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
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
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.
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.
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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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.
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.
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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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.
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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