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BDT Preamp build
BDT Preamp build
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Old 3rd November 2008, 09:04 PM   #11
jkeny is offline jkeny  Ireland
Join Date: Feb 2007
Location: Dublin
Yep, lifting an oscilloscope caused it, although I have had a number of incidents over the last 8 years - none as long standing as this one - they took just a day or two to clear.

You'll find metglas here: http://www.lessemf.com/mag-shld.html about halfway down the page "METGLAS® MAGNETIC SHIELDING FILM" at $2.50 per linear foot. This is wrapped around a funnel & is put over the 6AR8 tubes to shield from magnetic fields. According to Swenson, this worked wonders.

Thanks for the comments, Erik - do post your progress if you embark on this project!
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Old 4th November 2008, 08:37 PM   #12
jkeny is offline jkeny  Ireland
Join Date: Feb 2007
Location: Dublin
Finally Erik,
If you do try this project here's advice from John Senson for cheaper trafo alternatives that you might find useful -

You can use the Lundahl LL-1660 or
You could also try some small toroidal power transformers. You want the dual primary, dual secondary version with the highest secondary voltage you can get. For example ideal would be a design with the secondaries in series produces 120V (that would be a 1:1 transformer). Looking at digikey, an Amveco 1.6 VA one that has 44V with primaries in series, would work, its closer to 3:1 rather than 1:1 but it would still work, you just need to lower the output load resistors by the same ratio.
Go for low power transformers, they will have high inductance which you want in this case. BTW the primary goes towards the tubes and the secondaries go toward the output.

Also, I ordered 10 linear feet of the metglas & probably won't need all of this so could give you enough for covering two tubes when you have built this thing.

I presume you have seen this BDT thread? http://www.audioasylum.com/cgi/t.mpl?f=tubediy&m=141747
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Old 4th November 2008, 09:18 PM   #13
ErikdeBest is offline ErikdeBest  Switzerland
mostly thinkerer
diyAudio Member
Join Date: Mar 2003
Location: Zürich
BDT Preamp build
Hi Jkeny

Yes, I read through that thread and absorbed most (I think). Still Swenson does not tell if the BDT returned to his main system and why he is using the cascode. Therefore I asked those things to you!

Sometime ago I bought a set of second hand LL1680. Those are used in the Raven preamp by Olson and would also work well in the BDT.

One of my concerns is the unbalance in plate currents. In the first BDT thread here at diyaudio Swenson adresses this problem and recommends one to adjust balance by applying a DC voltage to the 'second grid' of the 6AR8. This surely works, but I would actually want to be able to feed a balanced signal to the unit, which makes it impossible to adjust DC at the unused grid. The LL1680 takes up to 5mA of current imbalance (it actually is a center tapped SE line output stage) so it would work, but toroidals would suffer! Or has Swenson adressed this inbalance problem as well...if so, the man is a true hero

About the Talema unit that you mention. Member revintage just built an line stage using it in parafeed, but measurements were terrible. He went to a larger model, but I have not heard back how it behaved.

Thanks for the offer on the metglas. I will keep that in the back of my mind for when I start this project.

my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
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Old 5th November 2008, 12:37 AM   #14
jkeny is offline jkeny  Ireland
Join Date: Feb 2007
Location: Dublin
Hi Erik,
I believe this thread from May 2008 will answer why he went with the Mosfet Cascode - for more gain & lower distortion. http://www.audioasylum.com/cgi/t.mpl?f=tubediy&m=141601

Edit: Where did you find the info on imbalance in plate currents?
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