http://music-electronics-forum.com/attachments/34706d1436035965-image.jpg
http://milas.spb.ru/~kmg/files/schematics/Engl/625/engl-fireball-amplifier-schematic_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
http://milas.spb.ru/~kmg/files/schematics/Engl/625/engl-fireball-amplifier-schematic_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
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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?
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.
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.
Alternatively, something is going wrong in the SS circuitry which appears to provide some sort of gain control via T1 J175.
This is a guitar amplifier thread and should really have been posted in Instruments & Amplifiers. Moving it... 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.
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.
In case anyone else has trouble, here is another link to the full schematic:
http://el34world.com/charts/Schematics/files/engl/engl_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.
http://el34world.com/charts/Schematics/files/engl/engl_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.
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Update, with a new matched pair 6l6 tubes, the bias is 74ma and 79ma at the lowest setting on the bias pot. I chop sticked every component on the entire board and cannot get the crackle to change. Once it is gone, I cannot get it to come back. T1 J175 is testing good, source and drain reading .571 and nothing with neg and pos reversed. I'm fairly certain it is a component. I ordered some freeze spray, maybe I'll find it with this method. As for the bias, I'm not getting any red plating, but is 74ma double what it should be? How can I lower the bias? On the bias pot circuit R86 is 22k, I thought lowering the resistance would give more neg voltage causing a lower bias, but that didn't work.
How are you measuring bias current? If indeed your current is that much, yes it is biased way too hot. Should be closer to 40-45mA for each tube. According to the schematic, it is voltage measurements between points A and C, B and C, 2.2-2.7 volts. I'm not familiar with this particular amp but I would tend to follow their directions. Also, what voltages do you have on pin 5 of both 6L6s? It should be about -45 to -50 or so. And adding resistance will increase bias voltage for that circuit. If they are biased too hot, you want to increase negative bias voltage. From -45v to -50v is an increase in bias voltage. You have a supply of -55 volts, so a lower resistance will lower bias voltage and bias the tubes hotter.
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