Community: Message List - bias voltage dt25

Re: RE: bias voltage dt25

support@line6.com — Sun, 11 Nov 2012 19:39:13 GMT

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

Re: RE: bias voltage dt25

support@line6.com — Thu, 20 Sep 2012 22:18:19 GMT

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: bias voltage dt25

support@line6.com — Sat, 25 Aug 2012 20:26:34 GMT

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: bias voltage dt25

support@line6.com — Sat, 25 Aug 2012 17:05:30 GMT

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

support@line6.com — Sat, 25 Aug 2012 16:50:55 GMT

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

support@line6.com — Sat, 25 Aug 2012 16:52:14 GMT

Pics for visualization:

Re: RE: bias voltage dt25

support@line6.com — Sat, 25 Aug 2012 15:31:06 GMT

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: RE: bias voltage dt25

support@line6.com — Wed, 08 Feb 2012 17:36:11 GMT

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

support@line6.com — Thu, 29 Dec 2011 08:46:07 GMT

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/

sounds like 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

support@line6.com — Thu, 29 Dec 2011 08:40:17 GMT

great!!

keep us informed abouth you're findings

grts Marcel