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-   -   300B SE cathode feedback to grid? (http://www.diyaudio.com/forums/tubes-valves/41700-300b-se-cathode-feedback-grid.html)

alan_982002 28th September 2004 11:52 AM

300B SE cathode feedback to grid?
 
2 Attachment(s)
Hi All,

I saw some 300B self-bias SE circuit that the cathode voltage is feedback to the grid via some potential divider (R4,R5, C3), would anyone tell me the operation theory?

Regards,
Alan :smash:

ThorstenL 28th March 2010 05:42 PM

Hi,

Quote:

Originally Posted by alan_982002 (Post 482268)
I saw some 300B self-bias SE circuit that the cathode voltage is feedback to the grid via some potential divider (R4,R5, C3), would anyone tell me the operation theory?

It works only with the preceeding stage and is taken originally from the WE 91 Amp, often seemingly without understanding. It "bootstrappes" the limited value cathode bypass capacitor to the grid, so it does not introduce local degeneration at low frequencies.

The driving stage must have very high output impedance for this to work, the coupling capacitor value also plays into this.

Ciao T

kevinkr 28th March 2010 07:21 PM

Quote:

Originally Posted by ThorstenL (Post 2134448)
Hi,



It works only with the preceeding stage and is taken originally from the WE 91 Amp, often seemingly without understanding. It "bootstrappes" the limited value cathode bypass capacitor to the grid, so it does not introduce local degeneration at low frequencies.

The driving stage must have very high output impedance for this to work, the coupling capacitor value also plays into this.

Ciao T

Which was the case since the preceding stage was a pentode connected pentode with a 100K plate load resistor.. Also I believe the cathode bypass cap in a WE91A was much smaller than in the example shown above. (I can't make out the value in the schematic linked to below, and my paper copy is in a filing cabinet in the garage. (Too cold today!): http://www.nutshellhifi.com/library/...tics/we91a.jpg

ThorstenL 29th March 2010 01:04 PM

Hi,

Quote:

Originally Posted by kevinkr (Post 2134581)
Which was the case since the preceding stage was a pentode connected pentode with a 100K plate load resistor..

It is possible that this is indeed the case for the full schematic of which an excerpt was posted above, however i would not suggest so with great confidence.

Quote:

Originally Posted by kevinkr (Post 2134581)
Also I believe the cathode bypass cap in a WE91A was much smaller than in the example shown above.[/url]

There where a few versions, one I have at hand for the 91B suggests it was 16uF, which is very similar to the 22uF shown above.

Ciao T


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