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Old 16th January 2011, 01:03 PM   #11
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For my 300B I made for heating RCR /20000 uF- 1 ohm - 20000 uF/ and tube is quiet in my 96 db speaker.
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Old 16th January 2011, 10:33 PM   #12
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Dear all. How much ripple is ok for this setup? (DC heating 300b) I am a bit new here.
50mV, 10mV, 3mV, 20uV ?
Are there any right answer for this?

Thanks in advance.

Last edited by bambadoo; 16th January 2011 at 10:35 PM.
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Old 16th January 2011, 10:52 PM   #13
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Quote:
Originally Posted by bambadoo View Post
Dear all. How much ripple is ok for this setup? (DC heating 300b) I am a bit new here.
50mV, 10mV, 3mV, 20uV ?
Are there any right answer for this?
There is no straight answer. Depends on your circuit and speakers. When you don't get any hum which is disturbing it is ok. Just try and modify until you are happy.

Thomas
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Old 17th January 2011, 10:09 AM   #14
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Quote:
Originally Posted by bambadoo View Post
Dear all. How much ripple is ok for this setup? (DC heating 300b) I am a bit new here.
50mV, 10mV, 3mV, 20uV ?
Are there any right answer for this?

Thanks in advance.
About 10 mV /measured with AVO-metter/
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Old 17th January 2011, 03:32 PM   #15
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Quote:
Originally Posted by Rod Coleman View Post
- the filament voltage affects the cathode-to-anode voltage along the filament's length. This means that there is a potential gradient of signal (MUSIC!) voltage across the filament.
Rod,

I've seen this talked about in various threads but am confused a bit. Do you mean that the current through the filament is the sum of the filament current and the anode current with a high-z filament supply? I'm having trouble seeing how that could be true but I'm curious if that is really what you are trying to say.
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Old 17th January 2011, 03:58 PM   #16
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Let's say you have the filament connected up as per the drawing in post #6: Filament + is returned to system power supply [GND]. The Filament - [NEG] terminal is floating, so far as the B+ power supply is concerned.

The filament heating current drops 5V [for 300B] from Filament + to filament -, and this forces a difference in HT potential between Anode to F+ [say, 360V] and Anode to F- [355V]. Between F+ and F-, there's a dc resistance of about 4 ohms, for a 300B.

With this arrangement, the anode *current* will all flow through the F+ terminal, but due to the geometry of the filament [crossing the widest dimension of the internal aperture of the anode] some of the electron flow will be emitted from different portions of the filament's length.

The fraction of electrons emitted at some distance from the Filament + terminal will be replaced by electron current sourced from the filament + terminal [electron flow, rather than conventional current - which flows in the opposite direction]. Of course, this fraction of the anode current must travel along the filament to reach the site of its thermionic emission.

So, we end up with a distribution of current along the length of the filament, which is slight heavier at the filament + end, due to the forced difference in HT between the two filament terminals (forced by the filament heat voltage).

Even though the current travelling through the filament is not 100% of the anode current , it is still added to the anode current at the Filament + terminal. This is important, because noise and hum in the filament HEATING current will distort this part of the current flow, and degrade the sound.

Also, this illustrates the problem with ordinary dc heating [with a capacitor across the filament]:

- the capacitor must show a small, frequency-independent impedance compared the 4-ohm of the filament
- must not add distortion.

THe only candidates to achieve the first requirement (47000uF or higher, electrolytic) are unlikely to meet the second!
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Old 17th January 2011, 05:21 PM   #17
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Hi!

Quote:
Originally Posted by azazello View Post
About 10 mV /measured with AVO-metter/
In which environment? Audible hum will be extremely different with the same ripple say between a 93dB and 103dB speaker.

Resulting hum on the ouput will also depend on the winding ratio of your output transformer. There will be a 3dB difference on the hum level at the loudspeaker terminal with the same amout of ripple between a 2,5k and a 5k output transformer.

That means the requirements for the ripple can be 10-12dB or even more different between systems. Thats a facto of 4.

Best regards

Thomas
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Old 17th January 2011, 05:39 PM   #18
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In my case - 96 db Fostex 206E
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Old 17th January 2011, 08:49 PM   #19
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Quote:
Originally Posted by Rod Coleman View Post
With this arrangement, the anode *current* will all flow through the F+ terminal, but due to the geometry of the filament [crossing the widest dimension of the internal aperture of the anode] some of the electron flow will be emitted from different portions of the filament's length.
I was thinking that the emission per unit length of the filament would be pretty much constant. Naturally, electrons would tend to congregate on the most positive portion of the filament as that is where they find the weakest repulsive forces.

I've often wondered how strong this distribution shift is in reality since there is a grid in close proximity that sits tens of volts negative of the entire filament.

I guess my confusion was that people seemed to be saying that there is a current flowing in the filament when it seems to me that there is a current flowing in the space charge through the vacuum. I was having trouble balancing the currents.

I guess my next question is: is my initial assumption correct that emission is constant across the filament or does the space charge distribution affect the emission of the filament across its length?
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Old 17th January 2011, 09:04 PM   #20
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Grid-to Cathode effective voltage will still vary from [say] 70V at one end, to 65V at the other, so I might expect a little difference, but the 300B has a large volume internal to the anode - this will give fairly gentle contours of E-field.

I don't think the emission will be wildly different across different portions of the filament, overall. This will still cause some of the current to flow within the filament's wire, I believe - the emission sites will be replenished from the filament core below.
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