Popping sound marshall jvm410H

[rbotn]

I run a small service workshop for electronics, especially tube amplifiers.
I have received a Marshall JVM410H which emits a pop sound in a non-linear pattern. There may be pop sounds at 40 second intervals or it may take up to 5 minutes before the sound comes. The sound does not depend on the volume controls. The sound can be reminiscent of someone pooping on a microphone. Any suggestions on where to troubleshoot?

 

[Kuga]

Could be the power tubes.

 

[EADGBE]

It might be the bridge rectifier.

 

[rbotn]

I have changed the 4 power tubes and also the two cathode resistors with the same value but as 3W. One was burnt. Have also adjusted the bias to approx. 45mA for each tube (pair). Might try to test with one by one of the old pipes to see if that helps.

 

[Kuga]

I have changed the 4 power tubes and also the two cathode resistors with the same value but as 3W. One was burnt. Have also adjusted the bias to approx. 45mA for each tube (pair). Might try to test with one by one of the old pipes to see if that helps.

After you replaced power tubes and resistors. Do It works properly?

 

[rbotn]

The popping starts after changing the tubes. That is what the owner said. I could not try it before, because of the burned resistor.

 

[Kuga]

Why do you installed 3W resistors? I'm not an amp tech but some resistors works like a fuse to protect circuit. Maybe current power tubes are bad because was installed with a burned resistors.

 

[dtier]

Have you viewed the power tubes while it pops? An arcing tube is usually accompanied by a flash. It sounds like the shorting tube that flamed out the cathode resistor has been changed. I would take a good look and measure the screen resistors as well.

 

[rbotn]

I will take a look at the tubes. To make it clear. I changed first the two catode resistors and then a put in the new tubes. i will also check the screen resistors.