Bad tube or need biasing

How can you get a great tone without a good understanding of what makes your amp tick? — EL34, EL84, 6L6, KT66, KT88, 12AX7, 12AT7, 12AU7, EF86, Output Transformers, Power Transformers, Capacitors, Resistors and everything else that it takes to make a tube amp sing!

Bad tube or need biasing

Postby shootingstarno on Sun Mar 06, 2016 2:07 am

Greetings. I have an all tube Bugera BC30 2x12 combo guitar amplifier. Whats wrong with a tube and or amp when turned on it has a "jet airplane" kind of noise. Are there bad tubes or can they just be in need or biasing. Its play still, fairly loud too, but that sound and it also seems to have a funny over sound now. Thanks for any help here. Gods blessings. And what does a microphonic tube sound mean. Whats happening there, if a tube goes microphonic.
shootingstarno
Peasant
 
Posts: 3
Joined: Thu Jan 17, 2013 3:04 pm

Re: Bad tube or need biasing

Postby OldSchoolDave on Sun Mar 06, 2016 4:36 am

Greetings!

Swapping out tubes, one at a time, is a good troubleshooting step. You should keep a set of spares on-hand, anyway. No need to spend a fortune (i.e. JJ's should be fine).

The sound you described might be caused by something else, but always check the tubes first.

A microphonic tube will pick up and amplifly sounds from other sources (radio stations, appliances, lights, etc.) - or make noise when moved. It can also emit a ringing sound when gently tapped with a drumstick or chopstick. Some circuits (eg. high gain preamps) and tube locations are more sensitive to microphonics than others. A tube that exhibits microphonics in one socket may behave fine further down the circuit.

How many hours have the tubes run?

Dave
...Hey! I played that song when it was new...
.
http://www.OldSchoolDave.com
OldSchoolDave
Prince
 
Posts: 4188
Joined: Sat Apr 29, 2006 9:46 pm
Location: HP, NC USA

Re: Bad tube or need biasing

Postby shootingstarno on Sun Mar 06, 2016 2:43 pm

Greetings Dave. Thanks for the response. I bought the amp new about five months ago. I dont have to many hours on it. Though I wonder about a few things. One. It was shipped UPS. The box looked old. And my brother accidently rolled it once because it was so heavy to push across the carpet. Also. Bugera says, and its true, that the amp is touch sensitive. Meaning the louder you pick, the louder it plays. So it has been getting some loud, sudden changes is loudness as I strum lightly or hit it hard.I havent liked that feature. If I hit it hard, it gets really loud. I wonder if this is the reason why the tubes seem damaged and or already spent/blown. Im new to tube amps. Though Im going to learn how to bias my own tubes and replace them because there are no techs close by where I live and just transporting it would probably damage any work done. Gods blessings. Ive got the tube voltages from Bugera for my power tubes to be biased. Now Ill just keep watching videos on youtube so I can safely bias them once I get a multi- meter etc. Paul WA. state.
shootingstarno
Peasant
 
Posts: 3
Joined: Thu Jan 17, 2013 3:04 pm

Re: Bad tube or need biasing

Postby Unit_1 on Mon Mar 07, 2016 1:38 am

Maybe you could make a video of the sound and post to youtube and post the link here. That might help a bit.

Tube amps in general have a lot of dynamic response to the picking level..... Actually sort of a good thing. If you're singing you can pick gently, and then in between verses you can play a bit harder. It's something you'll get used to and appreciate!

Be sure to ask questions when you're getting ready to bias if you have any. And be very careful not to touch any part of the circuit as there is lethal voltage powering the amp. 400 volts and higher is very common.

It's the same thing as power tools, fearing the tool is a GOOD THING as it keeps you from loosing a finger, hand, or your life.

AND Welcome to the Vintage Amps forum :thumbsup:
:mrgreen: :jam: :highfive:
Image
The ability to play/make music is a gift that not everyone gets. Those of us who have it should use it.

Asimov:Individual science fiction stories may seem as trivial as ever to the blinder critics and philosophers of today — but the core of science fiction, its essence, the concept around which it revolves, has become crucial to our salvation if we are to be saved at all.
Unit_1
Prince
 
Posts: 3713
Joined: Mon Oct 03, 2011 3:29 pm
Location: Kali4ñia (a small planet orbiting the west coast of the United States)

Re: Bad tube or need biasing

Postby OldSchoolDave on Mon Mar 07, 2016 1:56 pm

shootingstarno wrote:GIve got the tube voltages from Bugera for my power tubes to be biased. Now Ill just keep watching videos on youtube so I can safely bias them once I get a multi- meter etc.


Paul,

I'm not familiar with that particular model, but I don't think most EL84 amps (at least, the ones I own) have adjustable bias. biases? baisis? ;)

Either way, try the "chopstick test" on each tube (preamp and power) when the amp is idling and see if any make unusual noises when GENTLY tapped.

Dave

P.S. With the power off (and tubes cool), try reseating each one into their respective sockets. Intermittent connections can cause all sorts of trouble.
...Hey! I played that song when it was new...
.
http://www.OldSchoolDave.com
OldSchoolDave
Prince
 
Posts: 4188
Joined: Sat Apr 29, 2006 9:46 pm
Location: HP, NC USA


Return to Tubes, Transformers and Components

Who is online

Users browsing this forum: No registered users and 4 guests