These forums are read only, please use our new forums here.

Main :: DT50/DT25



bias voltage dt25
by MGblade on 2011-11-27 06:42:56

can,t find any specs on the dt25 el-84 bias voltage, a friend of mine is quite a skilled amp tech, i like to let him check/adjust the amps bias from time to time, so i need the recomended milivoltage setting.

hope someone can provide me with this info



Re: bias voltage dt25
by Nick_Mattocks on 2011-11-27 07:01:45

I don't believe Line 6 have released this information yet for the DT-25.

Nick



Re: bias voltage dt25
by Line6Tony on 2011-11-30 16:56:16

This is true. We have not published the bias voltage for the DT25 yet. I will try to get that and post it to the DT FAQ at http://line6.com/support/docs/DOC-2106.



Re: bias voltage dt25
by MGblade on 2011-11-30 23:57:44

thanks



Re: RE: bias voltage dt25
by Kneehow on 2011-12-01 14:41:51

I just updated our DT25/DT50 with the info you're looking for: http://line6.com/support/docs/DOC-2106

The DT25's EL84's need to be biased to: 25mV +/- 3mV.



Re: RE: bias voltage dt25
by spaceatl on 2011-12-01 16:07:07

+1...Thanks for getting that out...



Re: RE: bias voltage dt25
by andrewtorres on 2011-12-01 23:24:02

Hi, This is my first post. I purchased a DT25 head and cab last week. The amps sounds incredible but I am one of those guys that has to tinker with everything.

Anyway I had removed the back panel the night I got it home just to hear some different preamp tubes. I didnt find anything that made a dramatic difference. I ended up leaving a Tung-Sol 12AX7 in it and ordered some EL84 tubes online. They arrived today and so did the readings for the bias.

First off the bias on this head is very easy to test and adjust. You might even be able to test it and adjust it with the amp still in the cabinet and just the back panel off. I went ahead and pulled the head out of the cab by removing the 4 screws on the bottom. The tolex is super tight and sticky on this amp so you have to jerk on it a bit.

First I checked the bias with the stock EL84s. It was right in spec with at 26ish. I pulled out the stock electro harmonix and put in a matched pair of JJ EL844s. In case you haven't used these, they are a lower power EL84. They claim to be have 25% less power and have earlier break up etc etc. I just wanted to try them. . Anyway I put the EL844s in and the bias readings were around 16 so I adjusted each side to 25. I then put in a set of Groove Tubes GTEL84S. They have a 1 on them which I think represents the highest gain models. They registered around 30 so I brought the bias down.  Last I put in a pair of Sovtec EL84 and they were right around 22 or 23. All tubes were brand new.

I played the same guitar on all the tests. It is a Les Paul Traditional Pro with stock pickups.

I would love to say that a certain EL84 made this amp sound even better than it does out of the box. What I think I found out was this thing is pretty dialed in stock. I find this amp sounds better daily but I think it is the speaker breaking in.

For now I am going to try those JJ EL844s and a Tung Sol reissue preamp tube. Here is a write up on the EL844. http://www.ampgas.com/2011/01/13/a-look-at-jjs-new-6550-and-el844-tubes/



Re: RE: bias voltage dt25
by MGblade on 2011-12-02 04:56:29

Hi andrewtorres

can you post a little "how to bias the dt25 head" would love to do it myself,

is it easy to do for a first timer with some elektra expirience?

i've took of the back panel and saw the bias adjust pot, the ground hole and te two holes next te the tubes.

i saw some vids on youtube about biassing doesn,t seems to be that difficult.

would be great if you can post a little "how to"

grts Marcel



Re: RE: bias voltage dt25
by andrewtorres on 2011-12-08 22:27:42

Sure Marcel, I am by no means an expert. When I saw the bias locations I figured it had to be pretty easy. I have biased an amp or two in the past. Here is the steps I used in biasing  the DT25 Head:

1. Remove the upper panel on the back. It is six screws and then you might have to gently use a screw driver or some other object to pull the panel out. The tolex is sticky.

2. Remove the 4 screws on the bottom.

3. You have to use some force here but be careful and gently wiggle the amp out of the chasis. You might want to remove the tubes but it isn't required. Push from the front and pull from the back. Again the sticky tolex makes it difficult but it will finally come out. 

4. You will see round holes identified as Bias V2, Bias V3, Ground and Adjust Bias V2 and V3

5. Set your multimeter to DCV 200m or volts 200m

6. This is where I had to be creative. My multimeter tips were not long enough to protrude through the hole and touch the measurement point so 1 cut the tips off and stripped the wire back so about an inch was exposed. I then was able to stick the wire into the measurement hole. You will see there is a small goldish color hole down inside the hole cutout that you can stick the wire in. I stuck a black in the groud and red in the V2 and powered on the head. I let it warm up and then after making sure I had a speaker load and the other end hooked to my guitar I took it out of standby.

7. Make sure you are in AB mode and you should get a reading close to 25.0. If it's off, turn the adjustment screw with a small screwdriver until it is at 25 +- 3. I then put it in standby and moved the red lead to the V3 side and did the same.

So here is where are questions I have. The stock tubes were close to same voltage and within spec at around 25. I stuck in a matched set of Groove Tubes and it read around 29 on one side and 33 on the other. I adjusted them so one side was around 28 and the other 25. I then tried a couple other sets of matched EL84 tubes and had similar readings as the stock tubes. The tubes sound in my opinion was better the higer the voltage. Anyway for the questions:

1. Is it normal for matched tubes to be a couple volts off from one another?

2. Is biasing it up around 35 enough to cause amp damage?

3. I read Class A is Cathode and AB is Fixed. Does this mean Fixed is more sensitive to the bias readings?

4. I swear my amp is louder in Class A mode but Class A is supposed to be 10watts and AB 25 Watts. Am I missing something?

5. What are the advantages of running Class A over AB or vise versa.

Thanks everyone!



Re: RE: bias voltage dt25
by MGblade on 2011-12-09 07:37:05

this is great!!! thanks man

how do you like the JJ el844 tubes?? 

as for questions 4, class A seems louder becouse the tubes are wide open and wil saturate earlier so i think that's wy it wil sound louder earlier, i,ve noticed this too on the first time i switched from AB to A, but when the master volume is pushed class AB wil be louder with more clean headroom, but you have to go really(too) loud.

as for 1: i think a difference of  4 mv is too much for a matched set.



Re: RE: bias voltage dt25
by bozotone on 2011-12-28 12:18:35

I am an adicted tube roller and have a decent collection of old and new glass, so thank you so much for the specs.

I spent some time over the last few days swapping the 12AX7 phase inverter and EL84's. I first checked the bias on the stock EH 84's and found them at 31mV (both V2 and V3). I then rolled thru a set of Amperex's from '59 (they settled at 25mV and 28mV), a set of 1960 Mullards @26mV and 27mV, JJ's at 26mV and 26mV., and a set of real Teslas @27mV and 27mV. The Teslas in this circuit are, by a good margine, my favorites..great punch and smooth. The Amperexs came in second but a bit too much top end for an "overall" tube. If I were only doing AC30's or AC15's, this would have been killer. I am going to spend more time now with the P.I. and have a reissue TungSol 12AX7 in there now that works very well...



Re: RE: bias voltage dt25
by MGblade on 2011-12-29 00:40:17

great!!

keep us informed abouth you're findings

grts Marcel



Re: RE: bias voltage dt25
by Rowbi on 2011-12-29 00:46:07

andrewtorres wrote:

Hi, This is my first post. I purchased a DT25 head and cab last week. The amps sounds incredible but I am one of those guys that has to tinker with everything.

Anyway I had removed the back panel the night I got it home just to hear some different preamp tubes. I didnt find anything that made a dramatic difference. I ended up leaving a Tung-Sol 12AX7 in it and ordered some EL84 tubes online. They arrived today and so did the readings for the bias.

First off the bias on this head is very easy to test and adjust. You might even be able to test it and adjust it with the amp still in the cabinet and just the back panel off. I went ahead and pulled the head out of the cab by removing the 4 screws on the bottom. The tolex is super tight and sticky on this amp so you have to jerk on it a bit.

First I checked the bias with the stock EL84s. It was right in spec with at 26ish. I pulled out the stock electro harmonix and put in a matched pair of JJ EL844s. In case you haven't used these, they are a lower power EL84. They claim to be have 25% less power and have earlier break up etc etc. I just wanted to try them. . Anyway I put the EL844s in and the bias readings were around 16 so I adjusted each side to 25. I then put in a set of Groove Tubes GTEL84S. They have a 1 on them which I think represents the highest gain models. They registered around 30 so I brought the bias down.  Last I put in a pair of Sovtec EL84 and they were right around 22 or 23. All tubes were brand new.

I played the same guitar on all the tests. It is a Les Paul Traditional Pro with stock pickups.

I would love to say that a certain EL84 made this amp sound even better than it does out of the box. What I think I found out was this thing is pretty dialed in stock. I find this amp sounds better daily but I think it is the speaker breaking in.

For now I am going to try those JJ EL844s and a Tung Sol reissue preamp tube. Here is a write up on the EL844. http://www.ampgas.com/2011/01/13/a-look-at-jjs-new-6550-and-el844-tubes/

soundslike some great info there...

Just be careful with using different tubes.  the EL844 tubes will probably be fine, but I know that Bogner makes his tube amps to very tight specs, and also with all the smart switching in the amps poweramp section, I'd be tempted to just stick to EL84 tubes to ensure:

a, that nothing strange happens because the EL844 tube specs are a little different to what the digital controls in the amp expects to see...

and

b, that this doesn't void your warranty if you later get problems with the amp, and it then looks to be caused by using different tubes.

Just looking out for you chap

Cheers

Rowbi



Re: RE: bias voltage dt25
by joepaley on 2012-02-08 09:36:11

Total beginner here when it comes to biasing, couple questions about your post:

If the spec says 25, why not set them both to 25?  Why bias one at 25 and the other at 28?  Or are the two bias controls not totally independant?

Another question, elsewhere when I read about biasing, everyone is measuring their plate voltage.  Is that for some reason not neccessary for this amp?



Re: RE: bias voltage dt25
by Krontab on 2012-08-25 08:31:07

There is one bias adjustment screw so when you turn it both tube biases change. The spec says 25mV + or - 3mV. This means a range of 22-28mV is acceptable. This range is to accomidate the inherent variances in tubes so in his case the variance in the tubes was 3mV so he set V2 tube to 25mV and the V3 tube came out to 28mV. Theoretically he could have set V2 from 22mV to 25mV with an acceptable outcome as V3 would be ~ 25mV to 28mV given his variance. If you buy tubes in Duets or Matched pairs the variance should be minimal between the two.



Re: bias voltage dt25
by geppert on 2012-08-25 09:50:56

I am curious as to why Bogner only provided ONE bias adjustment for the TWO EL84's instead of one adjustment for each as they do in the SV's? This does not provide the ability to tweek the tubes to equal voltages if they happen to be different. I have a ~5.5 mv differential between my stock EH's and it bugs me that I can't tweek them right to equal. With a +/- 3 mv range of variance to the 25mv spec, I can "just" make into the window between these two tubes. I wonder perhaps if the stock tubes weren't actually "matched" in the first place? Anyways, I just bought a pair of matched Amperex EL84's so I have a spare set and also to have the ability try an alternative to the EH's. Once they arrive I'll put them straight in and see if the bias differential is closer than the stock EH's. I'll let you guys know the results. The truth I suppose in the end, is that this bias differential is quite acceptable and not a big issue. I just like the ability to fine tweek things to exactly the spec for each. I'll learn to live with it, and as my wife often says about a number of things I sometimes obsess over, "it's not the end of the world".



Re: RE: bias voltage dt25
by Krontab on 2012-08-25 09:52:15

Pics for visualization:

DSC_9479.jpg

DSC_9480.jpg

DSC_9490.jpg

DSC_9496.jpg

DSC_9482.jpg

DSC_9492.jpg

DSC_9491.jpg

DSC_9486.jpg



Re: bias voltage dt25
by Krontab on 2012-08-25 10:05:31

It is possible that the tubes were upset during shipping or that their tolarances at the factory for matched are higher than what we would buy as a consumer. To keep costs down I'm sure they work right within the functional limits, not aiming for optimal. In general matched seta are usually withing +/-3mV so +/-6mV max variance is more than enough. Why not 2 adjustment pots? I can't speak to the spider valve, but most amps I've seen tend to only have one bias adjustment per pair of tubes.



Re: bias voltage dt25
by geppert on 2012-08-25 13:26:34

Thanks for the info Krontab. I would have expected that on a $1000 head that each power tube would at least have it's own bias adjustment (my SV212 MKII does). The stock E.H. EL84's sound pretty good as does this DT25 amp in general. I still get more "hum" than I think is normal in Triode mode, and Triode + Class A mode but it is masked at band volume for sure. I'll be interested to hear the Amperex tubes when they arrive as an alternative. I don't expect it to be ground shaking however.



Re: RE: bias voltage dt25
by Stratman82 on 2012-09-20 15:18:20

Hey

I've just switched out the stock EL84s for JJ-Tesla cryo EL84s (a matched pair from Watford valves, UK). This was done to try and solve a "crackle and pop" problem which I deduced was probably due to the poweramp tubes.

The crackling problem is completely resolved so it seems the problem was indeed faulty stock poweramp valves (after approx. 7months use).

However, I'm new to tube amps and am now wondering if I should have the amp re-biased.... I don't think I'd want to venture this far myself so would probably pay for an amp tech to do it.

Is this really necessary and am I putting the amp and/or tubes at risk if I don't have it done? The amp sounds great following the tube swap.

Any advice would be much appreciated.

Thanks.



Re: RE: bias voltage dt25
by lef38 on 2012-11-11 11:39:13

andrewtorres wrote:

. Anyway I put the EL844s in and the bias readings were around 16 so I adjusted each side to 25.

They say these tubes must be biased 25% below the normal EL84 value , due to their nature. That is 18.75 +- 2.25 mV




The information above may not be current, and you should direct questions to the current forum or review the manual.