Loud crackling noise and no signal

[ES225]

I have a Champ 5F1 clone and it was working just fine until I started getting this static noise. At first it was intermittent and then a few days later became constant, to the point that I am not able to hear any guitar sounds through the amp, only the very load noise. Turning down the volume does not eliminate any of the noise.
I changed all the tubes but the same problem persists. Does anyone know whaat it could be, which component needs to be replaced? I would like to fix this myself and avoid having to take it far to a tech. I have no clue what it could be causing it. The amp was turned up all the way for about half hour just before this problem started but otherwise the amp was never abused or even taken out of the house. Any ideas?

[Chuck H]

Pull the preamp tube. Does the noise persist?

[dmeek]

The 100K plate resistor might be failing.

[ES225]

You mean just pull the preamp tube and not replace it with another one and see if I still get the noise? Turn the amp on without the 12AX7 ?

I will also try to replace the 100K resistor and see what happens.

I was thinking that it may be one of the filter caps gone bad but I don't have the experience.

[Chuck H]

You mean just pull the preamp tube and not replace it with another one and see if I still get the noise? Turn the amp on without the 12AX7 ?

Yes.

I will also try to replace the 100K resistor and see what happens.

Please don't do any arbitrary service. It only creates potential for new problems that may not be related and can confuse diagnosis. We should try to isolate and diagnose the problem as best we can before replacing any components. The time for trying anything without certainty does come up with these old amplifiers. But it should not be the first course of action for the reason I mentioned above.

[ES225]

OK, what should be my next move? I can try to pull the 12AX7 again and I was going to put in a new 100K plate resistor.
Otherwise I am stumped what went wrong with this amp all of a sudden.
I did add a small pot, which is a variable NFB loop. Instead of a switch, I used a pot. I forget the value now. That actually seemed to work fine just before the noise problem started.

[Chuck H]

OK, what should be my next move?

See post 2

I can try to pull the 12AX7 again...

"Again" ? Have you already pulled it and tested for noise? You did not report on that.

...and I was going to put in a new 100K plate resistor.

See post 5

Otherwise I am stumped what went wrong with this amp all of a sudden.

We are trying to determine that. But there has been no report on the requested test.

I did add a small pot, which is a variable NFB loop. Instead of a switch, I used a pot. I forget the value now. That actually seemed to work fine just before the noise problem started.

Well this may or may not be related to the problem. Sometimes new work with spotty contacts takes a little while to show itself. This is why I requested that you don't do any work before we've attempted some diagnostics. Arbitrary work has the potential to cause new problems and that can confuse diagnosis. We need to isolate and fix the problem at hand. That can only be done if it's known. Doing anything without knowing is not best practice. Well, that is until we can't discover anything. Then "trying" stuff is a last course of action.

[ES225]

I just pulled the preamp tube and turned the amp on. The same noise persists, like the amp is being fried. I remember this happening the same when I put in a new tube as well.
If I remember correctly last week I also changed the 100K grid resistor and still the noise remained. I hope I got the right resistor, there are two resistors the same value diagonal to each other connected on one end, I changed the one closer to the right side of the amp looking at it from behind, right?
What should I check next.

[ES225]

I need some clarification. I am not good at schematics. There are two 100K resistors. Which one is the plate resistor that I need to check. I checked the one closer to and pointing to the 12AX7 and it is not the problem. Is the one you suggest the one which is pointing away from the 12AX7 and toward the speaker jack? Neither of these 100K resistors look new and have no corrosion or leakage.
I looked at the filter caps and 2 of them seem to have dark spots at the ground end. Could it actually be these filter caps that have failed?

[Chuck H]

Since pulling the preamp tube didn't eliminate the noise you can ignore the 100k resistors for the moment (though you report changing one anyway when I asked that you DON'T). The problem is not in the preamp. By pulling that tube you have isolated the problem to the power tube or power supply circuit. The first thing I would check is voltages at the tube pins. If you're not familiar with safety matters measuring high voltage circuits you should look it up here or elsewhere for safety reasons. You will be leaving the preamp tube out for now please.

[ES225]

I have only limited experience with electronics. I am learning a lot as I go along. Now I know how to isolate preamp problems. Thank you.
So you would need to walk me though this. Check Voltages where exactly? What precautions to take? I did drain the caps. Is that what you mean? I have never taken voltages with my multimeter. I would need to know exactly what to do and where to do it. I do learn fast though. I appreciate all your help.
As I stated I did notice some staining on the ground side of the filter caps, like brown dots protruding the surface, not a lot but its there. They look like little burn marks. Could this be a sign.
Should I just order the 3 large filter caps and replace them?

[Chuck H]

Discolorations on the capacitors could be many things (some of which I admitedly couldn't know about). A clear sign of failure in the power supply capacitors (short of bench testing and only regarding "visual") would be actual leakage or bulging at either end of the capacitor.

I'm a big proponent of replacing any suspect electrolytic capacitor, BUT... We are still isolating the problem. Any arbitrary work without a clear reason can confuse diagnosis because more often than you might think problems are created as a result of the work. Cold solder joint, stranded wire hair not contained, change in lead dress creating instability or even a new part not being up to spec. plus other anomalies have put SO MANY amps on a closet shelf collecting dust awaiting further analysis. DO NOT DO ANY WORK ARBITRARILY!!!!

That said, there is a time when clear diagnosis fails that replacing "likely" suspect components is the only way to attempt progress. But not before that.

The measurements I'm requesting have to made in a live amplifier and high voltage on the circuits. A lack of experience in this area is problematic. More problematic would be a lack of confidence or even an over confidence. If you don't understand the danger you should not proceed. If you are afraid of the danger you should not proceed.

Ok. That's it for the horror show. All you'd be doing is applying your black probe to the chassis while applying your red probe to the power tube pins and reporting the readings. As long as you're not pawing at the live amp innards with bare hands you're safe. Just keep your bare hands or fingers from touching anything in the live amp. That's pretty much it. You'd be surprised how often that goes wrong and THAT is the reason for cautionary attitudes.

With the amp on the bench you'll want it connected to "the load". That is, tube amps require a load for the tubes to operate in a circuit safe environment. So whatever has to be done to make sure the amp output jack is connected to the speaker or a "dummy load" when it's outside the cabinet must be negotiated.

With the amp on and any standby switch in the play position, the meter set to measure DC voltages in the hundreds, you would take readings from the power tube pins.

Any questions?

[ES225]

The Champ not having a standby switch. Do I connect the negative of the multimeter to the chassis and the red positive to the positive of the filter cap and then turn the amp on once all this is connected? What setting do I set the multimeter? Do you measure each of the 3 filter caps the same way and take the reading? Please confirm and clarify?
It sounds like I do not need to touch the amp at all once the meter is connected right. All I need to do is turn the amp switch ON?

[g1]

Yes, that is a safe way to measure the voltage on the filter caps (though he was asking for voltages at power tube pins).
Meter would be set for DC volts. If it's not auto-ranging type, a range that can read up to 500V DC should be used.

[Chuck H]

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You seem to have an appropriate amount of respect for the circumstances. You'll do fine. Taking measurements in a live amp is something that happens all the time here. Best practice might indicate hooking up the ground probe connection before firing up the amp. I tpically just fire up the amp and then connect my black probe to the chassis with an alligator clip lead. Like I said earlier, don't touch any amp innards with your bare hands while it's live. That's pretty much the only criteria for personal safety, BUT...

There is a clause. If your amp does not have a three prong power cord or is improperly wired for chassis ground there's a small possibility that even the chassis could be live. In that case you would want to avoid touching both the innards and the chassis with bare hands until a zero voltage reading is taken from the chassis itself. That would be done by measuring for DC between the chassis and your home ground using the red probe on the chassis. And for that test it would be best to connect the red probe before powering up the amp. Access to your home ground should be avilable by poking the black probe into the ground hole on any AC outlet. Use an extension cord if needed.

EDIT: This assumes your homes grounding is done correctly.