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A last attempt

80.213.174.107
Posted on November 2, 2009 at 02:54:36
johs
Audiophile

Posts: 31
Joined: April 22, 2003



Hi folks,
I found a now deleted article of Mark Kelly about heater care. He had constructed a CCS/shunt regulator that looked interessting to me. However, the CCS part of it did not function and Triode Kingdom suggested changing R1,2 from 2K2 to 1K, I found 680R to be better for a input of 8,5V. Mark's circruit was not load independent on the output voltage so I put a TL431 on the base of Q3 to assure a stable voltage. For 6,3V/600mA use R1 should be 0R9 (650mV/I) with a shunt current of app. 100mA (Q5). In my dreams I have seen this construction in use feeding direct heated tubes, maybe with a choke like the Hammond 60mH/2A/0R7 at the end for power tubes. Are my dreams rubbish or will this circruit in any way compeat with f.i. Tent Labs filament supply???

The reason I removed my web pages, posted on November 3, 2009 at 03:28:02
Mark Kelly
Manufacturer

Posts: 6519
Location: Geelong, VIC
Joined: February 28, 2002
Contributor
  Since: November 3, 2004
 I couldn't be bothered with the argy bargy of trying to support these designs any longer.

BTW the CCS isn't my design, I believe it is due to the late great John Linsley Hood. To make it work you have to watch the Hfe of the transistors - the collector current of the amplifying transistor (Q1) must be well above the pass current divided by the Hfe of the pass transistor (Q2) otherwise Q2 will enter base starvation. If the Hfes of the two transistors are high enough that design of current source can achieve dynamic impedance well over 100k and is very quiet and stable.


I personally dislike the TL431 and will do anything to avoid using it.

Mark Kelly
RE: The reason I removed my web pages, posted on November 3, 2009 at 11:07:06
johs
Audiophile

Posts: 31
Joined: April 22, 2003
 Thank you Mark, it's always nice to be enlightened by people that are not unhappy hobbyists.

The Problem, posted on November 2, 2009 at 10:43:13
Triode_Kingdom
Audiophile

Posts: 623
Location: Travelin' Man
Joined: September 24, 2006
 The problem with the Tentlabs supply is that it doesn't do what you need. Says the manufacturer:

"The current source maintains the high output impedance (typically > 10kOhm)"

Unfortunately, this circuit doesn't really do that, at least not in terms of common mode signals. Common mode is the means by which most line noise is impressed on the filament. Common mode is also the means by which external components can adversely affect the sonic qualities of a DHT. This circuit offers little, if any, protection from such effects.

If you tell us the specific characteristics you want in the supply, someone here can probably point you to something more useful.
RE: The Problem, posted on November 2, 2009 at 12:03:08
johs
Audiophile

Posts: 31
Joined: April 22, 2003
 Thank you Triode Kingdom for pointing at a vital problem, common mode noise. That is absolutely on the unwanted list and the easy problem is hum.
I would be very pleased if someone can point me in the right direction. Ofcourse, batteries should solve this, but is big and heavy to handle. Also, there might be different solutions for heaters and filaments. Will choke supply be one way to go??

Common Mode Filament Chokes, posted on November 2, 2009 at 13:17:10
Triode_Kingdom
Audiophile

Posts: 623
Location: Travelin' Man
Joined: September 24, 2006
 I remember threads on this topic. You might try a search of the DIY Asylum. I'll be attempting to construct an isolated supply soon using a simplified switching power supply. The transformer in the supply will provide the high common mode impedance between the AC line and DHT. The question is whether it can be accomplished inexpensively...
Agreed, posted on November 3, 2009 at 03:32:33
Mark Kelly
Manufacturer

Posts: 6519
Location: Geelong, VIC
Joined: February 28, 2002
Contributor
  Since: November 3, 2004
 I dropped all the active supply stuff and went with batteries and chokes.

A cheap and easy way to construct the appropriate chokes is to buy an appropriate IE core power transformer, strip out the primary and wind a new coil as a continuation of the secondary. Adjust the air gap to achieve the best compromise between L and saturation and Robert's your esteemed relative.

Mark Kelly
Back to original plan?, posted on November 3, 2009 at 11:00:22
johs
Audiophile

Posts: 31
Joined: April 22, 2003
 Thanks guys!!
This seems to bring back my origonal plans for this preamp I am about to build? It uses only IDHT's and I bought Hammond 60mH/2A/0R7 chokes to be part of a CL circruit. Then I thougt that Mark's circruit might do the job and the chokes could be saved for an upcomming project of modifying my Myiabe PSE300B amp (yes - it's all about money) The treads I have found on heater/filament choke supplies also discusses the use of CMC's - rather small ones even. Mark, it would be interessting to wind my own chokes, however I have not got a clue how to do it, is there a resipy out there?
And, TK - I hope your project will be successful, it is always a good thing to have alternatives. Also thought Tent Labs supply was absolute state of the art, many well regarded audiphiles use it.
johs.

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