Hooking Up A Wall Wart To Test Heaters For Hum

[rp]

On my 5C8 clone I'm trying to figure out if the hum is from the wiring, PT, or from the 4 6SC7s or all of those. I tried playing with the ground positions and moving grid wires, and a humdinger too. Before I really start to dig in, or order a Tent Labs DC module I'd like to try a test with a 120/240 350mA to 3.5-12Vdc / 1500mA Power Supply I have, it's just little wall wart thing, a switching power supply. I'm wondering as if it is really robust enough for 4 6SC7 heaters and as it's switching will it add its own noise and throw my hum test off?

BTW does the tuner image indicate 50 cycle hum? I think so.

[Jana]

Instead of using a wall wart, hook up a 6.3 volt lantern battery. That way you will know for sure if it is from the heaters. If you use the wall wart, you may find it quiet but then again, maybe you still have hum. If there is still hum, now you are left with the question, is it something else or is the wall wart now putting its noise into the amp?

[martin manning]

Hook the power supply up to the amp and measure the AC on the filaments.

The tuner showing G could be indicating either G1 (50 Hz from primary or filament hum) or G2 (100 Hz from rectified secondary hum). Better to use a guitar and play G2 (98 Hz) on the low E string and see if it is the same or an octave higher than your speaker hum.

[martin manning]

Instead of using a wall wart, hook up a 6.3 volt lantern battery. That way you will know for sure if it is from the heaters. If you use the wall wart, you may find it quiet but then again, maybe you still have hum. If there is still hum, now you are left with the question, is it something else or is the wall wart now putting its noise into the amp?

Powering the heaters with DC will tell you if the noise originates in the heater circuit, but not whether it is from the heaters themselves (a tube problem) or from the heater wiring (a lead dress problem).