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BDT Preamp build

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Here are some pics of my convoluted build of this preamp: http://www.4shared.com/dir/9351477/a9615e53/BDT_Preamp.html

It's my attempt to make this into an integrated amplifier = BDT preamp + Baby Huey amp with the ability to use either separately. I'm building Zeus & SS Tabor amplifiers also & wanted to use the preamp with these - all differentially.

This pic is the final casing for it.

All comments welcome
 

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And the innards:
- Brass sleeved trafos are the Dave Slagle OPTs for the BDT (nearly as big as the original Rogers Cadet III OPTs (from which this whole chassis is extracted)
- The black box to the left of the PS trafo is a 12V SMPS for the CCS supply to the BDT tube heaters.
- One 250V voltage Reg (ala John S) is seen on perf board stuck to the inside of the front panel. The other 130V VR is to be mounted yet. Other little perf boards are
- one CCS for both BDT tubes (at the front of the case)
- two CCS boards for the diff input to the Baby Huey

Part of my slowness with this build was trying to fit it all into the Rogers Cadet case - an impossible task which I finally gave up on when the Slagle OPTs arrived (much bigger than I anticipated)
 

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Underside of chassis - in a disconnected state. I have lost one tube socket of the four in the Baby Huey section (broke too many pins & had to pull it). Also this section is in flux so ignore the wiring.

The part nearest the viewer is the front of the chassis & I've used two of the 3 tube sockets for the BDT preamp
 

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Hi Jkeny

Interesting build, have you made further progress?

Yesterday I was going through the net and stumbled upon the 'improved' version of the BDT by Swenson: the regulated power supplies and the cascodes on top of the 6ER8.

What I would like to ask, do you know if Swenson is using it in his main system again? I ask it because he mentioned in the earlier thread here at diyaudio that he was not using it anymore due to noise/distortion, having replaced with a switched attenuator. But now, an improved version with expensive transformers, etc...apparently it made its way back to his mains system?

Pity I forgot to buy some 6ER8's when I got the dampers from AES.

Erik
 
Hi Erik,
Unfortunately, I'm out of action with a sciatic nerve problem, which I'm finding difficult to get rid of - been with me for 6 weeks now.

I believe he is using it now as his preferred preamp after flip flopping between it and Gary Pimms Solid State Pentode (I'm also building this as a low power amp)

I think a couple of things in the end won the day for the BDT - the trafos, the use of metglas to magnetically shield the tubes & the new operating points that he is using with the Gary Pimm voltage Regs. I believe the tube microphonics also had to be tamed with some isolation/damping.

I am dying to get back to it as I was just about finished the build & he is so positive about its sonics.

The tubes are 6AR8 - about $7 in thetubestore - if you find them cheaper let me know.
 
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!
 
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
 
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.

Erik
 
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