input impedance

hennfarm · 3523

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Offline hennfarm

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on: April 15, 2010, 11:49:03 PM
A stock FP3's input impedance is determined by the 33k res, stopper and attenuators?  what happens if the 33k res are removed? What is a 12au7 or 6cg7 grid impedance? What will the source "see"?


Offline Grainger49

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Reply #1 on: April 16, 2010, 12:38:42 AM
The input impedance is the sum of the series dropping resistor, 33k ohms, and the "bottom" of the pot that is forming the voltage divider for your source voltage.  So at the zero volume setting on the pot, the series resistor is all there is between your source and ground.  If the resistor is removed your source ends up shorted to ground.  That is a bad thing.  It could damage your source.

The input impedance varies from 33k ohms to 48k ohms depending on the volume setting.

The 33k series value was chosen because it gives a good minimum load for solid state and tubed sources.

Why are you considering removing the series resistor?


Offline hennfarm

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Reply #2 on: April 16, 2010, 04:27:02 AM
i replaced the noisiest whispers with 50k stepped alps. the 33ks came out at that time.   so between the grig and cathode of say a 6cg7 is a short?


Offline Grainger49

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Reply #3 on: April 16, 2010, 06:07:03 AM
Quote from: hennfarm on April 16, 2010, 04:27:02 AM
i replaced the noisiest whispers with 50k stepped alps. the 33ks came out at that time.

If you wired the 50k Alps the same as the stock FP III attenuator then you still have a dead short at the input when the volume is at zero.  I.E. your source is shorted, again a bad thing!  Not knowing the configuration of your attenuators I can't comment on what happens one step, or more above "zero volume."

Quote from: hennfarm on April 16, 2010, 04:27:02 AM
So between the grid and cathode of say a 6cg7 is a short?

Between the grid and cathode doesn't apply here as it has nothing to do with the input impedance.  The Grid in a stock FP III is 15.22k to ground, the cathode is an LED forward bias to ground.
« Last Edit: June 08, 2010, 07:08:07 AM by Grainger49 »


Offline Paul Birkeland

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Reply #4 on: April 16, 2010, 08:27:40 AM
Quote from: hennfarm on April 16, 2010, 04:27:02 AM
i replaced the noisiest whispers with 50k stepped alps. the 33ks came out at that time.   so between the grig and cathode of say a 6cg7 is a short?

If you wired the alps without the 33k resistors, then you likely wired it as a standard attenuator, giving you 50k input impedance.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Grainger49

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Reply #5 on: June 08, 2010, 07:08:47 AM
He didn't answer.  I guess it was OK.


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