JTM45 Build Buzz Problem

[markammon1979]

Hi, I have finished building a JTM45 copy based on a metroamp schematic. I have tested the amp today. The sound is excellent and everything is working as it should apart from a loud buzzing that gets worse as the amps volume is increased. The buzz is there all the time even when nothing is plugged in to the inputs. I have checked over the circuit and voltages etc. and all seems well. Any ideas?

[markammon1979]

also...

Just to add that the buzzing disappears when the amp is run at lower voltages on a Variac. Any ideas anyone?

[Gtr_tech]

How do you have the gnds configured? Grounding is very influential on the noise floor. Keep the input jacks off the chassis.

[loudthud]

Does the buzzing stop immediately when the standby switch is switched off or does the buzzing fade out over a period of a couple of seconds?

[SoK66]

Obviously a ground issue. Your PT center tap, heater center tap, power supply filter cap, screen filter cap, PI filter cap and power tube cathodes should all ground to one stud on the PT. Ground your speaker jacks and the impedance selector switch on the PT stud, too. Attach all your preamp grounds and jack grounds to a buss wire across the pots. Another school of thought, common all of your pot, preamp and jack grounds on one wire off the board and ground it to the chassis. I prefer the former method, but both seem to work.

[markammon1979]

Solved!

Thanks to everyone who has replied. I checked through the amp again and realised that there were some grounding issues. Went back through the circuit and checked all the grounds and the problem is solved.