Help with hum in AC heaters

[HeadSh0T]

Hi

I have a tube output stage in my DAC which is humming. It's not a lot of hum I guess, but I can hear it in the background when my amp is turned to normal listening levels.

The tubes are a pair of ECC99s and a 6X4 in the power supply. All three tubes are AC heated. I ran the high voltage power supply through PSU Designer and it seemed pretty clean so I'm concluding it must be the AC heating that's causing the hum.

Is it possible to heat tubes with AC and eliminate hum entirely? or do I have to live with some hum?

What can I do to reduce/eliminate the hum?

[JOE DIRT®]

I am not a tube expert but I know from doing pro sound and dealing with guitar amps that maybe you have a ground issue either inside the amp or from the wall.....I would try another outlet that is on a different circuit first and if that does not cure your problem then try relocating your ground in the amp...another question arises that you did not say whether your amp is grounded to the mains or are you feeding it just the Hot and neutral wire>????

[fragman56]

Have you tried putting a 100 ohm pot between the AC leads with the wiper to ground, adjust for hum...this has eliminated some hum for me.

[wd40]

I am not a tube expert too.But in the DAC or CD tube output, suggest DC heated.

[Saurav]

Are your tube heater wires tightly twisted and far away from all signal circuitry? If any signal wires cross it, do they cross at right angles? And here's a trick that you can use with tubes like the ECC99 which might reduce the hum - build a voltage divider from B+ to ground such that the 'center point' is at about 40V, and connect this to the heater. You'll need to make sure the AC supply (i.e. the transformer secondary) is floating, and you'll probably also need to add a capacitor from the heater to ground. This should help with the hum.

Disclaimer: if you can't visualize what I'm trying to say, please don't try it. Also, this may not be an option if your tubes are set up as a mu-follower or SRPP or a direct-connected cascade or something like that. In those cases you probably already have a voltage divider raising the heater's potential.

[HeadSh0T]

Thanks for the replies..

Joe: I've even tried my DAC at someone else's house and it has the same issues. The DAC is grounded to the mains ground through a termistor.

fragman: I have not tried this but it sounds like a good idea.

wd40: i've considered DC heating but I don't have any more space for the parts required to do this!

saurav: no, my heater wires are not twisted. I guess I should do that. They don't cross any signal wires but they are about 5-10cm away from them..
I think I know what you're talking about but I can't really visualize it. I'm not sure what components to use for that.

[fragman56]

I think this is a given!

Twist those heater wires first/always. Get them away from signal wires. Jack around with placement while listening--agree completely with Saurav!

[fdegrove]

Hi,

I am not a tube expert too.But in the DAC or CD tube output, suggest DC heated.

10/10 for WD.

The voltage levels in a DAC are too low to eliminate hum entirely.

If hum is audible at normal listening levels then that's a lot of hum in my book.

While you're at it, regulate the heater feed and set it for at least 6V or 12V using a 78xx or similar or use a LM317 and set the output to the correct voltage.

Either way no more or less than 10% of the required voltage.

For the ECC99 I'd pt for the 12.6V heater arrangement which eases the current carrying demand.

Cheers,

[EC8010]

Originally posted by HeadSh0T
Is it possible to heat tubes with AC and eliminate hum entirely?

Yes. You need to do the heater wiring with tightly twisted (4-5 turns per inch) solid core wire, it needs to be pushed into the corners of the (earthed) metal chassis and arrive at each valve on a radial line. The order from the transformer must be valve with highest signal voltage first, least signal voltage last. Never make a loop between the conductors that encloses a valve base, keep the conductors twisted together at all times. The heater winding needs to have a centre tap to chassis. If you do all that perfectly, you can make an RIAA stage that doesn't hum using AC heaters.

[fdegrove]

Hi,

The heater winding needs to have a centre tap to chassis.

Shouldn't this work as well with 0-X volts xformer and 2 equal value resistors across the winding with the midpoint of the 2 Rs grounded?

I never tried this though....

Cheers,