#26 pre amp
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coolzero
diyAudio Member

Join Date: Sep 2009
Location: UAE
Quote:
 Originally Posted by Sheldon It's easier to understand if you think of current loops, and ground (or common) reference points as independent issues. There is a current loop for the filament. The current from the filament supply must return exactly to the filament supply. There is a separate current loop for the current through the tube, which also must return exactly to its supply. Let's say you are using cathode bias. In this case the current through the tube must go through the cathode resistor before it can return to the supply. And in cathode bias, the bottom of the cathode resistor is at ground (or common). Let's take an example, of a 5V, 1amp filament, and 10mA plate current and a 1k cathode resistor. Let's assume that the negative end of the filament is connected to the cathode resistor. With 10mA and a 1k cathode resistor, the potential at the top of the cathode resistor will be 10V. Now, the negative side of the filament will be at 10V. If we want to use filament bias, we take the filament return voltage from the bottom of the cathode resistor, instead of from the top of the cathode resistor. Now, the filament is in series with the cathode resistor, so both the plate current and filament current must go through the cathode resistor. The total current through the cathode resistor is now 1.01R (1A+10mA). If we want the same 10V at the cathode resistor, we must calculate the new resistor size (V/I=R or R=10/1.01=9.9R). So to get the same bias, we have reduced the cathode resistor by a factor of 100. The advantage is that we can eliminate the bypass resistor, since this low value resistor will cause very little cathode degeneration. The disadvantage is that any noise due to the filament supply ripple over the cathode resistor, will be amplified by the u of the tube. Sheldon
Thanks a lot Sheldon. Your explanation is an excellent education for me as usual. I put it all into a diagram; can you verify whether what I understood is correct.
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 21st June 2010, 09:43 AM #502 andyjevans   diyAudio Member   Join Date: Sep 2003 Location: London Hi Coolzero, I am using voltage regulation not current regs for filament bias - I would otherwise use current regs, but Thomas Mayer found this didn't sound so good as a LCL filament supply. I haven't gone into the supply yet, but voltage regulation is easy to implement here just to get it up and running and get the voltages right. Note that the 10 ohm resistor would benefit from being 11 ohm to keep the current through the 26 closer to 6mA. And a 20 watt part is really not adequate - think at least 30W or preferably 50W and that's better on top of the chassis. This resistor gets hot. On your diagram, if you have 10v on the top of the cathode resistor you have 11.5v on the other side of the filament, which is your supply voltage. I have a 11 ohm cathode resistor (10+1 ohms, 50W and 12W vitreous enamel) and 9.7v at the top of it, so supply voltage is 11.2v. Andy
 21st June 2010, 11:39 AM #503 Richard   diyAudio Member   Join Date: Apr 2005 Location: Melbourne Sheldon, Thankyou also for the explination and Coolzero for the schematic. Here's the link to Thomas' schematic on AA, if you haven't found it already. Tube DIY Asylum: RE: Filament bias for DHTs - how to do it? by Thomas Mayer Regarding filament supplies, after building a choke input one, I realised they were the way to go. I'm still using a LM350 after the LC but want to try a passive LCL when I get my hands on some 0.4H chokes Gianluca has given some insight using a LLCCLL supply and Thomas seems to use 400 - 600mH chokes in the + leg. Audio Asylum Thread Printer Has anyone here tried a passive filament supply along similar lines? Nicoch46, thanks for that interesting link, sounds like a quick weekend project . Rich __________________ "A little learning is a dangerous thing"
 21st June 2010, 12:24 PM #504 coolzero   diyAudio Member     Join Date: Sep 2009 Location: UAE Andy/Rich, Thanks for the information. However the Thomas Mayer’s schematic is not available it seems from that link. If someone is having it please share. One more thing. Can someone give an idea of what values to use in this LCL filament supply to work with the mentioned voltage? I have seen one before in the same thread but not sure whether it will suit for this.
 21st June 2010, 12:36 PM #505 andyjevans   diyAudio Member   Join Date: Sep 2003 Location: London Thomas writes as follows: "I'm using the new Lundahl filament chokes, the 2733. It has 0.1H/3,7A in parallel and 0,4H/1,7A in series connection. The filter chains themselves are LCL. The first LC section in the PSU chassis and the last L at the tubes. A small input cap can be used to tame the supply a bit, like 470uF. As cap between the chokes I use 40.000uF. I let the filament bias source end with a choke to AC decouple it from the signal path So looks like 470uF > 400mH > 40,000mF > 400mH I don't quite know how he adjusts the voltage here - remember he uses 801 not 26, so his filament supply is nearer 50v. "In my 801 linestage the bias resistor is only 33 Ohms. I still use the ultrapath connection. The B+ side of the lineout transformer is returned to the - side of the filament. So the AC signal actually bypasses the bias resistor (at least from the midrange on upwards)" Thomas Andy
 21st June 2010, 12:43 PM #506 Richard   diyAudio Member   Join Date: Apr 2005 Location: Melbourne Hi Coolzero, In the first link, see the end of the 3rd post by Thomas, for his filament bias. Also just scroll down in the last link, to the schematic with the 801 driving 801. The filament chokes are 0.6H. The Lundahl LL2733 seems to be the one developed for filament supplies. It has 2 0.2H (200mH) coils that can be used in series for 0.4H or parallel for 0.1H. From the schematic Thomas uses it in series just prior to the filament. Just wondering about the 1st choke in an LCL supply, is it better to use 1 coil in each leg + & - (ala common mode) or use them in series just on the + leg? Can anyone share some knowledge on this? Rich __________________ "A little learning is a dangerous thing" Last edited by Richard; 21st June 2010 at 12:56 PM.
 21st June 2010, 12:46 PM #507 Richard   diyAudio Member   Join Date: Apr 2005 Location: Melbourne Just beat me to it Andy. Also did you ever compare the Hammond 126C to the Lundahl 1660? Rich __________________ "A little learning is a dangerous thing"
 21st June 2010, 01:18 PM #508 andyjevans   diyAudio Member   Join Date: Sep 2003 Location: London Hi Richard, I have some Lundahls but haven't got round to comparing them. I doubt if they are worse!! Andy
Sheldon
diyAudio Member

Join Date: Dec 2003
Location: San Diego
Quote:
 Originally Posted by coolzero Thanks a lot Sheldon. Your explanation is an excellent education for me as usual. I put it all into a diagram; can you verify whether what I understood is correct.
Yes, with the voltage correction noted by Andy.

Sheldon

Sheldon
diyAudio Member

Join Date: Dec 2003
Location: San Diego
Quote:
 Originally Posted by andyjevans Note that the 10 ohm resistor would benefit from being 11 ohm to keep the current through the 26 closer to 6mA. And a 20 watt part is really not adequate - think at least 30W or preferably 50W and that's better on top of the chassis. This resistor gets hot.
Actually, if you have a wirewound resistor rated for 10W, it is technically adequate for 10W. This is usually assumed to be in free air at 25 degrees C. Often this means it will run very hot (300 degrees C or sometimes more - see this example: http://www.irctt.com/file.aspx?produ...type=datasheet ).

Now, you don't want to run at rated power, as that requires that everything be done just right. But note that at half the rated power, the temperature is still 200 degrees, which is fine for the resistor, but way to hot to touch.

So, the other disadvantage of this type of biasing, is that it does generate some heat, and waste some power.

Sheldon

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