[sdiy] Ken Stone Tube VCA question

[Simon Brouwer]

Neil Johnson schreef:
> Hi,
>
> cheater cheater <cheater00 at gmail.com> wrote:
>> Thanks a lot for the reply Neil!
>>
>> OK.. now I know what C3 does. But I don't know what R7 does :)
>>
>> I've read the preceding section, but they are talking about some sort
>> of voltage divider there - I can't see that in the schematic.
>> C3+R7 look like some form of feedback, but I don't know between what
>> things, and I don't know what this feedback accomplishes?
>
> Actually, JFET biasing is probably closer to what you are looking for.

Actually, cathode biasing ...

http://www.tpub.com/content/neets/14178/css/14178_43.htm

According to the data sheet of EF86
(http://www.drtube.com/datasheets/ef86-sed1996.pdf), for anode voltage of
150V you get 2mA of current at a control grid voltage of about -4V
(relative to the cathode).

As the control grid in this circuit is at around ground potential, the
bias is taken care of by R7 because 2mA through it gives this voltage of
about 4V.

If the current would rise the cathode-grid voltage increases, reducing the
current right back. This feedback keeps the quiescent current constant.

C3 ensures that the feedback works only at low frequencies so it does not
affect the gain at audio.

-- 
Vriendelijke groet,

Simon Brouwer
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