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Author Topic: Use of toroids in tank circuit of tube amp?  (Read 915 times)
WB0GAZ
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Posts: 31




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« on: September 06, 2008, 12:39:58 PM »

I'm looking for information on applying toroidal inductors in the plate tank circuit of a (large, 4-1000 based) tube HF amp. Are there special considerations in estimating core size and material type to avoid saturation, or other electrical considerations?

Very tks...
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NA0AA
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Posts: 1042




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« Reply #1 on: September 07, 2008, 03:27:27 PM »

I'm only responding to say good question.  I'm going to be very interested in the answer to this, but I will say:  I've seen the finals of transmitters from 10kw up to 500 kw output, and several amateur transmitters and none of them use a toriod in the tank - at least not that I recall seeing.

I'm going to guess that the inductance of anything but an air core either wildly overheats or is just way more inductance than the tank circuit calls for?

I'm waiting for someone smarter than me to answer...
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W2SSB
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Posts: 7




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« Reply #2 on: September 07, 2008, 06:04:14 PM »

Not an expert on this either. As I recall from way back a 4-1000a looks for approx 2 to 20 uh in its plate circuit for 10 to 80 meters. There are some whopper toroids available that are 1-30 mhz mix and about 6 inches in diameter and at least 1 inch thick. I think they average about 1 uh per turn and I would guess they can take a pretty substantial temp rise. Check out micrometals.com. They have free toroid modelling software that you can download, and you can also call them at no charge to discuss your requirements
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WB2WIK
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« Reply #3 on: September 08, 2008, 08:43:31 AM »

I see toroids used in high powered amps all the time.  Most HF amps built in the past 30 years or so have toroids in the plate tank circuit, if not for "all" bands then at least for 160m or maybe 80/160.

I didn't respond because I've never spent time trying to qualify or select proper materials for high powered use; Tom W8JI probably has, I'd ask him.

WB2WIK/6
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WD4HXG
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Posts: 184




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« Reply #4 on: September 08, 2008, 10:39:48 AM »

Powdered iron cores typically have a 55 degree C ambient temperature limit. Ferrites have differing temp limits. Some powder iron mixes are ususable up to 70 degrees C with the knowledge that the core's performance will degrade and life cycle will shorten. Further the larger the core volume the faster the core will age. (See

http://www.lodestonepacific.com/distrib/pdfs/Micrometals/desguide.pdf)

You will want to minimize your core losses. That is in part achieved by the following:

1 Selection of optimum iron powder mix for frequency of operation. You might use -2 material for 80 meters and -6 for 40 and 20.

2 The diameter of the wire used.

3 The number of turns used.

4 The spacing between the turns.

5 Using a single layer of turns.


If Micrometals still have the curves available look at the differing Q curves for differing core materials, number of turns, wire size and core sizes. A nice inductor with a Q of 250 at 4 Mhz can quickly sag at 5 or 7 Mhz to a Q less than 100. You can wind a toroidial inductor to provide less of a Q change over frequency but invariably you sacrifice the peak Q performance.

When looking at the core information on www.lodestonepacific.com you can see that different core materials are used at different frequencies. Generally -2 material is good for 80 meters. I personally prefer -6 material for use in the 7 Mhz to 30 Mhz range. -2 material can be used at 160 Meters but -1 is my choice in most applications.

Check the following url for Micrometals data on their cores.

http://www.lodestonepacific.com/distrib/pdfs/Micrometals/RFA_Cat.pdf

Enjoy experimenting.

73

Chuck WD4HXG  
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WD4HXG
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Posts: 184




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« Reply #5 on: September 08, 2008, 10:53:28 AM »

Here is Micrometals current test data for their toroidial product line.

http://www.micrometals.com/catalog_index.html
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