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Signal tube noise!

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M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
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!:confused:

Regards
M. Gregg
 

ChrisA

Member
2008-01-08 12:22 am
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!:confused:

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

Disabled Account
2010-06-28 11:04 pm
UK
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.
 
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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.
 
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M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
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.
 
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20to20

Member
2010-06-23 9:25 pm
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

Disabled Account
2010-06-28 11:04 pm
UK
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.
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
I think the H-K leakage as you say is the most probable!
You have got me thinking now I wonder what will happen if I reverse the DC on a "bad" tube?
Other guys use DC filaments I don't hear them having this problem.
 
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20to20

Member
2010-06-23 9:25 pm
I have had it happen to 3 tubes now NOS RCA, Mullard, Now Groove Tube. The voltage is spot on 6.3V DC.

I had a 12ax7 make a kind of hissy, sizzly, muffled poping noise. Not constant but aggrivatingly too present. I swapped sockets and it was there. Tossed it.

I've also had a dirty socket, and tube pins sound like that too. But wiggling the tube gives that one away.
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
The tube seems to go microphonic and noisy! If you tap it the sound comes and goes! I have changed the socket " Still the same. I thought it was an Elec cap so I changed that too! Its a recent build. I would bin the circuit, however I want to know whats happening might I come across it again!
 
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20to20

Member
2010-06-23 9:25 pm
I think the H-K leakage as you say is the most probable!
You have got me thinking now I wonder what will happen if I reverse the DC on a "bad" tube?
Other guys use DC filaments I don't hear them having this problem.

Since a good tube will usually work with a lower filament voltage, you might try dropping a volt to it if you can, for kicks, to see if it makes any difference at all.
 
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.

That can possibly apply to DHT's because the filament carries the signal current as well as the heating current. The positive side of the filament will carry more of the load than the negative side. Probably only an issue when tubes are run near current limits.

With a IDHT, the heater is not part of the cathode current flow, so this would not be an issue.

Sheldon
 
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