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Problem:
While playing recently, my DT-25 suddenly dropped volume significantly and had a slight burning smell.  I figured it was a tube(s) going out, so I bought some new ones and replaced them, but when I went to bias them, they were super low!  Instead of the 25 (+/-3), they were around 7-8.  I turned the bias, but then the weirdest thing happened - any turn up would start an unstoppable climb to 40-50, then it would suddenly kick down to 14 or less.  It would stay that way until I turned the bias up again, and then the same thing every time.

 

I checked the fuse, but it seems intact.  Since power was going to the tubes, I figure it can't be that anyway.

 

I may be wrong, but I suspected it may be kicking into Low-Volume mode as a safety feature.

 

Any idea what's going on?  How do I fix this?

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Can't bias the JJ's to the same bias voltages. The transfer

function curve and multiple topology gain structure of DT

series amps is not good for JJ's, Mesa's and Groove tubes.

 

I see too many DT series amps come across my bench because

of other tubes being put in the amp and biased to the same bias

volts as if they were EH tubes.

 

These are not tube amps of the 1960's and '70's, you can't get

away with the same kind of tube swapping like we used to do back

then. These are hybrid amps with analog and digital stages, both

solid state and tube. Can't get away with the same sort of things

like we did back then.

 

Please,... use the specified EH series. They've gone through a further

sorting through.

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Thank you, psarkissian, although the EH's were in the amp when the problem first occurred.  Was it a blown tube that cause the slight burning smell and the sudden drop in volume?

 

Also, and I know this is not the kind of question you were looking to elicit from you comments, but I'm curious what level bias you think JJ's should have.  Cooler?

 

Thanks,

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Depends on the smell. Whether it smelled like plastic, glass or metal.

 

The bias specs for the EH's has already been worked out in the designed

and test process, so it's a fixed spec.

 

As far as biasing JJ's, we have no spec for that. I would have to O-scope

it with a signal and dial it in to find where the bias point for the JJ's are.

 

Rheinhold's tube amp circuitry is not forgiving of deviations from the spec,

and matching our processing to it makes it even more hybrid and even less

forgiving of mods or deviations from the spec.

 

This isn't a tube amp from the 60's or 70's,... can't get away with the same

kinds of tube swapping we used to do back then. Use the specified EH's,...

it's the best bet.

 

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I hear ya, but it was with EH's installed by Line-6 themselves that this problem first occurred.  Not to mention this is the second time I've had an issue with the amp (and its tubes) and had to send it in to Line-6.  Not a sterling performance record on the EH's, so I think you can see my incredulity of the "reliability" of them, even as I get what you're saying about how the amp is spec'd.

 

The burning smell... I don't think it smelled especially metallic or or plastic.  Thoughts?

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Usage will determine how long the tubes last. Gigging musicians should

change them out every 12-18 months, bedroom, living room and weekend

rock stars ought to change them out every 20-24 months.

 

Over-biasing for more crunch, shortens the tube life. Making like John 

McGlaughlin or Al DiMeola is better for tubes than playing like a dark

master shredder king with the amp turned up to eleven.

 

I've seen fresh tubes (at other places I've worked) last two weeks in an amp,

or as long as five years (in that same amp).

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Usage will determine how long the tubes last. Gigging musicians should

change them out every 12-18 months, bedroom, living room and weekend

rock stars ought to change them out every 20-24 months.

 

Over-biasing for more crunch, shortens the tube life. Making like John 

McGlaughlin or Al DiMeola is better for tubes than playing like a dark

master shredder king with the amp turned up to eleven.

 

I've seen fresh tubes (at other places I've worked) last two weeks in an amp,

or as long as five years (in that same amp).

 

Does the master volume setting have any bearing on tube life?  What about pentode/triode?  i generally run in triode and always keep the master below 12:00.

 

(btw, only one g in mclaughlin, but i won't tell)

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You'd think after being a Mahavishnu Orch fan for four decades

that I wouldn't miss that one,... thanks.

 

Master volume, maybe just a smidge. Your settings seem fine.

Class-A Triode would run hotter than a Class-B Triode. And a

Class-B Pentode runs even more efficient.

 

If you gig regularly, the 12-18 months interval would be about

right for you (unless one goes prematurely). And the power tubes

have to be a matched pair.

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You'd think after being a Mahavishnu Orch fan for four decades

that I wouldn't miss that one,... thanks.

 

Master volume, maybe just a smidge. Your settings seem fine.

Class-A Triode would run hotter than a Class-B Triode. And a

Class-B Pentode runs even more efficient.

 

If you gig regularly, the 12-18 months interval would be about

right for you (unless one goes prematurely). And the power tubes

have to be a matched pair.

 

That's kind of interesting about Class-B Pentode being more efficient, just so happens I like that topology and exclusively use it. :)

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Update: So apparently my old EH tubes were not blown, and when i put them back in the same wonky stuff was happening, so something else was going on. Line 6 says there's no safety feature to kick it into quiet mode if the tubes get too hot, so i guess i gotta send it in :(.

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Update: So apparently my old EH tubes were not blown, and when i put them back in the same wonky stuff was happening, so something else was going on. Line 6 says there's no safety feature to kick it into quiet mode if the tubes get too hot, so i guess i gotta send it in :(.

 

Thanks for the update, I hope you get it fixed up. I'll be wondering how it's working out for you.

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Uuuuuuupdate!

 

So I sent it in to a local guy who replaced the tubes with JJ's (sorry, psarkissian, don't hate me) and somehow did not encounter the runaway bias drift I had.  Perhaps I was shorting something out with my cheap voltmeter?  Perhaps cleaning out the tube jacks really IS a big deal (I didn't try)?  

 

All I know is that was an expensive NON-problem to fix :P.

 

JJ's working great so far.

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No, I don't hate you, I just worry about you guys. Been doing this

four decades, so I know all the things that can go wrong.

 

Be careful, the JJ curves don't always match well with the EH's.

I love the Hard JJ's for certain application, but not for this.

 

If it blows, it may take down the processor board as well. I hope the

tech did the bias for the JJ's and not the normal bias used for the EH's.

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Thanks, I was being facetious, I could tell you care  ;) .

 

Fingers crossed, I guess, but so far so good.

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Yeah, I care. But I also don't want to see gear

come across my bench if it doesn't have to.

 

Luck.

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Thanks, I was being facetious, I could tell you care  ;) .

 

Fingers crossed, I guess, but so far so good.

 

I had JJ's in my DT25 for about 6 months before they blew and took one of the fuses with them. They sound great, but I don't think they work reliably in these amps. My suggestion is to keep a set of EH's on hand just in case.

 

Also, if you have a problem with the amp hopefully there is an authorized Line 6 service center near by. We have two in Portland, Or which are both excellent.

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How about replacing the preamp tube. Can a Bugera 12AX7B be used instead of the CHINA 12AX7B that is originally used? Just curious because I bought a burgera not being able to find the "CHINA" brand. Thanks guys

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"They sound great, but I don't think they work reliably in these amps. "--- Correct. JJ's are good tubes,

just not in these amps. They are not all analog tube amps, so you can't go tube swapping like we used

to do back in the 1960's and 70's.

 

 

I want to keep all your gear of of my repair bench. So use the specified tubes, please.

 

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Last week i replaced all tubes for the first time. But before doing that I measured the current bias. On V3 I measured 31mv and on V2 i measured 26mv. The 12AX7 was indeed a Chinese generic one and the two EL84 were the (probably) original Electro Harmonix ones. I swapped them out for an Electro Harmonix 12AX7 en two (matched pair) Sovteks. I could not believe my eyes when I measured 23mv on V2 and V3.

 

The original pair looked like the picture, one looked a bit "burned". Is this normal that there is such a difference?

 

 

burnt.jpg

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