Hum from on/off/volume control???

[Resonator Guy]

A while ago in some other thread I read that On/Off/Volume controls can cause hum. Enzo may have said that but I can't recall.
Can anyone tell me what that hum might be like. Tough question but I have a little MOT Noble amp that I recapped and there is a hum that is rather odd. It is relatively low up to about 6 on the dial, then disappears until it is over 8 when it returns, perhaps with a different frequency, much louder right up to 10 on the dial.
Could this be the control itself or should I just replace it and see? I want to keep it as original as possible, but I want it working reasonably correctly.
Thanks.

[g1]

Temporarily remove the wires from the switch part of the pot and connect them (power off of course). Dress the wires away from the pot and see if it solves the hum problem.
If it gets rid of the hum you will want to use some other type of switch for the power. If it does not help you can return it to original and search for the source of hum elsewhere.

[Jazz P Bass]

If the wires are long, twist them together.

I sh*t you not I had this resolve a hum issue on an old Fender where the on/off was part of the tone knob.

[Resonator Guy]

Thanks, guys. I will try both later today and see if it helps.

[Resonator Guy]

This is the second smallest chassis I have ever worked on. J P Bass, the wires weren't long enough to twist. One was just the lead from a cap on the preamp tube. No room at all. G-one, I tried the remote volume control and it seemed to work so I de-soldered all the other stuff on the switch and mounted a brand new one. It sounded great until I turned it up (not that I ever play loud) and it was humming away as loud as ever. Well, I know what the hum isn't.

[stokes]

The point g-gone was making was to install a power switch,a source of 60hz hum, away from the signal source,in this case,the volume pot.Installing a new pot/switch doesnt work,as you've seen.

[Resonator Guy]

Ah, but there is absolutely no room for a power switch. Like I said, one of the smallest chassis ever.

[stokes]

Can you not even fit one of those small "micro" switches?The problem is that the 60hz from the 120 vac line is getting into the signal via the volume control.If you cant find a place for a switch it may be worthwhile lengthening the wires that feed the switch so you can then try Jazz' suggestion of twisting the pair together.It will definately quiet the hum if not totally eliminate it.It is the same idea as twisting the heater supply leads.In such a small chassis I would even suggest twisting any ac pairs,like the HV secondaries to the rectifier and the 6.3 v heater supply from the tranny.

[Enzo]

First you need to determine if th switch wiring is REALLY the source of the hum. it may not be. That nis why it was suggested you take the wires OFF the switch terminals on the back of the control. COnnect them together so the amp is always ON, but position them away from the control as much as possible. NOW plug the amp in and see if it still hums the same. If it still hums, then the switch is not the issue. If the hum is gone, then we need to move the wires around somehow. Hint, a longer piece of wire can be installed to give more slack. We are doing this only temporarily, and only to determine of the wiring is the problem or not.


But i think your analysis might be wrong. The fact the hum disappears up to 6 on the dial then returns is not the control's fault. What you likely have is more than one source of hum. One source is before the volume control, so as you turn up the volume, that hum gets louder. It may not seem like it, but it actually just gets louder all the way up. BUT BUT BUT there is more hum getting into the system after the control as well. And due to the inverting nature of tube stages, it is "out of phase" with the other hum. So they try to cancel each other. When the volume is low, you mostly hear the hum from after the control. But when you get up to about 6 on the dial, the pre-control hum and the after control hum wind up about the same level, and being out of phase they thus cancel the most. So your hum overall sounds least. As you turn up even further, now we are amplifying the pre-control hum even more so you now hear more of that. And that also explains why the hums sound different below and above 6.

This may sound mysterious, but it is really very common.

There are many sources of hum in a tube amp, and each one has its own cure. You might have half a dozen sources of hum and that means we have to apply half a dozen solutions. A lack of shielding in the preamp stage might be one, while power supply ripple might be another.

[Resonator Guy]

Stokes and Enzo. I will try a few more things on this soon. I haven't even looked at a soldering iron for a while. This amp is primarily a shelf adornment but I would like it to sound right. It is a little sky blue three tuber that I don't intend to put an isolation transformer on, but it should perform properly to stay in my music room.

[Resonator Guy]

Well I finally got back at this and, before taking it apart again, I used all my electronics knowledge and tried the obvious. I put in another power tube. Almost all the various hums disappeared. There is some if turned up all the way, but that really is painfully loud. At half way, which is louder than I use it , there is no hum at all.
So on to the next one.

[Jazz P Bass]

"I used all my electronics knowledge and tried the obvious. I put in another power tube."
That gave me a chuckle.
Thanks.

[Resonator Guy]

[Bruce / Mission Amps]

***Not saying do this***
But, there was a reason lots of amps made with the on-off switch on the volume pot actually would break the neutral wire instead of the hot lead.

[Resonator Guy]

Bruce, would that be the same as just flipping the two prong plug over?