Patrick Turner wrote:
> In Addition to what I said below yesterday,
> some precautions need to be given when measuring the voltages outcome of
> shunt NFB when tried say between an output tube and input tube.
>
> There are two arms of the NFB resistance divider, R1, and R2.
> R2 is anode to grid, and R1 is grid to input.
>
> It is im****tant that the output resistance of the input tube be taken
> into account.
> This R is the Ra of the driving tube, its dc carrying RL, and a grid
> bias R
> for the output tube grid straight after the coupling cap between two
> tubes.
> There will also be a cap between output anode and R2.
>
> Suppose you have 1/2 a 6SN7 with Ia = 4 mA, then dc carrying 50k, and
> output tube biasing R of
> 200k after a 2.2uF coupling cap. Assume the 6SN7 is fully bypassed.
> Ra of the 6SN7 will be about 10k, so Rout of the whole input stage
> is 10k // 200k // 50k //, or about 8k.
>
> This 8k acts in series with R1, and IS REALLY PART OF R1.
>
This is basically what is says in RDH i.e that the source resistance of
the signal driving the input arm needs to be included in R1.
An alternate way of looking at it is that R1 is the input resistance of
the shunt/shunt NFB stage and thus the input signal is reduced by
Rout/(Rout +R1) where Rout is the source resistance of the driving
stage. This is something you should do anyway when calcualting overall
gain.
> But say one has 32k between input tube and output grid, then R1 is 40k.
>
> Suppose we would want ß = 0.1.
>
> Therefore 0.1 = 40k / ( 40k + R2 ), so R2 = 9 x 40k = 360k.
>
> Now the 8k od Rout for V1 is like an equivalent resistance in series
> with an imaginery
> voltage source.
>
> The model of the input tube needs to be considered, a voltage generator
> producing an output voltage of µ x Vg, with a series Ra = 10k, and at
> the output of the 10k
> there is the 50k and 200k shunting the output to fixed voltage points.
> Hence Rout = 8k.
>
> We do not need to know the gain conditions of V1, although its load
> should not be too low.
> We have 50k, 200k and the 32k to power into, and of course the 32k
> doesn't
> appear as 32k, it appears as a higher R because there is similarly
> phased voltages at each
> end of this R.
>
> But we do have V1 RL = approx 20k, rather low if we wish to swing about
> 30Vrms
> to drive an output tube with low THD.
>
> Now when we observe the working voltages in a real amp with two cascaded
> tubes
> the measurement of input voltage is impossible because its
> actually at the "front end" of ther imaginery 8k series Rout we
> calculated above.
>
> Therefore, for all **accurate** observations of the NFB voltages that we
> might wish to
> view, don't use a driver tube at all for experiments, use a low Rout
> signal gene,
> or something with negligible Rout of less than 100 ohms preferably.
>
Good advice.
Cheers
Ian
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