BDT preamp and Demagnetization

Posted by John Swenson (A) on July 12, 2004 at 01:00:23

Today I built a little demagnetizer for the 6AR8s. I ran some spectrums before and after, the top picture is before the demag and the bottom is after. I hope you can see what a big difference this made. There is still a large amount of 60Hz getting into the system, that will next thing I tackle. Thanks very much to smoking-amp for bringing up the need for demagnetizing these tubes.

The demagnetizer was vey simple, I used a spool of 30 gauge wire wrap wire from Radio Shack and a 6.3V transformer. The wire spool has a big enough central hole to pass the tube and the wire on both ends of the spool is available to make connections. The 6.3V transformer gives enough current to be useful but not enough to vaporize the coil! Do NOT use this for longer than a minute or else it will get too hot. I figure pretty much anybody hanging out here has several 6.3V transformers to use for this.

I taped the spool vertically to a board and taped the tube to a piece of plastic tubing that was just big enough so the pins would fit inside. The tubing is commonly available at hardware stores for sprinkler system use. You just need one of the short connector pieces, the 10ft long piece is a little overkill. I then plugged in the transformer and slowly fed the tube through the hole in the spool so it was 6 inches or so on the other side, then brought it back through and did this a few times, by then its time to unplug the transformer as the coil is getting HOT. The coil gets some time to cool down while I untaped the first tube and taped up the next.

As the spectrum shows it made a big difference on the performance. These spectra were taken with 2Vpp input with 10mA on the cathode. It was slightly less than a gain of 1.

Sound wise the sound is even better than before. The amount of detail being extracted from the recordings is phenominal. Its still very smooth sounding but now the musical texture has gone up quite a bit.

As some of you may know I am a singer with several choirs so I know what it sounds like to sing next to other people. I can sing along with the stereo system now and it sounds like singing with real people, this is the first time I've ever had that happen with a stereo system. There has always been something missing in the harmonic structure, with this system its not missing anymore. The expensive passive pre doesn't so it, the other tube and SS preamps I've had don't do it either, but the demagged BDT does.

John S.

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Topic - BDT preamp and Demagnetization - John Swenson 01:00:23 07/12/04 (18)

which tool was used for spectrum analisys? (nt) - Jenya 14:07:17 07/15/04 (1)

Cooledit - John Swenson 22:42:42 07/15/04 (0)

A question regarding linearity - Mark Kelly 17:57:20 07/13/04 (0)

BDT pic 7360, 6AR8..etc. - Bas Horneman 03:13:03 07/13/04 (0)

Idea.. - Owen 08:10:52 07/12/04 (7)

How big should R and Rd be ...roughly? nt - Bas Horneman 23:55:01 07/12/04 (6)

Should it look like this? - Bas Horneman 23:59:47 07/12/04 (5)

Re: Should it look like this? - Owen 01:25:04 07/13/04 (4)

200R if it is a 6channel amp? - Bas Horneman 02:02:21 07/13/04 (3)

Yep! - Owen 09:52:49 07/13/04 (2)

Addendum - Owen 09:55:43 07/13/04 (1)

Ok...thanks Owen! nt - Bas Horneman 10:53:45 07/13/04 (0)

Re: BDT preamp and Demagnetization - dave slagle 07:50:05 07/12/04 (0)

Please, could you clarify that graph a bit more? - arend-jan 05:29:22 07/12/04 (4)

Re: Please, could you clarify that graph a bit more? - John Swenson 08:51:27 07/12/04 (3)

Re: Please, could you clarify that graph a bit more? - marnold 08:59:37 07/12/04 (2)

The scale's on the right side of the graph (nt) - Allen Wright 09:32:00 07/12/04 (1)

Doh, thanks (nt) - marnold 09:39:29 07/12/04 (0)

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