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Crackling Sound On Startup

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My DT-25 makes a static like crackling noise, mostly shortly after startup. I have noticed that when I switch to class A, the noise disappers, Then it returns when I switch back to A/B. For the most part the amp seems fine, crackling does not appear to be effecting tone. I am using it at low volume in a apartment ...

 

 

All tubes were replaced a year ago. I realize they do need replacement after sometime. I am just wondering if the switching between classes points to a particular issue, (pre-amp tube, power tube, dirty connectors etc ..) or is just a interesting observation.

 

Thanks.

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My DT-25 makes a static like crackling noise, mostly shortly after startup. I have noticed that when I switch to class A, the noise disappers, Then it returns when I switch back to A/B. For the most part the amp seems fine, crackling does not appear to be effecting tone. I am using it at low volume in a apartment ...

 

 

All tubes were replaced a year ago. I realize they do need replacement after sometime. I am just wondering if the switching between classes points to a particular issue, (pre-amp tube, power tube, dirty connectors etc ..) or is just a interesting observation.

 

Thanks.

Tubes were replaced, but were they properly biased?

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Well the repairman stated that he biased them. It is the only Line 6 authourized repair shop around. I can hope he took the time to read the proper biasing procedure for the DT-25, but I can't prove it. :) 

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This is exactly what happened to me 18months ago. The crackling became permanent though....you could drown it out with playing loud but it was always there in the background.

 

 Solved by replacing the EL84 power tubes (did it myself without biasing) and haven't had any trouble since.

 

I discussed over the phone with an amp tech who suggested that it should be ok not to bias as I used matched tubes and it all worked perfectly after I changed them. My problem was that I bought online so would have to post the heavy amp back for inspection and warranty wouldn't include the tubes anyway. There is unfortunately no Line 6 certified amp tech in my area.

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Mine crackles - I call it "My Impatience syndrome"

 

When I turn the amp on and wait about 1 minute then turn the standby ON it is OK but when I turn it on and then don't wait too long I get the crackling.

Once warmed up it crackles no more...

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I discussed over the phone with an amp tech who suggested that it should be ok not to bias as I used matched tubes 

 

Just because tubes are matched doesn't mean you don't need to bias the amp.  You really do, unless by some miracle the replacement tubes were identical twins of the ones you replaced.  Biasing isn't hard.  Takes but a few minutes to do properly.

 

You might consider using a different tech. :)

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I

Just because tubes are matched doesn't mean you don't need to bias the amp.  You really do, unless by some miracle the replacement tubes were identical twins of the ones you replaced.  Biasing isn't hard.  Takes but a few minutes to do properly.

 

You might consider using a different tech. :)

I'm not saying biasing generally isn't required, I'm sure it is. I'm just saying I didn't bias that time and it all works fine, still going strong 18months later. It was more the "worked perfectly" part that the tech suggested was a good sign rather than the matched tubes.

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I'd say it's a power tube problem and would  just replace them and bias. Had the same problem a couple months ago, and replaced the tubes and have not had the crackling sound since. Stock tubes where junk. Maybe a bad batch, but others here have had simular issues. I may upload a video of the complete replacement and bias next time I change mine. Hardest part was getting the chassie out of the head housing. (glue)

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+1 Karl - good to see you are still around after all this time!

 

Line 6 reccomends using matched tubes and having them biased by a tech "in the know".  I have performed a number of replacement/biases on the DTs and found it pretty straight forward in comparision to amps I have worked on in the past.  If you have biased an amp it is pretty easy (25mv is reccomended for the DT25, 36v for the DT50).  Also be sure the amp is set to Pentode and "class A/B" mode, not in "standby" mode - fell for that once or twice ;)

 

As the bias point pins are pretty close together I often use a single header cable to go on the negative point, then use an alligator clip to connect to the meter - it is a little too easy (for me) to touch the pos/neg together and create a "0.000" reading on the meter.

 

If you haven't biased an amp PLEASE take all necessary safety precautions, as there is potential for injury (or worse) if not performed correctly. 

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I

I'm not saying biasing generally isn't required, I'm sure it is. I'm just saying I didn't bias that time and it all works fine, still going strong 18months later. It was more the "worked perfectly" part that the tech suggested was a good sign rather than the matched tubes.

If your new tubes test out at a lower voltage, they should last just fine. It's when they test higher than spec that you would see early failure. I had to put in 2 mismatched tubes to get by a month or so ago, so my voltage on one tube was 19 and the other was 28 just to get them close enough without individual bias pots. Sounded fine, but I pulled them as soon as my matched tubes arrived.

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When I first fired my DT50 Head (in Standby) it was quiet for a few seconds then it made a sizzle/fizzle sounds that lasted about 2 seconds after that It's been quiet ever since. I have about 10 hours on it now.

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