AC or DC current for 300B and 2A3 tube filaments ? - diyAudio
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Old 6th October 2004, 10:23 AM   #1
Kriskom is offline Kriskom  Indonesia
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Default AC or DC current for 300B and 2A3 tube filaments ?

Hi colleques,

Anybody helping me in deciding what is best for a 300B and 2A3 tube filament - an AC or DC current ?
Reasons behind is much appreciated as this will help adding my knowledge

Thank you
Paul Yahya
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Old 6th October 2004, 10:52 AM   #2
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While I have not used them personally, all replies I have seen to this question is "In any DHT, use of AC is mandnatory". It seems those who have tried DC, their project was doomed to crappy sound
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Old 6th October 2004, 11:17 AM   #3
Kriskom is offline Kriskom  Indonesia
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Thanks Gregg t3h GeeK
Thus I need to provide a "humpot" to counter the possible hum .

Question :
People say that the humpot MUST be placed as near as possible to the filament pins. True ? Why ?

Yrs

Paul
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Old 6th October 2004, 11:23 AM   #4
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Hi,

Quote:
"In any DHT, use of AC is mandnatory". It seems those who have tried DC, their project was doomed to crappy sound
Sorry but that's just not true....
Another one of those urban myths I'm afraid.

Cheers,
__________________
Frank
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Old 6th October 2004, 11:34 AM   #5
Kriskom is offline Kriskom  Indonesia
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Thanks Frank.
Since it is a "myth", then DC supply can do ?
There seems two categories of people : DC and AC fans, and thus both do have their justifications ?
What is it then ? if some colleques can advise me ?
Thanks a lot
Paul
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Old 6th October 2004, 11:43 AM   #6
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If you have a valve that only needs a small amount of -ve grid bias, then the differential dc voltage across the heater can be a significant fraction of the bias, meaning that the emission across the filament varies as the effective heater-to-grid voltage will vary along the filament.

As most DHT triodes require a high -ve bias voltage, the small dc change in effective bias voltage resulting from a 6V (say) heater supply won't result in too much variation in emission across the heater.
It will still have some effect though, so it might be audible or measurable.
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Old 6th October 2004, 11:49 AM   #7
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Quote:
Since it is a "myth", then DC supply can do ?
It's a myth only as far as absolutes go. It's possible to design a very complex multiple choke/high quality caps/special diodes dc heater supply but it's not easy. Building a simple DC source is guaranteed to sound as crap.
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Old 6th October 2004, 12:49 PM   #8
Previously: Kuei Yang Wang
 
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Default Re: AC or DC current for 300B and 2A3 tube filaments ?

Konnichiwa,

Quote:
Originally posted by Kriskom
Anybody helping me in deciding what is best for a 300B and 2A3 tube filament - an AC or DC current ?
AC will cause a mudulation of the signal which represents itself as hum and intermodulation. That gives AC it's charateristic sound. Most DHT's sound by far to opaque and hum by far too much with AC. AC heated Amp's often are also replete with parts having a forward/bright sound signature, cutting through the Fog created by AC heating.

DC causes it's own problems and needs to be implemented with great care, or Amplifiers will sound strident and unpleasant, moreso if an Amp designed and voiced for AC heaters is converted to DC.

So, the answer is obvious.

If you do not wish to tolerate too much noise the choice is DC. Then design and voice the Amp to work well with DC. You can get as low as <0.3mV noise levels out of a sensibly implemented DC heated 300B Amp, or > 80db below 1 Watt.

If a hum level around 50-60db below 1 Watt (which is the best achievable with AC Heating and non-extreme hum cancellation circuits) is acceptable to you (the average for AC Heated SE Amp's with 2A3 and 300B is around 3-5mV) then go AC and voice the Amplifier to work well with AC.

Sayonara
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Old 6th October 2004, 04:09 PM   #9
Kriskom is offline Kriskom  Indonesia
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Arigato Kuei -san for yr explanation.
Very interesting. I am still a rooky.
But I still need to make a decision based on the following progressing 300B project technical specs :
Power supply trafo :
Primary 0-220-230VAC
Secundary :
2.5-0-2.5V / 3A for the filaments of two Russian Flying C 5U4GTB rectifiers.
380-0-380V/ 300mA for the high current
two 5V/2A for the two 300B heater filaments . There is no centertap on this winding
two 6.3V/3A for the two 6SL7 drivers

For both channel high voltage I am using two chokes of 10H/200mA , two 20uF/600V oilcaps , two 40uF/600V oilcaps and to come to a B+ of 403VDC (6SL7) and via two 51K/0.5W resistors to B++ 455VDC for the 300B
While the 6SN7 heaters will go on the 6.3V standard converted to DC via a rectifier diode and some caps.

I need to decide whether using AC or DC for the 300B heaters.
Basedupon this scantily information, what wuld yu suggest ?
I have provisionally provided a humpot of 100R/4W pAralelled pin 1 & 4 of 300B with 39R/2watts to the pot centerpin , and via a 1K/20W resistor to ground.
Ideally null hum is preferred , but I am prepared for little hum as ong as it is not much audible during silent passages.

Thanks for yr attention
Konbanwa
Paul
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Old 6th October 2004, 05:01 PM   #10
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Konnichiwa,

Quote:
Originally posted by Kriskom
the 6SN7 heaters will go on the 6.3V standard converted to DC via a rectifier diode and some caps.
Why? You are introducing unneccary rectification noise back into the mains transformer and the 6SN7 does not need AC, just make sure the 6SN7 heater is biased suitably to avoid parasitic rectification effects.

That said, why on earth a 6SN7? It would be at the very bottom of my list of Valves to use in a 300B Amp (actually, it would be OFF THE LIST).

Quote:
Originally posted by Kriskom
I need to decide whether using AC or DC for the 300B heaters.
Basedupon this scantily information, what wuld yu suggest ?
You don't need to commit youself. Leave space for a decent DC supply and start with AC. The 5V will rectify to around 6V DC using schottky diodes and a 10,000uF Capacitor, add a nice filter choke with around 0.75 Ohm DCR and a second capacitor and you have a pretty decent DC Heater supply. Then use what sounds better to you.

Quote:
Originally posted by Kriskom
I have provisionally provided a humpot of 100R/4W pAralelled pin 1 & 4 of 300B with 39R/2watts to the pot centerpin ,
That is fine and will work for both AC and DC.

Quote:
Originally posted by Kriskom
and via a 1K/20W resistor to ground.
Why 1K? I feel 880R is more apropriate at the HT you will be ending up with. Parallel 7K5/5W with the 1K resistor and you have pretty close to 880R.

Quote:
Originally posted by Kriskom
Ideally null hum is preferred , but I am prepared for little hum as ong as it is not much audible during silent passages.
Okay, lets see.

If your speakers are 90db/2.83V sensitive you will get between 30db and 40db absolute SLP of 100Hz hum in 1m Distance. A pair of speakers in stereo and at around 3m listening distance will result in 27-37db SPL of hum at the listening position. I personally find the 27db tollerable but not the 37db and in late night "quiet" listening even the 27db absolute can cause irritation.

Now if your speaker are more sensitive than 90db/2.83V/1m you will likely experience significant audible hum.

Sayonara
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