Valve headphone amp humming through headphones

[bbking83]

Hi

I have a little dot mk3 which is humming through the headphones, i have removed earth issues by cheating the earth cable, making sure it is unplugged from all sources etc.

Now the problem is this hum, it does not get louder as i change gain switches, neither does it get louder if i use the volume control.

Something i noticed last night, if i let the amp go cold it takes 18 seconds to start humming again, yet if i power on the amp from warm it starts humming almost straight away.

Anyone certain this could be a valve issue, or should i send this back to the guy I bought it from?

many thanks
Simon

 

[Mooly]

From a cold start and the valve heaters are also cold meaning that the valves are 'non operational' in that state. So only hearing hum after a few seconds after a cold start is normal, and hearing it immediately following a hot start is also normal (because the valves are up to temperature and operational)

Hard to say what the issue is but if the hum is present with no inputs connected (and ideally with the input selected also having shorting plugs in place) then it suggests a design issue (or fault). The fact it hums with the volume on minimum reinforces that point of view.

Sorry... probably not what you wanted to hear.

 

[bbking83]

I forgot to add that one of the valves is slightly microphonic, but from what i read there is no way a microphonic valve could cause hum.

So you don't in any way think any of pre or power valves could present this humming problem??

 

[Mooly]

I don't think a valve will be faulty as such, but different valves could alter the perceived hum level. You would only know that by substituting them.

I've done a quick web search and it seems there are several reports of 'hum' to varying degrees.

 

[316a]

Probably not the power supply as there would be buzz rather than hum . The hum could be radiated from the mains transformers which look very close to the output valves .

316a

 

[azazello]

I thing....hummm is 50-60 Hz riples and it caused bad filtering. Do You have chock in PSU? Or maybe after some time, the current of output tube is rising and riples in PSU also are rising....
You can check bias track for output tubes. If You have multimetter, check U on the first grid or on cathode resistor of output tube in first moment after switch- on and after hummm rising. U must be the same.
If You don't understand me, ask for specialist or better send it back.

 

[Sheldon]

Probably not the power supply as there would be buzz rather than hum . The hum could be radiated from the mains transformers which look very close to the output valves .

316a

Mains hum can also be picked up by having inductive coupling between unshielded or untwisted signal wiring close to, or parallel to mains or heater wiring. Mains and heater wiring should also be tightly twisted. Sometimes caused by inductive coupling of power transformer with output transformers - but in this case it would be in the output channel closest to the power transformer.

Sheldon