Miller Capacitance of triode strapped pentode

coldcathode · 9th September 2009, 02:55 PM

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.

Wavebourn · 9th September 2009, 03:28 PM

Expect something in range 40-100 pF.
That means, for 50 KHz pole you need at least 30 kOhm.

Bandersnatch · 9th September 2009, 03:35 PM

hey-Hey!!!,
There is also the internal g2-k capacitor. As triodes, this now becomes part of the plate circuit.
cheers,
Douglas

Yvesm · 9th September 2009, 03:35 PM

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.

Yvesm · 9th September 2009, 03:36 PM

Slower !
Douglas wins !

coldcathode · 9th September 2009, 03:43 PM

Thanks for the replies but I still don't have a definitive answer.

What is the formula for this type of calc?

Merlinb · 9th September 2009, 03:49 PM

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 · 9th September 2009, 03:54 PM

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

Wavebourn · 9th September 2009, 04:06 PM

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.

coldcathode · 9th September 2009, 04:15 PM

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??

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, 03:35 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

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9th September 2009, 03:28 PM	#2
Wavebourn 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	#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?

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??