Signal tube noise!

[M Gregg]

Just a quick question.

Has anyone had this! I have found that signal tubes ECC 81 etc when used with DC heaters start making a sound when amplified through the speakers! Sounds like someone dragging their nails across the back of the speaker cone. If you tap the tube it has become "mechanicaly noisy". If you change to AC heater supply it does not happen!

Regards
M. Gregg

[hidnplayr]

Have you tried elevating your heaters (dc) ?
To answer your question: no, I dont use ECC81 or DC heaters at ground potential

[M Gregg]

Originally Posted by hidnplayr

Have you tried elevating your heaters (dc) ?
To answer your question: no, I dont use ECC81 or DC heaters at ground potential

I must admit I assume that AC heaters or DC heaters would be at Gnd due to hum bucking. However I will try lifting the heater chain.

Thank's for your thoughts.

[ChrisA]

Originally Posted by M Gregg

Just a quick question.

Has anyone had this! I have found that signal tubes ECC 81 etc when used with DC heaters start making a sound when amplified through the speakers! Sounds like someone dragging their nails across the back of the speaker cone. If you tap the tube it has become "mechanicaly noisy". If you change to AC heater supply it does not happen!

Regards
M. Gregg

Two things, you have to elevate the DC, just like you would with AC and if you are not careful the DC wires can act like a highway system for transporting noise. Signals can couple to the DC wire at one place and conduct some distance away and then couple again to something else. I think you have to still twist the wires, watch layout and maybe even regulate and bypass the DC.

[M Gregg]

I must admit that I find it strange that the problem only occurs with the DC. The circuit works great for about a month then the noise happens. It sounds like the heater is dropping "sliding" inside the cathode. Only happens with DC not if I use AC.

[M Gregg]

Might sound like a daft question. The heaters are 6.3V or 12.6 AC thats RMS? Does that mean the DC value is different?

[Sheldon]

Originally Posted by M Gregg

Might sound like a daft question. The heaters are 6.3V or 12.6 AC thats RMS? Does that mean the DC value is different?

The integral of power for AC expressed as VRMS, is the equivalent of VDC. So the same value would apply for both.


Sheldon

BTW, if your system is quiet with AC filaments, then why use DC? If you have some hum at AC, and haven't lifted the filaments (15-30V or so), then that's the first thing to try. Only if that's not quiet enough, would I go with DC.

[M Gregg]

I find it "sounds" better with DC filaments. I must admit this has got me puzzled! Someone said to me in the distant past that the heater can age in a non uniform manner with DC. I don't want to post old wives tales, it's probably a load of hog wash.

[20to20]

Originally Posted by M Gregg

I find it "sounds" better with DC filaments. I must admit this has got me puzzled! Someone said to me in the distant past that the heater can age in a non uniform manner with DC. I don't want to post old wives tales, it's probably a load of hog wash.

I can't speak from experience on this but here goes a theory.

The DC voltage is high there ALL the time. So if the heater to cathode seperation is starting to break down and "leak" with noise, this is what may be happening. With the AC heater, the voltage is cycling down to zero and only peaks momentarily so the opportunity for the leakage is smaller. So if it is DC H-K leakage it may speak to the quality of the tube.

[M Gregg]

Originally Posted by 20to20

I can't speak from experience on this but here goes a theory.

The DC voltage is high there ALL the time. So if the heater to cathode seperation is starting to break down and "leak" with noise, this is what may be happening. With the AC heater, the voltage is cycling down to zero and only peaks momentarily so the opportunity for the leakage is smaller. So if it is DC H-K leakage it may speak to the quality of the tube.

I have had it happen to 3 tubes now NOS RCA, Mullard, Now Groove Tube. The voltage is spot on 6.3V DC.