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Old 30th January 2003, 03:35 PM   #11
Joel is offline Joel  United States
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Default Re: Slew rates, op-amps, and glass

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Originally posted by EC8010
...When I have done this, I invariably feel that I would like a lot more current through my driver to keep the distortion down (even if it is ultrasonic).
So, are you saying that the necessary slew rate of a 2A3 at 20,000Hz is not 11.3V/mS?
Because if you accept that, then we know exactly how much current is needed. No more, no less.
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Old 30th January 2003, 03:43 PM   #12
7N7 is offline 7N7  United Kingdom
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Default Re: ...the art of assumptions...

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Originally posted by James D.
Tim,

I given two differnt ones for Miller cap today myself. The first where no reference to a specific circuit applies uses mu as the amplification. The second uses the formal derived expression where gm*R' represents the amplification. The third uses A for the amplification...

Three expressions - one meaning...but then you knew that....

I do not think that this is true; A seldom equals mu - certainly not in triodes - this depends entirely on the chosen operating point, which also determines gm and therefore ra.

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Old 30th January 2003, 03:51 PM   #13
Joel is offline Joel  United States
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A= voltage gain

So, actually in reality the current requirements would even be a fraction lower than calculated.
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Old 30th January 2003, 03:59 PM   #14
7N7 is offline 7N7  United Kingdom
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Quote:
Originally posted by Joel
A= voltage gain

So, actually in reality the current requirements would even be a fraction lower than calculated.
Absolutely, since it is A that is used in the miller calculation not mu

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Old 31st January 2003, 05:44 PM   #15
EC8010 is offline EC8010  United Kingdom
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Joel,

what I'm saying is that I don't feel that expressing slew rate in volts per microsecond (s for second, S for Siemen) is particularly useful in a valve circuit. As my earler post describes, we can calculate exactly how much current is required to drive the input capacitance of the output valve. My key point is that it is well worth plotting this on the loadline of the preceding valve because the ellipse may take the valve too close to cut-off for linearity, and that may cause you to choose a higher quiescent current.
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Old 31st January 2003, 07:26 PM   #16
Joel is offline Joel  United States
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EC, If you're saying hey, throw in a .5mA extra to compensate for discrepancies - that's cool with me. My "rant" I guess was that certain respected "gurus" make wild proclamations about the necessary grid current demands for such tubes as 300-B's and 2A3's. They also make statements like "5 to 10 times the calculated value is a good rule of thumb". This is illogical, since the calculated value is already in itself an extreme worst-case-scenario!

I also have had to listen to people critque mine and other people's designs based on these "rules of thumb" - people who have no idea what these numbers mean, or how they were derived.
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Old 31st January 2003, 10:01 PM   #17
EC8010 is offline EC8010  United Kingdom
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Joel,

I'm saying that you may decide that you want a lot more current. Bear in mind that when you move vertically on a triodes's anode characteristics, the valve really isn't very linear at all - especially near cut-off. I really suggest that you do the sums for an amplifier (perhaps your own), plot the currents, and draw the elliptical loadline. The first time I did it, it gave me a very nasty surprise, and I increased my quiescent current considerably in order to restore linearity.
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Old 31st January 2003, 11:00 PM   #18
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mebbe I have to say something already known-but rule of thumb from ol' good books is-driver stage must have at least 1/10 dissipation of driven stage .
say-we need driver for VT4C with 85 W diss;
for that sort of beast,we need at least 8.5W burned in driver.
believe me-good rule of thumb.
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Old 31st January 2003, 11:04 PM   #19
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Ah, so I *am* spot on with my 6CL6 > quad 6146 amp, eh?

Tim
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Old 31st January 2003, 11:15 PM   #20
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Default NO ARGUMENT HERE.

Hi,

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Ah, so I *am* spot on with my 6CL6 > quad 6146 amp, eh?
He,he...start building and rock that house.
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