Crackle and static on warm up - diyAudio
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Old 5th July 2015, 02:26 PM   #1
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Default Crackle and static on warm up

http://music-electronics-forum.com/a...5965-image.jpg

http://milas.spb.ru/~kmg/files/schem...ematic_new.gif

Hi, I am having some trouble with my Engl Fireball 60. It recently had a blown preamp tube in the V3 spot. After replacing the tube there was very low output from the preamp. I found and open 100k plate resistor on V3b, R29. Replaced R29 and restored output, but now there is the static and crackling. Without an instrument plugged in, the amp crackles for about 10-15 minutes until it warms up. I replaced all the tubes one by one and all together in case there is more than one bad one. I've cleaned and re soldered all the sockets and re soldered all the components in the V3 section of the preamp. Im focused on this area because I shorted pins 2 and 7 on V1 and V2 and the crackle is still there, so it has to be further in the circuit. If I put a signal straight into the return of the fx-loop, the crackle stops. I have this area marked on the schematic pic above. C17 is a 10% WIMA MKS4 100nf capacitor that sounded almost microphonic when I tapped on it, so I removed it from the board and it tested 106nf, this should still be good right? Could it be leaking DC voltage and still read good on a meter? I'm not very knowledgable when it comes to tube amplifiers. I have worked on some electronics and I have done my research on safety precautions, but right now it is a guessing game for me on which components to replace. Any advice or suggestions would be greatly appreciated. Thanks

Last edited by johnathansrowe; 5th July 2015 at 04:25 PM. Reason: edit jpeg image unable to view
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Old 6th July 2015, 01:45 AM   #2
AllenB is online now AllenB  Australia
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What you describe resembles the behaviour of some carbon composition resistors. Maybe through age, or manufacturing tolerances, or previous heat damage, these resistors could change physically as they warm up.
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Old 6th July 2015, 02:28 AM   #3
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Thanks Allen, I checked all the resistors and they seem to be in spec, except for the one I replaced. This is weird though, I put the amp back together after changing C17 and there was no change in the crackle. I put different power tubes back in and one seemed to be brighter, so I turned the bias pot all the way down and checked the bias on the tubes. I think I am doing it right, I measured the DC voltage on pin 8 of each tube. One was 44mv and the other was 72mv. I thought it was supposed to be around 36mv. I turned the bias pot up and checked it. One was 72mv and the other was 100mv so I quickly turned it off. Could this be the whole problem? I'm not sure where to start looking for bad components that would cause this in the bias section. Do the screen grid resistors control the bias or only if it is fixed non adjustable bias?
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Old 6th July 2015, 04:40 AM   #4
AllenB is online now AllenB  Australia
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Brighter, you say? A heater running bright could change a few other things.

Regarding resistors, I've introduced this issue swapping in NOS CCs that measured within spec so I'd suggest there's no easy way to tell without tracking it down.
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Old 6th July 2015, 12:41 PM   #5
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What components are controlling the bias? Should I be focused on the capacitors and resistors coming from the plates on the preamp tube V4 to the power tubes screen grid or the diodes and resistors on cathodes of the power tubes to the OT. This is definitely a learning process for me, but it's great, as I want to build my own amp someday.
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Old 6th July 2015, 01:39 PM   #6
DF96 is offline DF96  England
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V3A is in a weird circuit: an active attenuator. Could it be that this stage has parasitic oscillation? At RF this is just a cathode follower, so instability is possible. That would explain the apparently microphonic capacitor. Is R26 right on V3 pin 7?

Alternatively, something is going wrong in the SS circuitry which appears to provide some sort of gain control via T1 J175.
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Old 6th July 2015, 01:54 PM   #7
kevinkr is offline kevinkr  United States
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This is a guitar amplifier thread and should really have been posted in Instruments & Amplifiers. Moving it...

Hopefully additional insights will follow.
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Old 6th July 2015, 03:28 PM   #8
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I can't get the second link to open but from your first link and posts, it seems as though the problem is still in your V3a stage or before since you have no noise when inputting a signal in the effects return(V3b). But I don't have the full schemo in front of me.

The fact that the noise goes away after warming up tells me it's a bad solder joint or component. If this is a PCB amp, there is a chance the trace isn't in good contact with the solder joint if you are sure of your soldering. Try using a chopstick or other non-conductive tool to push on each component to try to make the noise. Don't forget about R26. As DF96 said, hopefully it's mounted on the socket.

In regards to your output tubes, the one that is at 72mV and later at 100mV is probably faulty or definitely not a good match. Try to put in tubes that are within 5mV of each other. If the whole problem is in this stage, the noise would be there no matter where you input the signal because it has to pass through the power amp section no matter what. I would put the original power tubes back in and bias accordingly. Screen grid resistors have nothing to do with bias.
As I said before, I don't have the full schematic, so I can only talk in general terms here.
As DF 96 said, it could also be T1 J175. it is a JFET and usually is just used as a switch to change the circuit structure for more or less gain.
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Old 6th July 2015, 04:43 PM   #9
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Ok thanks guys, I'll look into this tonight. It's weird you can't open the second link, as that is the full schematic. It's the first image when you search engl fireball 60 schematic. I tried the link and it opens fine. Oh well, I'll keep working on it and hopefully come up with something.
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Old 6th July 2015, 08:48 PM   #10
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In case anyone else has trouble, here is another link to the full schematic:
http://el34world.com/charts/Schemati...l_fireball.pdf
It's a hi res. image, just keep magnifying it.
Seeing the full schematic, I don't think I would change my response for now. See if the chopstick method will work after re-tubing to match power tubes.

Last edited by boobtube; 6th July 2015 at 08:55 PM.
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