Question on the installation of LL1668 plate choke - diyAudio
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Old 25th October 2006, 06:10 AM   #1
Ruach is offline Ruach  Singapore
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Default Question on the installation of LL1668 plate choke

Hi guys,

I need a quick advise regarding the LL1668 plate choke.

I am planning to install this plate choke in my tube preamp.

My tube preamp design is based on a simple plate follower design and it uses only one 5687 or 6N6P tube. The B+ is about 200V and the current is about 10mA for each channel of the 5687 or 6N6P.

The LL1668 has two coils. Can I use just one piece of LL1668 as plate choke which will be connected to both plates of my single tube i.e. one coil of LL1668 to plate 1 of 5687 and the second coil of LL1668 to plate 2 of 5687? Will this connection work or do I need two pieces of LL1668 to make this installation work.

Thanks for your help.
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Old 25th October 2006, 05:09 PM   #2
kevinkr is offline kevinkr  United States
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You'll need two separate chokes as the coils are magnetically coupled to each other and you'll end up with a mono pre-amplifier with quite a lot of fighting between the two sections when the signals on each plate are not identical > read distortion..

Put the two coils in each choke in series (not parallel) for best performance unless the 5687 idle current exceeds the recommended idle current for this configuration. Don't use just one coil either.
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Old 27th October 2006, 05:41 AM   #3
Ruach is offline Ruach  Singapore
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Thanks Kevin for your help.

For the LL1668, the max allowable current is 25mA and for the LL1667 (15mA) is 15mA. Am I right?

If I am biasing my tube at 15mA, will be it ok to use the LL1667 (15mA) or should I provide for some allowance and use the LL1668 instead?

I would assume that 270H is better than 100H as a store of energy for the plate.

The need for two separate chokes will make this mod really expensive and I hope that this mod will make a signifcant difference.

Thanks again for your help.
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Old 27th October 2006, 04:23 PM   #4
kevinkr is offline kevinkr  United States
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Were it me I'd use the LL1628 with the windings in series, the additional current margin should slightly improve linearity and later if you wish to do something with say the 5842 (another good candidate for this design) you will be able to reuse the chokes.

At these low rps 100H ought to be just fine..
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