6_pin3.jpg

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There was no way for me to get into the chassis in my measure measurements...I think I posted a pic of empty socket with the probe in it...

 

I generally keep the load connected anyway just because I am going to need it connected later when the tubes are back in and I don't like to forget that...just a good habit methinks.

 

BTW, technically it is "possible" to safely power up a tube amp without a load as long as there is zero signal applied as there would be little flyback voltage to burn up the output tranny...But not a good idea. However,  with no power tubes, there is actually nothing going through the output tranny because the circuit is open...so the presence of the load makes no difference with the power tubes removed...

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Yes,... what spaceatl said.

 

As soon as you put a power tube in and power up, there is a current running thru

the primary side of the transformer. If the secondary isn't correctly loaded on the output,

the transformer fails at the primary. I've seen red plating, blown impedance matching

and power tube plate resistors burned.

 

Best leave it to the pros. Would hate to see someone void their warranty 'cause they got

curious or wanted to hot rod an already hot rod.

 

And these are hybrid amps, not like the analog tube amps of the 1960's. These Bogner

designs are not forgiving when it comes to mods or hot rodding, like tube amps of the 1960's.

And Rheinhold Bogner is already a hot rod master of tube amps. So stick to specified tubes.

 

For those unaware of the source of the term "hot rod", it comes from the 1950's and 60's.

Tricked out race cars had engines where the intake, exhaust and piston rod got hot when racing.

To "throw a rod" is to have a rod break down in the engine, and sometimes blow thru the engine

block (throw a rod), and "blow the engine" in the process. It's not pretty, and is very messy.

 

So, please don't "throw a rod" in your Bogner design L6 tube amp. And don't void any warranties.

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I generally keep the load connected anyway just because I am going to need it connected later when the tubes are back in and I don't like to forget that...just a good habit methinks.

 

Please add/edit

 

Measuring plate voltage on a DT 

1) Remove power tubes with amp unplugged from AC.

2) AC Plug in, speaker connected, power and standby both on.

3) Class A/B, Master vol down

4) DMM set to DCvolts @ 500V

5) Black clip to chassis, Red to pin 3

6) Note voltage

7) Power down, unplug AC and put tubes/everything back together again. 

 

From there, math and Ohm's law tells me where to set my 70% bias for my DT.

 

Izzat about right?

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So, please don't "throw a rod" in your Bogner design L6 tube amp. And don't void any warranties.

I'm guessing you're probably under corporate obligation to dissuade modifications for liability purposes. I'm not trying to mod the amp, and my warranty's been over for a long time. So - just trying to make the DT run lean. I want it to last forever. I love this amp. I would never hurt my baby. Which is why I turn to you experts on the DT forum.

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"I'm guessing you're probably under corporate obligation to dissuade modifications for liability purposes"----

 

No. I'm under no obligation to be here at all. I'm a tech,... here on the forums as a courtesy. I'm trying to keep

everyone's gear from ending up on my bench. I've been dealing with tube electronics since the 1970's.

 

There are those out there who may know a thing or two. There are also those who don't, and they are the

ones I worry about. Trying to keep your product experience a positive one.

 

I see too many tube amps end up on my bench because someone didn't take my advice. Just trying to

keep everyone's gear from ending up on my bench. I don't need the extra job security (for anyone wondering).

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"From there, math and Ohm's law tells me where to set my 70% bias for my DT. Izzat about right?"----

Sort of,... at least for an analog tube circuit like the old Fender circuit schematic I saw in an earlier post.

Can't treat a hybrid like this with a mix of analog and digital, in quite the same way as the old 1960's/70's

amps. It's an assumption that gets people in trouble, and gets their amp onto my bench.

 

It's an amp, play some rock music thru it,... don't mess with it.

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... don't mess with it.

 

 

 I'm glad to have you address this thread, but you're not being specific about any of your vague warnings against taking a measurement on my amp that will inform me about where to set my bias. The OP was of the mind that setting bias at 70% was the way to go. And assessing the plate voltage was part of that measurement. It seems like a really simple and relatively safe procedure, IF you know the procedure. 

 

Is my procedure wrong for measuring the plate voltage on the DT?

Is measuring the plate voltage correctly on the DT somehow harmful? (for the amp OR the operator)

Do the calculations for obtaining your 70% value pertain to the DT? e.g. ((Max Dissipation / Plate Voltage) * 0.7) * 1000 = bias mv

 

I'm paraphrasing here but, OP claims that 70% lets the amp run cooler and could be beneficial to the longevity of the DT. And 70% bias setting has been kinda the "standard" bias setting for tube amps for decades.  We know that the published bias recommendation for the DT25 is 25mV+-3. OP's measurements indicate that's up in 90% range (for his amp, anyway) 70% would obviously be more in the 21mV range. Which would presumably run cooler. 

I am using "cooler" in thermodynamic terms, although I understand that there is a tonal component as well.

 

Is there any truth to the notion that 70% will run cooler on the DT? e.g. 21mV bias runs cooler than 25mV bias

Are you saying that running your bias at 70% could be/is harmful to the DT? 

 

Or are you saying, officially, that a 25mV bias setting will run plenty fine on my DT25 for years and years of awesome, juicy tube goodness and that it'll save me the time and brain power if I just don't bother with it cause it won't make any difference for the better?

 

If you answered those important specifics, or any of the specifics I've brought to the discussion, we could maybe put this investigation to bed and get back to rocking out.

 

I appreciate your expertise. 

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The specified bias is what's optimal, that's why it was set as the bias.

It has nothing to do with running cooler. Bias is used to keep the signal

and the transfer function within linear regions to prevent non-linear distortion.

 

Under biasing is just as bad as over biasing. Under bias can also lead to

drop-outs at certain frequencies at certain power levels. Not to mention the

non-linear distortions.

 

The 70% solution is useful in 1960's analog tube system with one gain topology.

This is a hybrid system with four topologies, and can not be treated the same way.

 

Use the specified tubes at the specified bias and everything runs well. Bogner

designs are very unforgiving when you try to go outside of the specs.

 

I have these kinds of talks with all the tube people (Bogner, Pittman, Matthews, et al)

at the trade show every year.

 

My expertise,... this is not a 1960's one topology analog tube system. Don't mess

with it, stick to the specs.

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And as far as tubes for DT series amps go,... stick with the EH version from Electro-Harmonix.

And before someone says that all an EH is, is a Sovtek with EH painted on it,... EH owns the

Sovtek factory in Russia. The EH's are further scrutinized premium versions for clients like us

who want a level of reliability in the tubes we get.

 

I pushed for better QC in tubes a few years ago with the Spider Valve series, and that's carried

on to through to the DT series. Mike Matthews (owner of Electro-Harmonix) sees to it that QC is

top notch with the EH series tubes. I talk with him about this every year at the trade show. He's

quite the gadget geek.

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My expertise,... this is not a 1960's one topology analog tube system. Don't mess

with it, stick to the specs.

 Ok - I'm fine with taking your advise about using the recommended 25mV bias and not bothering with setting at 70%. And you offered plenty of specifics on why I should, so thanks for that - but you still didn't address any of the specifics of the procedure for checking the plate voltage on the DT. I probably won't bother at this point now that I understand the lack of benefits, but just for science's sake I'd like to know since this is still somewhat mysterious. I'm just trying to educate myself and by proxy the other members of this forum, and right now theres just a bunch of my speculation posted on that procedure. Can someone please, PLEASE address that, so we can put this to bed entirely? Thanks in advance...

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I can't get into circuit level stuff with an end user.

We don't do self servicing circuit advice. I can't walk you thru this stuff.

 

I only get down to this circuit level stuff with factory authorized service centers

if they need a tech assist or query. And they already know this stuff to begin with.

 

Knowing the plate voltage would be useful in calibrating a UREI tube compressor

or limiter (which I used to service and calibrate).

 

Beyond that, it's of no use here, and I've never needed to check plate voltage on

any of our tubes amps, accept to assist in putting together a service manual for

the service techs.

 

Sorry I can't get any deeper into this with you.

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Sorry I can't get any deeper into this with you.

 It's cool - I appreciate your followup and input. (pssssst - you could always mssg me regarding my pv procedure question wink wink nudge nudge simple yes or no even - I'll keep it off the forum.)

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