Why does the 6AS7 have such high Miller capacitance ? - diyAudio
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Old 18th September 2006, 01:27 PM   #1
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Default Why does the 6AS7 have such high Miller capacitance ?

Just a simple question. My understanding of Miller Capacitance is that it is made up of grid to plate capacitance multiplied by the gain of the 6AS7 +1. Since the 6AS7's gain is just 2x how come it has such a high Miller capacitance. What am I missing.

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Old 18th September 2006, 01:36 PM   #2
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What makes you think it does?
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Old 18th September 2006, 01:43 PM   #3
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Experience suggets it might,, and various things I read about 6AS7 in OTL amps. I could be wrong though.

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Old 18th September 2006, 02:25 PM   #4
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It is a big tube and big tubes must have high Miller capacitance! Seriously I think many are assuming that just because the tube is big and have high Cgp and high Cgk it must have a high Miller capacitance, but as you are writing Miller capacitance is dependant on the gain of the tube in the application according to Cmiller = Cgk + A*Cgp where A is the gain of the tube in the circuit, note A is not the mu of the tube.

For OTL's you are probably using many 6AS7 in parallell so you would have quite a lot of capacitance but not excessively so.

The same rumour about high Miller and high grid current is spread about 6C33C but in reality Miller is similar to a 300B and grid current is not higher than a standard bem power tube like EL34 or similar but a big envelope like the 6C33C make people think in strange ways.

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Old 18th September 2006, 03:04 PM   #5
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Useful.

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Old 18th September 2006, 03:06 PM   #6
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Hi Hans

Can you tell how much current a 6C33C draws when drived into class 2 operation?

Many thanks

Erik
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Old 18th September 2006, 04:29 PM   #7
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Quote:
Can you tell how much current a 6C33C draws when drived into class 2 operation?
No I have never measured it, I don't use class 2 operation as I think it put very high requrements on the driver stage which has to cope with driving an impedance which is not constant but varies between very high and very low in the kohm range.

My comment about grid current in 6C33C where obviously refering to negative voltage operation where the drive requirements are very low. The grid current at -0.5V is specified to <5uA and the max allowed grid leak is specified to 200kohm for fixed bias and 1.5Mohm! with cathode bias.

I would expect that grid current with positive grid voltage is rather high for this kind of tube with high Gm.

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Old 18th September 2006, 04:42 PM   #8
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Quote:
Originally posted by tubetvr
It is a big tube and big tubes must have high Miller capacitance! Seriously I think many are assuming that just because the tube is big and have high Cgp and high Cgk it must have a high Miller capacitance, but as you are writing Miller capacitance is dependant on the gain of the tube in the application according to Cmiller = Cgk + A*Cgp where A is the gain of the tube in the circuit, note A is not the mu of the tube.
. . .
Regards Hans
Yeap, that's curious.

It seems that very few people takes care of the terrible effect of an 1M pot in the grid of a plate loaded 12AX7
Vintage sound ?

Yves.
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Old 18th September 2006, 04:42 PM   #9
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Hi Hans

Thanks for the reply. I am in no way in the position of designing anything, so I would not even try it...but I thought, let at least ask - and in the case of a 6C33C you know lots!

Erik
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Old 18th September 2006, 06:11 PM   #10
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Default Re: Why does the 6AS7 have such high Miller capacitance ?

Quote:
Originally posted by Shoog
Just a simple question. My understanding of Miller Capacitance is that it is made up of grid to plate capacitance multiplied by the gain of the 6AS7 +1.
Yes.

Quote:

Since the 6AS7's gain is just 2x how come it has such a high Miller capacitance. What am I missing.

Shoog
Here are the capacitances of the 6AS7:

Input: 6.5pF
Output: 2.2pF
Reverse Transfer: 7.5pF

If using each section in parallel, double those values. For the 845, the respective internal capacitances are:

Input: 6.0pF
Output: 6.5pF
Reverse Transfer: 13.5pF

Considering that one 845 has a Pd of 75W, and two sections of a 6AS7 have 26W, yeah, that's pretty bad. As for what you're missing, it's this: the 845 was designed to function primarily as an audio power amp that can also operate as an RF amp (though that's not often done). The 6AS7 was designed to operate as a DC amplifier for voltage regulators, either as a series pass device or a parallel regulator. Being that it's primarily a DC device that's incidentally used as an audio PA, reduction of interelectrode capacitance obviously wasn't a high priority.

Quote:
Originally posted by Yvesm
Yeap, that's curious.

It seems that very few people takes care of the terrible effect of an 1M pot in the grid of a plate loaded 12AX7.

Vintage sound ?

Yves.
There's a lot of that going around. They then connect the output of a pipsqueak like a 12AX7A to the grid(s) of a 6L6: slew rate problem

Even though it ain't RF, those "small" capacitances do matter.
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