Miller Capacitance of triode strapped pentode
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 9th September 2009, 02:55 PM #1 coldcathode   diyAudio Member     Join Date: Jan 2009 Miller Capacitance of triode strapped pentode Ok, I have searched the forum and found very little definitive info on this subject. Here is my quandry I am designing a PP output stage using 2 x 829B's. They will be Triode strapped and paralleled. (don't ask why these tubes..I just like a challenge) I am having a tough time calculating the Miller Cap of these tubes. It gets complicated because a: the two sections are parallel and B: its a pentode "triode strapped" Inter-element caps are as follows per section Cgk 14.5 Cak 7 Cga 0.12 Stage gain will be about 8-9 maybe. I realize that some of these values will double due to the parallel connection but can someone help me with the method of calculating it? I need to know how low I need to get the output impedance of the driver stage to keep response up to 25Khz or so.
 9th September 2009, 03:28 PM #2 Wavebourn   Designer & Technologist diyAudio Member     Join Date: Sep 2006 Location: Pleasant Hill, CA Expect something in range 40-100 pF. That means, for 50 KHz pole you need at least 30 kOhm.
 9th September 2009, 03:35 PM #3 Bandersnatch   diyAudio Member   Join Date: Jul 2003 Location: Ann Arbor, MI hey-Hey!!!, There is also the internal g2-k capacitor. As triodes, this now becomes part of the plate circuit. cheers, Douglas __________________ the Tnuctipun will return
 9th September 2009, 03:35 PM #4 Yvesm   diyAudio Member     Join Date: Jul 2004 Location: Ardeche Those tubes sometimes integrate an internal bypas capacitor btwn screen and cathode. Near 200pF IIRC. But it appears accros the load, not as a "Miller multiplier" ! Yves.
 9th September 2009, 03:36 PM #5 Yvesm   diyAudio Member     Join Date: Jul 2004 Location: Ardeche Slower ! Douglas wins !
 9th September 2009, 03:43 PM #6 coldcathode   diyAudio Member     Join Date: Jan 2009 Thanks for the replies but I still don't have a definitive answer. What is the formula for this type of calc?
Merlinb
diyAudio Member

Join Date: May 2006
Location: Lancashire
Quote:
 Originally Posted by coldcathode Thanks for the replies but I still don't have a definitive answer. What is the formula for this type of calc?
In theory it is:
Cin = Cgk + [(Cg1a+ Cg1g2) * gain]

The problem is in finding a datasheet which gives you a genuine value for Cg1a. Cg1g2 may also be hard to find, but is probably less important.

Bandersnatch
diyAudio Member

Join Date: Jul 2003
Location: Ann Arbor, MI
Quote:
 Originally Posted by Merlinb In theory it is: Cin = Cgk + [(Cg1a+ Cg1g2) * gain] The problem is in finding a datasheet which gives you a genuine value for Cg1a. Cg1g2 may also be hard to find, but is probably less important.
Cg1-a is very small in pentodes, and Cg1-g2 is typically 20-30% more than the Cg-a of a comparable triode. With a pentode, g2 becomes part of the anode and is multiplied in the Miller effect.
cheers,
Douglas
__________________
the Tnuctipun will return

Wavebourn
Designer & Technologist
diyAudio Member

Join Date: Sep 2006
Location: Pleasant Hill, CA
Quote:
 Originally Posted by coldcathode Thanks for the replies but I still don't have a definitive answer. What is the formula for this type of calc?
I estimated Cag1 and multiplied it on stage's voltage gain.

 9th September 2009, 04:15 PM #10 coldcathode   diyAudio Member     Join Date: Jan 2009 Well the data I have been able to find is as follows Cgk = 14.5 (also listed as input C) so I am assuming Grid 1 to cathode Cak = 7 (internal capacitance of plate to cathode) Cga = 0.12 grid to plate I found spec that shows the capacitance of G2 to Cathode including the capacitor is approximately 65pf G3 is tied to the cathode. So, with these numbers what do I do? G2 tied to the plate and is part of the load so where does the 65pf fit in? Ignored correct? Cak also load it is ignored also? Do I ignore the G1 to G2 capacitance or make one up? so if I take the Cga and double it (for parallel) and multiply by 10 (gain 9 +1) I get 24pf Then add twice the Cgk = 29pf I get 53pf??

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