Dumble SE

[tele_player]

No, it's not a PI of any kind. What's referred to as the PI in a push-pull amp isn't needed in a single ended amp. Of course, it does invert the phase of the signal, but that doesn't matter.

[Aharon]

Thanks Tele........I thought it might've been a "fake" PI to make up for the fact that SE amps don't have one...............for more gain.
I read about the fake PI somewhere and I have been looking for the thread.
Thanks again
Aharon

[tele_player]

If you find something discussing the fake PI, I'd like to read it.

[Aharon]

If I find anything I'll post the link.
Aharon

[nickt]

Interesting that the tone stack is inside the feedback loop. I wonder what effect that has on the tone? - just musing...

Thanks for posting Funkman.

[Aharon]

Maybe it would be ok to replace the voltage divider after the first stage with a 1M pot as in the second stage?

Also......why a 250K master volume?.

I was going for a SE amp and this might be it.

Aharon

[GatesofDawn67]

.I thought it might've been a "fake" PI

I think that the third stage is a filter. Here is my analysis.

Using superposition:

1) short out the cathode and then the transfer function of the output network using the plate as the input is 470k/(Ztube+100k+(1/s*0.01uF)+470k).

Since tube output impedance is negligible compared to the other impedances, we can simplify by leaving it out. Expressed in terms of the reference designators.

R2/(R1+1/sc+R2)

= sCR2/(sC(R1+R2+1/sC))

= sCR2/(sC(R1+R2)+1)

= sCR2/(sC(R1+R2)+((R1+R2)/(R1+R2)))

= (R2/(R1+R2))*sC/(sC+(1/(R1+R2))

= (R2/(R1+R2))*s/(s+1/(C(R1+R2))

The first term is an attenuator and the second is a high-pass filter.

The plate's output is inverted has a gain > 1, we will call this -k

-ksR2/[(R1+R2)(s+1/(C(R1+R2))]

2) short out the plate and then the transfer function of the output network using the plate as the input is (100k+(1/s*0.01uF))/(Zfollower+470k+100k+(1/s*0.01uF))

Since follower's output impedance is negligible compared to the other impedances, we can simplifiy by leaving it out. Expressed in terms of the reference designators.

(R1+1/sC)/(R2+R1+1/sC)

=(R1+1/sC)/(R2+R1+1/(sC*(R2+R1)/R2+R1))

= (R1/((R2+R1))*(( s+1/(CR1)/(s+1/(C*(R2+R1))

The first term is an attenuator and the second is a low-pass shelf.

R1( s+1/(CR1))/[(R2+R1)(s+1/(C*R2+R1))]

3) the total transfer function of the network is 1) + 2)

-ksR2/[(R1+R2)(s+1/(C(R1+R2))] + R1( s+1/(CR1))/[(R2+R1)(s+1/(C*R2+R1))]

=(-ksR2 + R1( s+1/(CR1)))/[(R1+R2)(s+1/(C((R1+R2)]

=(-ksR2 + sR1+ R1/(CR1)))/[(R1+R2)(s+1/(C((R1+R2)]

=(-ksR2 + sR1+1/C))/[(R1+R2)(s+1/(C((R1+R2)]

=(s(-kR2 + R1)+1/C))/[(R1+R2)(s+1/(C((R1+R2)]

= {(-kR2+R1)/(R1+R2)}*{s+(1/(C(-kR2+R1)))/(s+(1/(C(R1+R2)))}

=[(R1-kR2)/(R1+R2)]*[(s+(1/(C(R1-kR2)))/(s+(1/(C(R1+R2)))]

Since -kR2 >> R1, The first term becomes -k[R2/(R1+R2)] which is is gain of the third stage and an attenutator.

In the second term the zero breaks first making it a high-pass shelf. However the zero is negative, so it is non-minimum phase; i.e., it's like a combination shelf and all-pass.

[Structo]

Whhaaaa.......what?

[ChrisM]

Scary thing is I actually know what the means now

Guess school is finally paying off

[dartanion]

I think what Aharon is talking about is mimicking the gain structure in a LTP in the driver stage of an SE amp. This has been done with some of the SE Wrecks that have graced these pages. Also, I use a full PI in my Absinthe amp (SE parallel octals) as I wanted to retain the character of the Express PI. One side of the PI isn't used.