According to Radiotron 4th ed., there isn't anything wrong with under-powering the filament/heater. However, you do have to decrease the plate current accordingly since fewer electrons are being emitted from the filament or cathode. This leads to a smaller space charge available to supply peak currents. Depleting the space charge is what's bad.
I would think that under-powering the filament/heater might have some benefits such as less thermalnoise due to less heat. (I'm just hypothesizing here, I run my heaters at full voltage.)
SpreadSpectrum said:
Yes, this is true, also as in pre-amps one seldom goes anywhere near maximum Ia. But the advantage is not dramatic, in my experience. (I would believe the greater advantage might be in longer life of the heater! - though I cannot recall ever having had a tube heater burn-out in my life.)
I have had no issues with running my heaters slightly lean, 6V down from 6.3V with no adjustment to the circuit.
I have been running some PCC88's at 6.3V for about two years and no issues. I expect them to go on for a long time.
It has been suggested that this slight under heating will increase tube life and improve linearity.
If running autobias doesn't the operating point adjust to compensate for the underheating and reduction in plate current ??
Shoog
Shoog,
From my reading of the first chapter of Radiotron 4th (I'm no expert on this subject by any means), it sounds like if the operating conditions of the circuit demand higher current than the filament/cathode can provide (from the space charge around the filament), the cathode will provide those electrons directly. This is not good for an oxide coated cathode, but tungsten cathodes don't mind. I don't think that underheating will limit current by itself. I think that it's up to the circuit designer to ensure that an underheated tube is also run conservatively.
If you don't have access to this book I can post the paragraph that addresses this if you are interested.
ErikdeBest said:
...like me.
I use regulated AC with current limiting. It prolongs life of filaments due to small in-rush current causing less local overheating of filaments, working points of tubes are more stable, and level of hum is less than possible with AC.
SS components are very cheap compared to what has to be spent to obtain transformers!
Edit: here is one possible sample (C2/R3 time constant instead of current limiting):
SY said:
About 30yrs in my case and I am with SY on this one.
Given the efficiency of the speaker systems I typically use I need less than 1mVrms hum and ripple (I actually measured less than 400uVrms hum, ripple and noise measured with grounded inputs) at the output of my power amplifier in order not to hear hum from this source. (Unfortunately my current line stage has magnetic coupling into the opts and electrostatic coupling into the 26's- noise at the output is under 200uVrms and I can HEAR it because the following power amplifier has 20dB of voltage gain.)
I use CCS to heat 5V power dhts exclusively, a mix of ac or dc heating as appropriate for other applications. Signal path in my case all dc heating.
My hearing is typical for the 50yr old I am except in one area, my threshold of hearing for noises below 100Hz is about 20dB better than average (for all ages) which makes me absolutely maniacal about hum. I can hear the 50VA Hammond power transformer in our gas boiler from 10' away or more. I listen with all fluorescent lamps in the room shut off because I can hear their magnetic ballasts in the background. My wife and most of my friends surprisingly cannot.. Go figure..
Is this a blessing or a cursekevinkr said:
My hearing is typical for the 50yr old I am except in one area, my threshold of hearing for noises below 100Hz is about 20dB better than average (for all ages) which makes me absolutely maniacal about hum. I can hear the 50VA Hammond power transformer in our gas boiler from 10' away or more. I listen with all fluorescent lamps in the room shut off because I can hear their magnetic ballasts in the background. My wife and most of my friends surprisingly cannot.. Go figure..
I am.
Learning never ends and I'm always ready to learn.
I was reading on another thread about an issue with AC which I had never taken into account. I think it was "rdf" who pointed out that line hash on the heater AC could bleed across into the cathode and produce all sorts of modulation distortion. It seems a very reasonable point. However a good line conditioner should address this problem and make the AC benign.
Shoog
Shoog,
If you don't have this book, you must download it.
I was looking at paragraph 5 of page 3, "If the filament or heater is operated for long periods at reduced voltages, the effect is a reduction in emission, but no damage is generally done to the valve unless the cathode currents are sufficient to exhaust the "space charge." Low cathode temperature is, therefore, permissible provided that the anode current is reduced in the proper proportion."
And on the bottom of page 2, "In the case of one having a pure tungsten filament no damage is done to the filament if all the electrons emitted are drawn away immediately to the anode. This is not so, however with oxide coated cathodes..."
I had very little understanding of the physics of how tubes operate until I read chapter 1 of this book. Very well written and understandable. I mean, its definitely a reference book and is kind of a boring read cover to cover, but it has got many jewels of information that dispel many of the myths floating around. This is the authoritative work from the period when tubes were the technology and major money was being spent on tube R&D.
If you don't have this book, you must download it.
I was looking at paragraph 5 of page 3, "If the filament or heater is operated for long periods at reduced voltages, the effect is a reduction in emission, but no damage is generally done to the valve unless the cathode currents are sufficient to exhaust the "space charge." Low cathode temperature is, therefore, permissible provided that the anode current is reduced in the proper proportion."
And on the bottom of page 2, "In the case of one having a pure tungsten filament no damage is done to the filament if all the electrons emitted are drawn away immediately to the anode. This is not so, however with oxide coated cathodes..."
I had very little understanding of the physics of how tubes operate until I read chapter 1 of this book. Very well written and understandable. I mean, its definitely a reference book and is kind of a boring read cover to cover, but it has got many jewels of information that dispel many of the myths floating around. This is the authoritative work from the period when tubes were the technology and major money was being spent on tube R&D.
In Europe the standard line voltage is 220V, in Britian and Ireland the standard line voltage is 230V. Britian no longer makes many normal Tungsten light bulbs, we get most of ours from Europe where they are spec'd for 220V. Tungsten light bulbs generally last significantly less in Britian than in Europe (by a factor of up to 3x). This shows that even slight overvoltage can significantly reduce filament life. The opposite has been shown to be true in that a Tungsten light bulb can last 300% longer if slightly undervoltaged. The same principles apply to valve filaments.
It would seem that its nearly irrevelant to consider heater life in normal operation. Underheating could be a problem if the tube is run hard. Potentially overheating is a much more serious issue and could shorten tube life dramatically. The only serious issue is whether slightly underheating does infact improve linearity, or whether this is purely a audio myth. Heater failure is practically unknown.
So DC removes potential voltage problems and can slow inrush currents. It has to be questioned whether the potential gains are warrented in light of the ruggedness of heaters. DC should remove line hash (if done properly - if done badly it could introduce bad ringing and current spiking).
AC line hash should be removable with a simple ceramic cap on the heater and a good line conditioner.
Overall I can't see that there are enough advantages for me to consider going from AC to DC unless dealing with microphone or low output phonos.
I think very slightly underheating may have enough advantages to be used.
Shoog
It would seem that its nearly irrevelant to consider heater life in normal operation. Underheating could be a problem if the tube is run hard. Potentially overheating is a much more serious issue and could shorten tube life dramatically. The only serious issue is whether slightly underheating does infact improve linearity, or whether this is purely a audio myth. Heater failure is practically unknown.
So DC removes potential voltage problems and can slow inrush currents. It has to be questioned whether the potential gains are warrented in light of the ruggedness of heaters. DC should remove line hash (if done properly - if done badly it could introduce bad ringing and current spiking).
AC line hash should be removable with a simple ceramic cap on the heater and a good line conditioner.
Overall I can't see that there are enough advantages for me to consider going from AC to DC unless dealing with microphone or low output phonos.
I think very slightly underheating may have enough advantages to be used.
Shoog
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