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Offlinestefan
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problem with tube amp: crackling/buzzing after heating up
    #7994920 - 02/08/08 05:13 AM (15 years, 11 months ago)

My guitar tube amp has a disease of some sort:smirk:

When I turn it on everything works fine, but after heating up (whan I've played for 15-30 minutes) it starts crackling and buzzing, probably a short circuit.

I've had a guitar technician look at it, but he couldn't find anything. Obviously that isn't really weird since the amp only does it when it heats up after a while of playing. I'm not an amp-tech by any means, but from what I've read it could be the old tube sockets that can short circuit when they heat up and need to be replaced. Could this be right, or could it be something else?

I hope someone here can help me out, or at least point me to another forum where someone can!


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OfflineSaulGood
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #7994942 - 02/08/08 05:32 AM (15 years, 11 months ago)

i'm no expert by any means but it sounds like something tube related.... try these guys... they can probably come up with an answer.

http://www.amptalk.com/

do you know the make/model/age of the amp? is it a combo amp or just a head?


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Offlinebeneath
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #7994944 - 02/08/08 05:34 AM (15 years, 11 months ago)

maybe old tubes?

mine does it and my tubes are ollllld.


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OfflineSeussA
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #7994947 - 02/08/08 05:37 AM (15 years, 11 months ago)

After heating up, does it crackle/buzz all the time, or only when you are playing something? If only when you are playing something, does the crackle occur only on loud notes, or even when soft notes are played? Is the amp a monoblock or stereo? If stereo, does the crackle occur on both channels, or just one?

If I had to guess, with the limited info available, I would suspect a decoupling capacitor is going bad (or a cap in the feedback loop). It could also be a cold soldier joint that is losing connection with thermal expansion. (I am assuming that you have already swapped/tested the tubes.)

It would help to know details about the amp...


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OfflineSaulGood
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Re: problem with tube amp: crackling/buzzing after heating up [Re: beneath]
    #7994955 - 02/08/08 05:45 AM (15 years, 11 months ago)

yeah i got an old gk tube guitar head from the '70s, back when they still made guitar heads, and it occasionally gets temperamental (gain channel wont work after a while) but it sounds awesome so i just keep it on clean and use pedals (i run it through a Marshall 4x12 JCM 800 lead). Tubes are great but they can be high maintenance at times, especially vintage stuff.


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Offlinestefan
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #7994996 - 02/08/08 06:19 AM (15 years, 11 months ago)

wow so many replies after such a short time, great:D


details:

-it's an Ampeg VL-503 ( http://www.leejackson.com/VL503.htm )
-combo guitar amp (mono)
-tubes are new (JJ E34L) but first I did something wrong with the bias so maybe they're fucked? *
-it does the crackling buzzing thing all the time after 15-30 minutes of playing (when the amp is heated more I guess). In the first 15-30 minutes it sounds good! Could this be the tubes, or most likely some short circuit?




* First the B+ fuse (1A slo blo) blew (first great sound, then after a while crackling/buzzing (also while not playing) for a few minutes, then no sound anymore-->the fuse blew). According to forums and a call to a guitar technician 90% chance it was bad end-tubes. so I ordered a fresh set of tubes (also preamp tubes since I figured they were old too) and put them in. The amp can be biased on your own since it has small knobs and leds that light up green when properly biased. So I did that, even better sound than before! but after a while bzzcrchhhtsskkk... same fucking fuse blew. Now I couldn't figure it out by myself easily so I brough the amp to a guitar/amp technician. And gave him the schematics which he found very helpfull. He looked at it (measured it trough and stuff) and called me on the phone saying nothing was wrong, but that there was something weird with the bias system. When you turn the bias knobs from down to up the voltage was way too high (36 mA in stead of 26 or so, which probably blew the fuse. When you tusned the bias knobs from up to down the voltage would be good. He thought it was a design flaw or something.
well I picked it up and played for a while, everything worked fine (also put the pre-tubes in I bought). Now I would hear a certain nice timbre in my guitar tone that I didn't hear before, I adjusted the EQ to see where it was 'located' because I was curious. Well it stayed and I suddenly noticed that I was hearing the sound of the wood my guitar was made of, amazing! (never had a tube amp before)
Is it possible that the tubes got ruined because of the too high voltage? In a way that they now work fine, until they heat up and start buzzing and crackling?


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OfflineSaulGood
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #7995044 - 02/08/08 06:55 AM (15 years, 11 months ago)

you may just have a defective amp. contact ampeg/your vendor and see if you can get a replacement.


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Offlinestefan
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Re: problem with tube amp: crackling/buzzing after heating up [Re: SaulGood]
    #7995056 - 02/08/08 07:04 AM (15 years, 11 months ago)

bought it second hand..


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OfflineSaulGood
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #7995089 - 02/08/08 07:31 AM (15 years, 11 months ago)

...damn.

You might be able to get decent results from a noise gate pedal, but if you fry your amp every time you use it then that wouldnt work either.

verdict:
it sounds like you may be fucked. try a noise gate, if that doesnt work i would just sell it off, count my losses and buy a new one. do you know the person you bought it from? if so you can go complain to them and see what they have to say.


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InvisibleWhiskeyClone
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #7995879 - 02/08/08 11:25 AM (15 years, 11 months ago)

Post your problem here: Harmony Central - Amps

It's a very high-traffic forum with a ton of amp people.


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Welcome evermore to gods and men is the self-helping man.  For him all doors are flung wide: him all tongues greet, all honors crown, all eyes follow with desire.  Our love goes out to him and embraces him, because he did not need it.

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:heartpump:


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Offlinestefan
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Re: problem with tube amp: crackling/buzzing after heating up [Re: WhiskeyClone]
    #7998874 - 02/09/08 01:14 AM (15 years, 11 months ago)

I tried that and it appears to be so high traffic that in 15 minutes your post isn't even on the front page anymore.. I guess I should bump it again:smirk:


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InvisibleWhiskeyClone
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #7999312 - 02/09/08 07:30 AM (15 years, 11 months ago)

Yeah it's very high volume there


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Welcome evermore to gods and men is the self-helping man.  For him all doors are flung wide: him all tongues greet, all honors crown, all eyes follow with desire.  Our love goes out to him and embraces him, because he did not need it.

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Offlinestefan
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Re: problem with tube amp: crackling/buzzing after heating up [Re: WhiskeyClone]
    #8003665 - 02/10/08 04:44 AM (15 years, 11 months ago)

a few people there suggested that it was most likely a cold solder joint

Quote:

Check the grounding of the bias circuit, that and the cathodes of the output tubes and some of the staked connections on the PT have been recurring problems with those amps. As suggested, Cold Solder Joints.




I guess I'll have to open my amp again and see what I can find :shrug:

Quote:

I'll tap around and see if I can find it that way, sometimes, sometimes not. If I can make it happen or stop by moving components then good. I'll inspect then solder that and any others that may look suspect. but you can't really tell by looking unless it's blatant



there is a possibility that I'll have to run the open amp until the problem occurs and then start 'poking' all components very lightly one at a time to see where the cold solder joint is. Ofcoarse I'll use a non electricity guiding something and not my bare hands. good idea, or just plain stupid?

let's say if I find a cold solder joint.. is the way to fix this just resolder the part and then it's fixed or..?


someone else suggested
Quote:

Nobody mentioned plateload resistors yet??? Geez. WTF over?






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Offlinestefan
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #8007570 - 02/11/08 02:55 AM (15 years, 11 months ago)

well I think those guys were right about the cold soldering joints!

Today I've been playing with a table fan pointed at the back of my amp and the popping and hissing takes way longer to show itself and to a lesser extent too

so that must have been it right?

I still need it fixed though since it isn't really reliable like this:smirk:


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OfflineSeussA
Error: divide byzero


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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #8007580 - 02/11/08 03:07 AM (15 years, 11 months ago)

> so that must have been it right?

Quote:

I would suspect a decoupling capacitor is going bad (or a cap in the feedback loop). It could also be a cold solder joint that is losing connection with thermal expansion.




Could be a bad capacitor as well. Both cold solder joints and failing caps can cause the problem you described. Reflowing all the solder is going to be easier than replacing caps, so start there. Rare to see, but it could also be a bad tube socket.


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Offlinestefan
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Re: problem with tube amp: crackling/buzzing after heating up [Re: Seuss]
    #8007621 - 02/11/08 03:45 AM (15 years, 11 months ago)

:thumbup:

if it's a capacitor, would the problem still occur when cooled?

I'll look at the soldering first:cool:


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OfflineSeussA
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #8007635 - 02/11/08 03:58 AM (15 years, 11 months ago)

> if it's a capacitor, would the problem still occur when cooled?

Not always. See http://www.justradios.com/safetytips.html for a good discussion on caps in tube amps...


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InvisibleDiploidM
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Re: problem with tube amp: crackling/buzzing after heating up [Re: stefan]
    #8007793 - 02/11/08 07:10 AM (15 years, 11 months ago)

Something to look for is high-voltage arcing from the plate circuit. Tube plates (anodes) operate at hundreds, or even thousands of volts. These voltages are typically carried by special wires similar to spark plug wires on car engines. When they get old, the insulation can crack and cause arcs to jump across air gaps. If this is happening, it would definitely cause all sorts of noise.

Next time the amp starts acting up, turn off the lights and take a look inside. If there's arcing, you'll see it.


--------------------
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1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.


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OfflineSeussA
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Re: problem with tube amp: crackling/buzzing after heating up [Re: Diploid]
    #8007819 - 02/11/08 07:27 AM (15 years, 11 months ago)

> Tube plates (anodes) operate at hundreds or even thousands of volts.

The EL34 tubes operate around 450V, typically. If a coupling cap goes bad, you can get positive voltage on the control grid resulting in internal arching (short) which can cause horrid distortion in the output signal.


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Offlinestefan
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Re: problem with tube amp: crackling/buzzing after heating up [Re: Seuss]
    #8041854 - 02/19/08 07:54 AM (15 years, 11 months ago)

update:
opened the amp, so that I could see everthing and started playing (ofcoarse I was carefull not to touch anything inside). After a while the noise started so I put my guitar down and started looking at the insides of my amp.. nothing unusual to see there really.
I started poking wires very lightly with a 'pin' (sp? what are those things called) of my multimeter (touched the isolated plastic of the wire only, just to move it a little). And a wire close to the guitar input seemed to be the problem: when I toughed that wire the noise got louder and worse.

I poked it another time, a little harder and the noise stopped. Played for a while and there wasn't any noise anymore:cool: So far my minimal technical abilities did do a good job haha:tongue2:


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