Cracking Noise in Princeton Reverb Copy

[Johnrcurry]

I am looking for help with a Weber Princeton Reverb copy (layout and schematic diagrams attached). I built in 2018 and it has been reliable until about a week ago when it suffered a power transformer failure. I replaced the power transformer with a new one from Weber. After replacing the transformer and carefully checking everything. I could not find any other fault that may have caused the PT failure.

Now that the amp is back together it has developed a new fault. After the amp has been on for several minutes, it starts to make a crackling noise, like static. The noise is present regardless of whether there is anything plugged into the input. It is present with the volume turned all the way down, although it goes up in volume as the volume knob is turned up. I have swapped in different tubes in every position with no change. I tried removing the tubes one at a time see if the noise would go away. The only tube that makes a difference is V3. The noise goes away when that tube is pulled, but does not go away when the others are pulled. I have visually inspected and prodded every solder joint with no effect. I reflowed a couple of dodgy looking joints with no effect. I have checked the values of all resistors and all are within specs (although I supposed one could be faulty with the fault only showing up when warm).

I welcome any suggestions as to what to try next.

Thanks

[Helmholtz]

The only tube that makes a difference is V3. The noise goes away when that tube is pulled, but does not go away when the others are pulled.

Yery unlikely that the noise doesn't stop with V4 pulled?

[Johnrcurry]

Helmholtz, you were right to question me on that. Based on your response, I went back and re-did the my "pull-one-tube-at-a-time" experiment. As you suggested, the crackle goes away when either V3 or V4 is removed.

[Helmholtz]

Ok, now short/jump V3B grid pin 7 to ground. Any difference?

Then remove the pin 7 short and ground V3A grid pin 2.

[Johnrcurry]

Thanks, Helmholtz. I did as you suggested. Grounding pin 7 killed the crackle and created a hum. Grounding pin 2 had no effect. What does this tell us, as far as identifying the faulty part(s)?

Again, thanks.

[Helmholtz]

Thanks, Helmholtz. I did as you suggested. Grounding pin 7 killed the crackle and created a hum. Grounding pin 2 had no effect. What does this tell us, as far as identifying the faulty part(s)?

It means that the crackle is not generated in the power amp including V3B. And its not caused by V3A.

Wondering about the hum when shorting V3 pin 7.
Do you have a scope?

As a start replace R7.

[Johnrcurry]

Here's the latest, I replaced R7 and it had no effect. I don't have a scope.

[Helmholtz]

Ok, now ground V1B grid pin 7.

If still noise, ground the point where C15 and R14 connect.

[Johnrcurry]

Thanks, Helmholtz. I did as you suggested and here are the results. Grounding V1B pin 7 does not kill the crackle. Grounding the junction between C15 and R14 does kill the crackle. What does that tell us?

[Helmholtz]

Could mean a bad (leaky) coupling cap C15.

Does the reverb control have have any effect on the noise?

[Johnrcurry]

The chassis is out of the cabinet so the reverb tank is not connected. With the tank disconnected, the reverb control has no effect.

[Johnrcurry]

Also, can I just clip in a new C15 to see if tha'ts the problem?

[Helmholtz]

Also, can I just clip in a new C15 to see if tha'ts the problem?

Wiring another cap in parallel won't stop the leakage current.