Metropoulos Forum

Ok, first problem with my recently built JTM. Hopefully someone can help me diagnose and fix it.

Problem: distinct, sharp popping sound during attack on a single note or the the first note of a chord at medium to high volumes.

Operating conditions:
* jumpered channels
* popping happens with both volumes at 12 o’clock and anywhere above that, but not at low volumes
* guitar straight into amp, I also experience the problem when I use my Weber load box
* popping just started today, i.e., it wasn’t happening yesterday
* tubes have about 50 hrs on them

I’ve also experienced a distinct popping sound right when I turn on the standby switch, and this has been happening since day one, but never during playing.

Thanks in advance for the help.

Mike

If you play the amp in a dark room, do you notice any arcing in any of the tubes? May also be a bad solder joint somewhere in the circuit.

No arcing. I just finished testing for microphonics - tap on the tubes and listen for for the tap on the speaker.

I found something, but I can’t explain it. First time through I found that v1 when tapped is very clear through the speaker. All other tubes ok. Swapped with two other tubes that I know are good - same problem. I haven’t opened anything up yet, but my gut says loose solder joint.

Any thoughts on what my next step should be?

Thanks,
Mike

So after swapping tubes around you've isolated the problem to V1? If the microphonics follow the tube, you probably have a bad tube.

If the problem remains with V1, try "chop-sticking" the wires to the socket. I'm betting you will get some microphonic type sound. I spent two weeks chasing this very symptom on a JCM-800 clone I built for my son, thinking it was a bad signal wire shield, leaky coupling capacitor, grounding issues, until I found it was just a cold solder joint on V1 socket. Re-flowed the joint and viola - no noise !

Could also be an issue with the socket itself, but I've never experienced that.

Good luck with your amplifier .

It is normal to hear the tapping through the speaker when tapping v1. It is the most sensitive spot and the tapping sound gets amplified by the successive stages. If the tapping produces a ringing or howling sound, then you know the tube is microphonic. Some amps are more sensitive than others.

If you are comfortable working inside a live chassis, you could try chopsticking every component and solder joint with the volume up to see if the problem occurs, Be aware that some points around the v1 socket will cause noise through the speaker when tapped.

Thanks for the responses. I’ll try these suggestions and hopefully that gets things back to normal, which by the way, is spectacular.

I hbave also read of folks having issues with the standby switch going bad and causing trouble like this. Most switches are not rated to switch the high voltage DC that we see in amps. Bad contacts could cause noise problems that would get amplified by the circuit. You could remove the switch temporarily and see if the noise goes away. If your switch is a typical two lug Carling style, simply unsolder the wires and connect them together. Make sure the caps are drained and doublecheck with a meter before doing the work.