Sharing Fairchild Data

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DaveP

Well-known member
Joined
Nov 8, 2005
Messages
3,019
Location
France
I had to work this stuff out for my Fairchild project so I thought I'd share it with the community.

This link shows the Compression Ratios for the curves in the Fairchild 670 factory manual and the German test results on an example showing just the Factory setting curve 3 at various positions on the dial.  There is a small difference (3.9 to 4.5) on curve 3 due to manufacturing tolerances and factory settings I guess.  http://i41.tinypic.com/2u3xlok.jpg

This link shows my calculations for the associated gain of the control amp in order to get maximum compression to produce the curves in the manual.  http://i42.tinypic.com/25q9lio.jpg (50V ac gives 70V dc for the 6386 grids)

The -3.2dB column shows the reduction due to the pad between the signal amp output and the control amp input.
The ac control column shows the position of the front panel control which is, just on 0.1, or full on 1, as described in the manual.  The 12AX7 gain column shows the effect on the gain of the 12AX7  by adjusting the dc control pot.  The positions of this control are described in the manual and they check out to these gain values as far as I can see.  The amp gain column covers the combined effect of 12BH7 gain, 6973 gain, output TX reduction and feedback reduction.

The 100k dc control pot can be replaced with a string of resistors instead. http://i42.tinypic.com/2sb1j84.jpg  The values given produce the gain required in the table for the factory curves.  The 12AX7 cathode voltage has to be trimmed first by adjusting it to 3.6V (cut-off) with a suitable resistor, say 100k.  A switch gives a more reproducable  result than trying to remember how the pot was set.

I hope this info is as helpful to you guys as it has been to me in understanding how this magnificent beast works.
best
DaveP

 

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