K&K Audio / Lundahl Transformers

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Confusion....

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Posted on October 27, 2002 at 08:16:19
Story
Audiophile

Posts: 10453
Location: NJ
Joined: December 11, 2000
 I'm alittle confused and want to get this right, I have no reference book to follow and a couple of the articles I've been reading by builders seem like they are misinformed, confusing me more. Basically speaking, if I'm using an interstage that calls for a primary impedance of 3.5k at .02ma (max), we are talking about the load on the other end that would give .02 ma current draw through a tube representing that resistance in parallel with the trans. primary of 3.5k? I understand the difference between imp. and res., but now I've steered myself into a mental whirlpool, it still doesn't make sense in trying to figure out the tube choice. Thanks for any help. Tony

 

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correction, posted on October 27, 2002 at 08:33:19
Story
Audiophile

Posts: 10453
Location: NJ
Joined: December 11, 2000
 not in parallel. That is the load on the tube to draw .02ma, 3.5k? This would represent the dc figure (70 volts on the plate), but not impedance. And the impedance rises with frequency, so how do you calculate? I'm lost.....

 

Re: Confusion...., posted on October 27, 2002 at 16:03:16
KevinC


 
Tony,

Basically, the transformer recommendation calls for a tube with a plate impedance of 3.5K or less when it is operated at no more than 20mA current. It has nothing to do directly with the reflected impedance or frequncy dependence of impedance. To start a project looking at using a "new" (to me) tube, I look up the tube characteristics to see if at some nominal operating point the tube will be in the neighborhood of the recommended impedance at a current that won't saturate the transformer. To use your example of 3.5K and 20mA, I could use a 5687 or a 6DJ8 with no problems. I might not like the sound of either, but they will work.

Does this help?

Kevin

 

Re: Confusion...., posted on October 27, 2002 at 18:07:16
Story
Audiophile

Posts: 10453
Location: NJ
Joined: December 11, 2000
 Hi Kevin, thanks for your help, I've been looking at an article with a few key typo's in it. Wasted time. Now all I need are more plate curves or resistance charts. Thanks for your help, I'll be back! Tony

 

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