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Old 24th April 2007, 05:35 PM   #1
Katapum is offline Katapum  Europe
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Default Roll off frequency.

Hi all,

Id like to find Jadis roll off frequency.
I think main formula is f=1/(2*pi*C*R). What are R and C values or what to do to find them?



I appreciate your help.

Katapum
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Old 24th April 2007, 06:44 PM   #2
Tweeker is offline Tweeker  United States
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R is in ohms, C is in Farads. To convert microfarads (uF) to farads, divide by 1 million.

This page is handy for determining the cutoff (-3dB) of an RC circuit.

This wont necessarily give you the amps actual limits, the OPT comes into play here too.
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Old 24th April 2007, 07:02 PM   #3
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HF rolloff will be determined by OPT or whenever the pair of 12AX7 sharing .8ma runs out of steam trying to drive 3 6550s in UL. I wouldnt expect it to be very high.
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Old 24th April 2007, 07:28 PM   #4
Katapum is offline Katapum  Europe
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For now Im only concern about two 12AX7 and 3 6550. So how can I exactly find C and R?
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Old 24th April 2007, 07:35 PM   #5
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The dominant roll off will be .47uF and 474k.

474k is a very high grid leak for what appear to be fixed bias 6550s.
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Old 24th April 2007, 07:40 PM   #6
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And Miller capacitance doesnt play any roll?
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Old 24th April 2007, 07:57 PM   #7
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The .47uF and 477k is for LF roll off. Miller capacitance enters into HF and is much more complicated to calculate. It is often worse than it appears as the 12AX7s must supply the current to charge this capacitance, the 12AX7s output impedance is not the only factor. Calculating Miller capacitance for UL op is involved.

Designing Common-Cathode Triode Amplifiers.
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Old 24th April 2007, 08:05 PM   #8
Katapum is offline Katapum  Europe
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From another thread:

Quote:
Posted by fdegrove

One caveat, the amp is seriously limited in high frequency response by its driver stage, it's starts to roll off gently from ~ 16KHz.

So how can I find 16kHz value?
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Old 24th April 2007, 09:49 PM   #9
PCC88 is offline PCC88  United Kingdom
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This page details the effects and calcs for Miller Effect

http://www.aikenamps.com/MillerCapacitance.html
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