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DT25 Tube Bias

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Hey everyone,

 

This is my first time swapping out tubes. I ordered a matched set of EL34's with 24-hour burn in.

 

I put the new tubes in, let it sit in standby for a few minutes, took it off of standby. I measured the bias on V2 and adjusted to 25 mV, I then measured V3 and was reading 35mV. I adjusted it to 25mV and then measured V2 again and it was 15mV.

 

So basically I have a 10mV difference between the 2.

I assume this is not normal. Any way to adjust them independently?

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Are you using the specified Electro-Harmonix versions of the tubes?

It's not an all analog tube amp, so using the specified tubes are recommended.

 

Also check with the Knowledge Base here on the Forum regarding bias voltages.

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Are you using the specified Electro-Harmonix versions of the tubes?

It's not an all analog tube amp, so using the specified tubes are recommended.

 

Also check with the Knowledge Base here on the Forum regarding bias voltages.

Yes Electro Harmonix brand

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I just looked at my receipt and thought that I had purchased a matched set.

Turns out they weren't a matched set.

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Hey everyone,

 

This is my first time swapping out tubes. I ordered a matched set of EL34's with 24-hour burn in.

 

I put the new tubes in, let it sit in standby for a few minutes, took it off of standby. I measured the bias on V2 and adjusted to 25 mV, I then measured V3 and was reading 35mV. I adjusted it to 25mV and then measured V2 again and it was 15mV.

 

So basically I have a 10mV difference between the 2.

I assume this is not normal. Any way to adjust them independently?

 

As far as I know there is no way to adjust them independently. I did the same thing you did when I had an issue with sound fading off after 15 minutes of play which culminated into no sound all together. All the fuses checked good so I ordered an ultra matched set of EH's and after making the bias setting had 4mV difference and I thought that was abnormal. I was thinking the tubes should read the same but I suppose they don't most of the time.

 

I'm not sure what the bias was new since I wasn't equipped to check it. I had another problem anyway which Line 6 fixed.

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Matched set is a must.

And be certain you have a stable AC main voltage when doing bias on a tube amp.

 

10mV difference is a bit too much.

From the Knowledge Base--- DT25 bias is 25 mV +/- 3 mV.

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I just looked at my receipt and thought that I had purchased a matched set.

Turns out they weren't a matched set.

 

That sucks.

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Yeah, sucks.

Take them back, see if they'll exchange it or something.

 

Not having a matched set would be too unstable, like running

a hot rod car at near red line,... with a mal-adjusted timing belt.

  • Upvote 1

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So I got a new set of MATCHED EL84's from Tubedepot.com (I highly recommend them) They are cheap and reasonable!!

Nonetheless, I put them in and they each biased to 25 mV +/- 0.5 mV.

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I hope they are EH brand EL84's. These amps are not forgiving of modifications.

They might come highly recommended for general use, but are they, when it comes

to a DT amp?

 

These are hybrid amps (analog and digital sections), so you can't do tube swaps like we use

to do in the 1960's and 70's, on an all analog tube amp.

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So just for future reference, how do you check the bias voltages on a DT25?

 

Craig

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Best to have an authorized service center do it, as they will have a VariAC

and service info to properly calibrate and bias with. 

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So just for future reference, how do you check the bias voltages on a DT25?

 

Craig

 

 

If and ONLY if you have previous experience and skills working on tube amps you might find this link helpful.  Voltages in there can kill you so be careful.

 

http://line6.com/supportarchivenew/thread/75238/

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Most definitely radatats. I've been dealing with tubes (valves) since my teens, 40+ years,...

... I still get zapped from time to time. And it hurts. Can't emphasize that enough.

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Thanks, it was more curiosity than anything else. I am an engineer by trade and I know that valve amps can bite very hard.

I have a Burman 502 combo which is in need of some tlc but it's far too scary for me :-)

 

Craig

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