Transformer Induced Hum

[hesamadman]

It would be my assumption that if you get a slight hum even with volume all the way down, it would be a PT inducing into OT. Does this make sense?

[tubeswell]

Hum has many possible sources, e.g.to name a few possibilities; unwanted AC coupling into the signal path from EMF surrounding AC wires that are in close proximity to signal wires, PT coupling into OT where the cores are in alignment, unbalanced heaters, unbalanced output tubes in a PP amp, ripple current from insufficient power supply filtering, SS diode switching spikes coupling back into the PT secondaries and thence into other secondaries and coming out as noise from the filaments that couples into the signal path, poor grounding.

[HotBluePlates]

It would be my assumption that if you get a slight hum even with volume all the way down, it would be a PT inducing into OT.

It could be anything after the volume control. The easiest way to narrow it down is to pull tubes. If this is a push-pull amp, start with the phase inverter tube.

[jojokeo]

I had a hum once with the vol turned down and it ended up being the bypass cap across the load resistor on a preamp pentode in the V1 position. Go figure-

[hesamadman]

Well....im gonna dive into this. I guess I have just been worried about my tranny layout. Anyone suggest the headphone trick when laying them out? Any good source of info for that?

[tubeswell]

Well....im gonna dive into this. I guess I have just been worried about my tranny layout. Anyone suggest the headphone trick when laying them out? Any good source of info for that?

You want the PT to be working for this test. Bolt it onto the chassis and wire the mains side in and set up the 6V secondary winding with a dummy heater load (something that emulates the total current draw from all the filaments of the types of tubes you anticipate using). Then switch it on.


Hook a set of high impedance headphones (say 150R) up to the OT secondary and sit the OT on the chassis. If you hear any hum, move the OT around until the hum diminishes. But if you can't really be bothered with all that palaver, then having the xformer cores oriented in different axial planes to each other will prevent unwanted EMF coupling between them.

[jojokeo]

I've done this without bolting the PT down or putting any load on the filaments, just put wire nuts on any un-used wires to be sure not to short out anything. The reason being I don't drill any holes in a new chassis before I've done this step for a new build. Which reminds of something, hesamadman - what is your chassis made of? Aluminum or steel? Also you don't want your power tubes to be too close to the PT's core especially with the core laminations lined up in the same direction as the power tubes. Some tubes and manufactures want you to line up the tube's plates in a certain orientation to each other also. Have you considered this? Another important considering that can't be underestimated is your grounding scheme and if it's implemented properly?

[alerich]

My mini JCM 800 build hums even with the power tubes pulled and the standby off. The OT and PT are almost touching each other due to tight quarters on the chassis and in the cabinet. The PT seems to be inducing hum in the neighboring OT. It is pretty small and I can live with it.