CJ

Fairchild 670-DC Threshold?
« on: August 17, 2005, 03:51:19 PM »
I was wondering what this DC threshold does on the 670.
Also, there was some mention that the 12AX might be seeing class B and possibly C operation.
Explain.

Thanks!

The DC Threshold is the single pot in the middle.

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


bcarso

Fairchild 670-DC Threshold?
« Reply #1 on: August 17, 2005, 03:54:54 PM »
What's the voltage on the fat line at the bottom?

CJ

Fairchild 670-DC Threshold?
« Reply #2 on: August 17, 2005, 03:56:34 PM »
sorry for the crop job, but the file is huge.
Thats ground. Also, I added more to the top of the pic.
Let me know if you want the whole schematic.

cj
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

CJ

Fairchild 670-DC Threshold?
« Reply #3 on: August 17, 2005, 04:12:19 PM »
Having problems getting the big file to download.
My site wants an FTP type  deal.

I can email it if you want.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Larrchild

Fairchild 670-DC Threshold?
« Reply #4 on: August 17, 2005, 05:35:22 PM »
Assuming the grids are essentially grounded to DC, the cathodes are being shifted by that 100k pot to change their operating point from class a for compression to class b or c for peak limiting.

The effect is that in class b, only the tips of the waveform get amplified.
Hence, Peak Limiter via smaller conduction cycle.

fat line is ground .... top line is reg b+

bcarso

Fairchild 670-DC Threshold?
« Reply #5 on: August 17, 2005, 05:56:18 PM »
Quote from: "CJ"
Having problems getting the big file to download.
My site wants an FTP type  deal.

I can email it if you want.


I'll PM you with the addy.  I believe Larrchild would also appreciate a nice clean copy.

PRR

Fairchild 670-DC Threshold?
« Reply #6 on: August 17, 2005, 09:40:38 PM »
> what this DC threshold does

I'm sure I explained that once. Probably here.


 

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