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Old 27th November 2010, 06:16 PM   #11
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Join Date: Apr 2003
Location: Southern Tier NY
Redid the cover and bonded it to the shield ground better. onlything left now is a little 60Hz.
1) Photo of Test Bed with improved Cover.
2) FFT of test bed with power and filament off
3) FFT of test bed with power off and filament on.

Not going to complaign about it any more. Time to start testing.

Can see a little 120Hz as a result of the rectification of the filament power. Using some temporary caps right now since I accidently blew up the good ones I had. FYI 10V rated caps won't work here when you have 12Vac. Duh!!!!
Attached Images
File Type: jpg DSC02831.JPG (147.1 KB, 22 views)
File Type: jpg baseline with plate and simple cover bonded no fil.jpg (181.3 KB, 20 views)
File Type: jpg baseline with plate and simple cover bonded w fil.jpg (182.9 KB, 19 views)
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Old 27th November 2010, 06:54 PM   #12
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Join Date: Apr 2003
Location: Southern Tier NY
Default Summary of what worked

To sum it up.
FFT 1. Original test result. Had ground loops and picked up environmental noise.

Step 1. Broke ground loop between earth ground and circuit ground via ying/yang diodes. Seen in FFT 2.

Step 2. Loosley shielded and bonded shield to earth ground. FFT 3.

Attached FFT's are with the 12b4 fully on at 20mA and 140V anode.

Will make me think twice about building a simple wooden chassis! Certainly one needs to incorporate at least some shielding internal to the box as it will certainly drop the noise floor.

The final 2 FFT shows the test bed with a GE 12b4a jan tube running at 1kHz and a Sylvania 12b4a. Setpoints were not adjusted just rolled tubes. Good to have a clean measurement to see the differences.

Now on the the quest of building my next preamp.
Attached Images
File Type: jpg dirty w gl baseline fully on.jpg (192.4 KB, 18 views)
File Type: jpg dirty no gl baseline fully on.jpg (187.3 KB, 16 views)
File Type: jpg clean baseline fully on.jpg (182.8 KB, 5 views)
File Type: jpg clean 1kHz fully on.jpg (186.5 KB, 5 views)
File Type: jpg clean 1kHz fully on sylv.jpg (185.3 KB, 7 views)
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Old 27th November 2010, 08:18 PM   #13
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Join Date: Aug 2009
Location: Sacramento
Hello SGregory,
Thanks for posting your trials and tribulations. Looks like computer, lighting and any other power supplies in the area grow grass.
Try going to the GoodWill store to the cookie tin department. I have a preamplifier built in a 1991 OREO Christmas cookie tin. The tin fixed the audible hum. Punch a couple of holes, solder a ground lead to the bottom.
The best option is to buy a new cookie tin and eat the Christmas cookies.
DT
All just for fun!
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Old 27th November 2010, 08:39 PM   #14
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I like the cookie idea.
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