Dt 25 blowing tubes and fuses, unequal bias on matched tubes

[Martijndrift 0]

Hello everyone,

 

I have been a proud owner of a dt25 combo for almost 3 years.

Since a few months ago I started having problems:

 

The first time, the tube protection fuse blew, I noticed the tubes were damaged, so I put a new set of matched JJ EL84 and a new fuse in.

When I wanted to bias the amp I noticed a difference in the bias rating of the tubes, where 1 tube would be at 21 mv the other was at 29 mv.

I immediately returned the tubes, the shop owner measured the tubes at different plate voltages, and everytime they were matched perfectly (I have the test reports). The tubes did not seem to be the issue

 

I used these tubes for about 3 months, with no sound issues whatsoever (4 hours a week at rehearsal volume) after which they blew again (along with the fuse).

 

I decided to order a set of matched sovtek el84 (like the standard ones), put them in and noticed where one tube would be at 22 the other would be at 28 mv. Still a difference, from that moment on I was pretty sure that it was a problem with the amp.

 

These tubes lasted for about 2 months after which one tube failed (one side of the tube was covered in a mirror like metal finish on the inside).

 

I took the amp apart, measured some components for the bias setting circuit, this seemed to be in order. Did not see any components that were visually damaged. 

 

I just assumed that I must be having alot of bad luck, I put in new tubes: sovtek el84m, again a matched, again a difference in bias voltage. These tubes lasted only about 10-12 playing hours.

 

At first there was a drop in volume as if one tube failed, 40 mins later the fuse blew and I noticed one tube being damaged as well.

 

I don't know if blowing the fuses/tubes and the difference in bias voltage are related, but I do suspect they are. I have quite a decent understanding how the power section of this amp works so I would like to fix it myself.

Also the amp was always in class ab pentode mode during failure (this is the only mode I use it in)

 

Has anyone had similar problems in their amps (or other amps with a class a/b push pull power amp for that matter). Or does someone know what the problem could be or which components I should start checking?

 

Thank you in advance!

[radatats 921]

you seem to know what you are doing but I didn't hear you mention actually adjusting the bias to match them up... I hope you did...  If not see the link below...

 

http://line6.com/support/topic/3327-dt-50-25-tube-biasing-tips/?do=findComment&comment=21863

[Brazzy 441]

Hello everyone,

 

I have been a proud owner of a dt25 combo for almost 3 years.

Since a few months ago I started having problems:

 

The first time, the tube protection fuse blew, I noticed the tubes were damaged, so I put a new set of matched JJ EL84 and a new fuse in.

When I wanted to bias the amp I noticed a difference in the bias rating of the tubes, where 1 tube would be at 21 mv the other was at 29 mv.

I immediately returned the tubes, the shop owner measured the tubes at different plate voltages, and everytime they were matched perfectly (I have the test reports). The tubes did not seem to be the issue

 

I had a similar issue with trying to get micro matched tubes balanced in the bias area. I started out with 36 and 32 on my DT50 and after rechecking it after approx. 10 hours of use I had 36 and 34. Seems like they are closing in. Don't know why it's like that, I can only think that the amp's design is in play here. Meanwhile I have more than 20 hours on it and have yet to recheck the bias to see where it stands. Did I mention it still sounds great and everything works fine. I've had this amp for about 10 months and have approx 120 hours on it currently and this is it's first set of replacement tubes. You can check out  this thread I created when for the first time I expereienced tube failure >>> http://line6.com/support/topic/6699-dt50-head-output-just-went-silent-any-ideas-why/?p=61241

[Martijndrift 0]

you seem to know what you are doing but I didn't hear you mention actually adjusting the bias to match them up... I hope you did...  If not see the link below...

 

http://line6.com/support/topic/3327-dt-50-25-tube-biasing-tips/?do=findComment&comment=21863

Yes I did, thats why I tried to balance the difference in bias voltage as close as I could to the 25 mv that line 6 implies.

Independently of the set bias value, the tubes should give the same value. The only reasons I could think of that affect the measured bias voltage (from a mechanical engineers point of view, I might forget something?) are:

 

Difference in the 1 ohm resistors, I measured these and they were withing the 1% deviation that is associated with these resistors.

Difference in tubes, ruled this out by using different sets of tubes.

Temperature of the tubes (different resistance with temperature), I let the amp warm up for about 20 mins before setting the bias, so I doubt this is the cause.

A defect screen resistor maybe?

 

@Brazzy, it's odd that there is a similar problem with your dt50, but then again I would be happy if I got 120 hours on the powertubes :P

 

Dispite of the difference in bias, which with my previous set of tubes was within the +-3 mv range that line 6 gives, it should not damage the tubes up to the point where they will blow in a couple of hours right?

[Brazzy 441]

@Brazzy, it's odd that there is a similar problem with your dt50, but then again I would be happy if I got 120 hours on the powertubes :P

 

Dispite of the difference in bias, which with my previous set of tubes was within the +-3 mv range that line 6 gives, it should not damage the tubes up to the point where they will blow in a couple of hours right?

 

Just to clarify, the orignal tubes that shipped with it "I think" lasted me apprx. 100 hours, then I started having wierd noise issues and finally no sound at all. After that it would work normal for about 10-20 miutes at that point there was no sound and the power tubes were out and cooling down. That's when I geared up for the power tube service. I'm still using the recommended replacement tubes. No fuses in the amp were blown.

 

As far as tubes lasting only a few hours? I would say it's possible to get tubes that aren't quite mnufactured correctly. As others have mentioned tubes can be a hit or miss kind of thing. I'm sticking with Original Equipment tubes for now.

[radatats 921]

When you put the new tubes in, check the bias.  Turn it all off and swap the tubes and recheck the bias.  If the hi/low volatage doesn't move with the tubes then the issue is probably in the amp.  Do they bias out evenly with adjustments?  If so that shouldn't be an issue either.  Recheck the bias after a few hours playing just to see if anything is changing.  Did you change the 12AX7 as well?  It has a role in the power section even though its not a preamp tube.  Are you certain you got the right fuse for the amp?

 

I am sure you did all these things, just brainstorming a few things to be certain...  If it keeps blowing tubes it needs a repair ticket...

[Brazzy 441]

When you put the new tubes in, check the bias.  Turn it all off and swap the tubes and recheck the bias.  If the hi/low volatage doesn't move with the tubes then the issue is probably in the amp.  Do they bias out evenly with adjustments?  If so that shouldn't be an issue either.  Recheck the bias after a few hours playing just to see if anything is changing.  Did you change the 12AX7 as well?  It has a role in the power section even though its not a preamp tube.  Are you certain you got the right fuse for the amp?

 

I am sure you did all these things, just brainstorming a few things to be certain...  If it keeps blowing tubes it needs a repair ticket...

 

I'm not having the same exact issue as the OP, although I did what you've posted....

 

I switched the tubes and checked after heating them up for more than 15 minutes, same results. I replaced all the tubes with EH, the recommended tubes and still had the same results/readings. leading me to the amp is the cause of the imbalance somehow. Although, like I said the bias balance has improved a wee bit with time, it's not exact, which I think it should be pretty close to being equal on both tubes. Even on the bias check after 10 hours when the bias seemed to be closing in a bit, I tried switching the tubes and the same readings. Switching the tubes didn't help.

[Martijndrift 0]

When you put the new tubes in, check the bias.  Turn it all off and swap the tubes and recheck the bias.  If the hi/low volatage doesn't move with the tubes then the issue is probably in the amp.  Do they bias out evenly with adjustments?  If so that shouldn't be an issue either.  Recheck the bias after a few hours playing just to see if anything is changing.  Did you change the 12AX7 as well?  It has a role in the power section even though its not a preamp tube.  Are you certain you got the right fuse for the amp?

 

I am sure you did all these things, just brainstorming a few things to be certain...  If it keeps blowing tubes it needs a repair ticket...

Thank you for generating some ideas!

 

Unfortunately there is only 1 bias adjust pot, this means that when I set one tube at 25 mV, the other is way off. So I try to set it in a way where both tubes are equally far off from the implied 25 mV. I had the same thought about the phase inverter, say if one side of it is damaged, than one tube in the poweramp might not be getting the same current. When I talked to a friend of mine about this he seemed quite convinced that the phase inverter would not cause a difference in bias voltage. But I wanted to try it anyway.

I have used fuses with a slightly lower rating (3.15 ma as opposed to 3.75) I could see this being the cause for blowing a fuse, but then again it only blows after a tube breaks, so I doubt this is the reason for my problem. I also only started using these after the original fuse blew. Just to be certain I'll order the right value.

 

Yesterday during rehearsal I put in some old sovtek el84 and the phase inverter that came stock in there. It did live for 3 hours, so I'll have to see in the next couple of weeks if it stays alive. The bias difference is still there though.

[podman999 0]

Hi, I have a DT25 combo with a similar problem. The stock tubes measured 31.3 in V2 and 22.8 in V3. I put a brand new set of JJ EL84 matched pair and a 5751 (12ax7). Switched it on and started playing. Got about 5 miniutes playing and the tube fuse blew. It happened before I had the chance to check the bias of the new tubes. 

Stuck a new fuse in with the old tubes and its fine. I'll try and re-bias the new tubes after the weekend. I have a gig to play. 

I too am surprised by the different bias values for each valve and cant get them to match. I'm also surprised that a new set of EL84's would cause so much of a problem? 

Valves bought from Watford Valves. Never had a bad one before.

 

Thanks.