A question on hum ?

[catnine]

I never noticed before but on my SF champ build and my 6G2 build which is a P-P if I have either amp on, vol on 0 ,nothing plugged in and put my ear about 6 inches from the speaker I can hear a slight low tone hum , it sounds the same on both amps . They don't have old caps and have three wire cords . If I am away from either amp by a foot I can't hear it then . Is this normal ? The heater wires are twisted and both have center tapped heaters. The tubes are not old either. I thought about adding one more filter cap on the 6G2 build but for other reasons and I am not certain how many filter caps a 5Y3 can handle.

[Enzo]

Hum is a generic term for mains frequency related noise. Hum comes from MANY sources, not just power supply filters. EACH source has its own cure. Adding a zillion filter caps will do nothing at all to help hum from heater wiring. And all heh heater wiring changes in the world will do nothing to reduce power supply ripple hum. You have hum coming from within the amp and you have hum from the world around it, like the flourescent lights over your head.

It is normal for most amps to have some sort of low "background" hum. All the hum abatement strategies you know are efforts to minimize hum. But hum is always there on the scale somewhere. Once you have quieted most of it, each additional decibel of reduction is harder to come by. Look at what NASA has to do to pick the signal out of the noise coming from its satellite probe at Jupiter or Saturn versus your table radio trying to pick up New Orleans at night. Ever larger antennas, ever quieter circuits, ever more this and that.

If twisted heater wires kills 99% of the radiated heater hum, that still leaves 1%. If elevating the heaters to some DC reduces cathode/heater hum 99%, it still leaves 1% of that. And so on. COuld you add more filters? Sure, and they would get rid of MORE of whatever ripple is left, but not ALL of it, and we donl;t know that ripple is what you are hearing anyway.

You have a single ended amp and a push pull. And you say they sound the same in hum. But since all hum is mains freq related, that doesn't tell us much. Hum all tends to sound the same anyway. SO the little hums in your Champ might be power supply related, while the hum in the P-P amp might not.


If you are a foot away, you cannot hear it? The take your ear out of your speaker and just enjoy your amps.

[Jazz P Bass]

Use a volt meter & measure the 'hum'.
Place the meter in Vac mode , clip the leads to the speaker, turn on the amp & measure the Vac ripple.
A tube amp will not be less than 4mvs.
I start to worry if it is approaching 10mvs.

[Gtr_tech]

Single ended amps like Champs, AC4s and the like will normally have a bit more hum than push pull amps. This is unavoidable. Push pull amps cancel supply hum via the output transformer, and single ended amps don't. For push pull amps, hum can come from several sources and is not always easy to track down due to the design of the amp.....