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PostPosted: Fri Nov 18, 2005 8:59 pm 
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Holy Ghost
Holy Ghost
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Joined: Sun Mar 13, 2005 4:10 pm
Posts: 7519
Location: Canada
Very Important Construction Tips (lessons learned) from my first Lightning project.

To minimize hum, here's what worked for me:

A) Two star grounding points.

For the PT and 6.3V heater CT, and the El84 cathode resistor. The 6.3V center tap is very important.
For all the remaining grounds in a star configuration.
The two star ground points were located at opposite ends of the board. The preamp one, was located beside the input jacks.

B) Used a 14H, 50 mA choke instead of a resistor. Some people say "just use a resistor instead of a choke", but in my case, a choke was absolutely necessary. So follow the schematic, there's a reason it was designed that way.

C) Electrically isolate the input and output jacks from the chassis using shoulder washers.

D) Make sure there is clear and good separation of the OT leads. The output leads go around the 'power' end of the board. The OT input leads go directly to the OT from the EL84s, via underneath the board. The output leads go to the speaker jack which is also electrically isolated from the chassis.

E) Twist all AC wires (heaters, power leads, HV AC and switch leads.) Kept them all at the opposite end of the chassis and tight to any iron.

F) Tube Heaters were wired in parallel, from tube to tube.

G) All power leads went along the 'rear' of the chassis and resistors from tag strips went directly to the tube pins.

H) Used shielded cable from the input jack to the tube. Grounded it to the chassis at only one end - beside the tube it was attached to. (I now do it at the jack end)

I) Steel chassis approx size 16" X 5"X 2" with bottom cover. (aluminum works too)

J) I used an 8K output transformer from the donor amp. Sounded good to me - goes from Clean to Dirty, but maybe I'll try a 4K OT some day.

Results: With no input connected, this has resulted in a Lightning design, with almost NO hum.

_________________
Stephen
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