Hi!
When a power amp with indirectly heated tubes hums on AC, there is a problem in the design or layout of the amp. It would be better to fix the real problem instead of going to DC heaters.
Different story on preamps. For phono I use DC too. In linestages it depends on the tubes used.
Best regards
Thomas
When a power amp with indirectly heated tubes hums on AC, there is a problem in the design or layout of the amp. It would be better to fix the real problem instead of going to DC heaters.
Different story on preamps. For phono I use DC too. In linestages it depends on the tubes used.
Best regards
Thomas
Me too. In addition to DC CCS for heaters, I use separate transformer for heaters, good RF filtering after the rectifier and elevating the heaters' potential above the cathodes'. I also use separate CCS for each tube. This is for phono stage and pre-amp. For power amp final stage, AC heating is fine.
I think the point is you have to step back occasionally and decide what you are trying to achieve and how you are trying to achieve it. In a power tube like the 6AS7 there will be no measurable benefit in going to DC - so why even consider it.
Many times I see people going to ridiculous lengths to optimise a substandard circuit, when they would be far better off going back to basics and choosing a better circuit in the first place.
Its very easy to fall down the rabbit hole of audiofoolery simply for want of a bit of basic knowledge.
Shoog
Many times I see people going to ridiculous lengths to optimise a substandard circuit, when they would be far better off going back to basics and choosing a better circuit in the first place.
Its very easy to fall down the rabbit hole of audiofoolery simply for want of a bit of basic knowledge.
Shoog
Operating filaments on DC will cause metal migration that will short the its life, specially on direct heating tubes, something that we can see as well on car lighting running on DC.
Also, on transmitting tubes with direct heating running DC filaments, it was usual to reverse the polarity of the filament supply once a week or so to make sure the wear as uniform.
Now, who cares about shortening life on tubes? I don't, and I use DC every time in my applications.
But of course I digress, You are talking about regular low voltage tubes using indirect heating, so You may surely use AC without causing any hum from there; as stated by others, huming is coming normally by poor layout of the components on the chassis and some times from wrong earthing.
And if You go DC on indirect heating, the only thing that can kill your tube is the inrush current every time You power on the system. So go ahead and use DC with a current limiter, and stay within the maximum rating of the tube.
Cheers, and Happy New Year for everybody!
Also, on transmitting tubes with direct heating running DC filaments, it was usual to reverse the polarity of the filament supply once a week or so to make sure the wear as uniform.
Now, who cares about shortening life on tubes? I don't, and I use DC every time in my applications.
But of course I digress, You are talking about regular low voltage tubes using indirect heating, so You may surely use AC without causing any hum from there; as stated by others, huming is coming normally by poor layout of the components on the chassis and some times from wrong earthing.
And if You go DC on indirect heating, the only thing that can kill your tube is the inrush current every time You power on the system. So go ahead and use DC with a current limiter, and stay within the maximum rating of the tube.
Cheers, and Happy New Year for everybody!
For what it's worth, I recently built a phono amplifier for moving magnet cartridges with AC heater supply and found that the AC heater supply is not the dominant source of hum at all. I took a couple of precautions, such as using an EF86 as input valve, which has a heater to grid capacitance of only 2.5 fF (0.0025 pF), decoupling its cathode with a big decoupling capacitor of 220 uF, wiring up the heaters with thin professional audio cable (shielded twisted pair), using ceramic valve holders and using the well-known trick with two equal resistors because the transformer had no centre tap on the heater winding.
I used a positive heater bias of about +20 V, but noticed no difference when I shorted the 20 V, not even when the power transformer was placed at a large distance from the circuit. Shorting one of the two "centre tap" resistors clearly increased the hum.
The one thing I could not quite get under control was pick-up from the magnetic field of the power transformer. By properly rotating the transformer I could get the hum level subjectively below the noise level on one channel of the stereo amplifier, but not on both. I could only get it below the noise on both channels by putting the transformer at a large distance from the circuit.
By the way, interesting to hear that electromigration has been known since the early days of light bulbs. I only knew it as a limiting factor for the current handling of wires on integrated circuits.
I used a positive heater bias of about +20 V, but noticed no difference when I shorted the 20 V, not even when the power transformer was placed at a large distance from the circuit. Shorting one of the two "centre tap" resistors clearly increased the hum.
The one thing I could not quite get under control was pick-up from the magnetic field of the power transformer. By properly rotating the transformer I could get the hum level subjectively below the noise level on one channel of the stereo amplifier, but not on both. I could only get it below the noise on both channels by putting the transformer at a large distance from the circuit.
By the way, interesting to hear that electromigration has been known since the early days of light bulbs. I only knew it as a limiting factor for the current handling of wires on integrated circuits.
Hi!
This is a myth IMHO....
I've been running 801A filaments in a linestage on DC for more than 10 years.
My experience shows no evidence of tube life shortening due to DC heating.
The same in my phonostage with IDHTs on DC heaters.
Tubes loose their emission way before any effect of the DC heating in my experience
Best regards
Thomas
This is a myth IMHO....
I've been running 801A filaments in a linestage on DC for more than 10 years.
My experience shows no evidence of tube life shortening due to DC heating.
The same in my phonostage with IDHTs on DC heaters.
Tubes loose their emission way before any effect of the DC heating in my experience
Best regards
Thomas
I did a study on 5V DC vs 6.3V AC. as long as the tube wear was concerned the 5V wore consistently from triode to triode section (12ax7 match gain and transconductance sections) compared to the AC dual triode. and the 5V tube did not drop gain till 3500 hrs passed compared to the 6.3V @2200 hrs and the gain reduction was not as great as the 6.3V So I can see the way of small signal amps with heaters DC operated being an advantage.
Why didn't you compare 6.3v DC with 6.3V AC. That would have produced more meaningful results. I consistently run my small signal valves at between 5.7 - 6.0 V AC to extend valve life, and it seems no surprise that your experience of running heaters lean produces an extended service life.
Shoog
Shoog
Maybe metal migration is a myth these days, but it is physics as well, and so many tubes had suffered from this in the past.
Of course tube developments have mitigated the shortcomings and since long time ago AC is good enough for indirect heated tubes (IHT).
There were two good main reasons why tube manufacturers started to use IHT - one was to use AC on the heaters as it is not in the signal path, and the another one was to have uni-potential cathodes.
However I agree that for practical purposes we can use DC on filaments (I do), and one reason to do it is to make sure no induced hum will come in into the signal path due to wire radiation on the high impedance control grids.
Of course tube developments have mitigated the shortcomings and since long time ago AC is good enough for indirect heated tubes (IHT).
There were two good main reasons why tube manufacturers started to use IHT - one was to use AC on the heaters as it is not in the signal path, and the another one was to have uni-potential cathodes.
However I agree that for practical purposes we can use DC on filaments (I do), and one reason to do it is to make sure no induced hum will come in into the signal path due to wire radiation on the high impedance control grids.
Hi Guys,
Please read another opinion from other guy that uses a Switch to reverse the DC filament current on the DHT output tubes... really picky, and worthless, right?
211 single ended triode Tube amplifier using lundahl interstage double c core,kt88,300b,845 direct heated triode,mkp, ...
Please read another opinion from other guy that uses a Switch to reverse the DC filament current on the DHT output tubes... really picky, and worthless, right?
211 single ended triode Tube amplifier using lundahl interstage double c core,kt88,300b,845 direct heated triode,mkp, ...
So this answers my question posted on other threads here...that using 5Vdc on heaters will not affect tube performance hence prolonged the tube life as well, especially for low power tubes?
I had ask this since i've seen datasheets says 5.7 to 7v ac or dc min and max for tubes. I've tried using 6.3VAC on my Aikido preamp and no hum at all. I had many compact smps 5.1vDC so i want to give them a try.
Junm, unless DavesNotHere did a statistically significant sample size, and continued to run the test till gain reduced below an 'accepted' low level, and identifies the valve type, then you should really just take that result as an indicator only of what you may experience - not as an answer
Junm,
why go DC if you do not have any problems with hum at all? Adding a SMPS can even introduce some HF noice or hash issues.
I'd be careful with underheating IDHTs so much. There have been also reports of reduced lifetime if you underheat too low! If I remember correctly underheating can cause 'cold' islands on the cathode surface which will drop in emission. The other warmer parts of the cathode try to keep up und will wear out quicker.
Someone with more knowledge on this topic may correct me if I wrote something wrong
Thomas
why go DC if you do not have any problems with hum at all? Adding a SMPS can even introduce some HF noice or hash issues.
I'd be careful with underheating IDHTs so much. There have been also reports of reduced lifetime if you underheat too low! If I remember correctly underheating can cause 'cold' islands on the cathode surface which will drop in emission. The other warmer parts of the cathode try to keep up und will wear out quicker.
Someone with more knowledge on this topic may correct me if I wrote something wrong
Thomas
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