Equivalent circuits and more signal flow diagrams

Discussion in 'Amp Tech Center' started by peteb, Jun 4, 2018.

1. petebFriend of Leo's

Apr 25, 2003
Equivalent AC circuits remove components like capacitors that don't affect AC.

The B+ power supply is equivalent to an AC ground.
The tube is an an AC signal generator, the cathode and plate load resistors are connected and togethor they provide the load the tube works into.

Last edited: Jun 4, 2018
2. petebFriend of Leo's

Apr 25, 2003
Adding in an AC ground reference and show the tube as a signal generator.

3. petebFriend of Leo's

Apr 25, 2003
The signal voltage is discontinuous between the cathode and the plate inside the tube, but not outside.

In a cathodyne phase inverter with equal plate and cathode loads, it is shown below how equal and opposite voltages develop at the plate and cathode across the plate and cathode resistors.

This one shows how a small unbypassed cathode resistor will develop a smaller cathode signal voltage than the plate signal.

Here, the cathode signal voltage transitions from positive to negative while the plate signal voltage does the opposite.

4. petebFriend of Leo's

Apr 25, 2003

These sources are DC analysis but apparently the circuits in posts 1 and 2 are a Thevenin equivalent circuit.

Thevenin's version says the source is a voltage generator and the load is in series with the internal load of the volatage generator.

Norton's version says the source is a current generator and the load is in parallel with the internal load of the current generator.

Last edited: Jun 4, 2018
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5. petebFriend of Leo's

Apr 25, 2003
I favor Norton in that the tube generates signal current within the tube and the signal voltage develops across the external load resistors.

I favor Thevenin because the load in series with the tube makes more sense to me.

6. petebFriend of Leo's

Apr 25, 2003
Is this right? Is this wrong?

If this is wrong I would like to know, and know why.

Thanks

7. petebFriend of Leo's

Apr 25, 2003
Consider the need for the cathode bypass capacitor.

This AC circuit has a small cathode resistor.

8. petebFriend of Leo's

Apr 25, 2003
The small cathode resistor puts a small signal voltage on the cathode, which hurts the gain.

9. petebFriend of Leo's

Apr 25, 2003
A bypass capacitor is added to the small cathode resistor

10. petebFriend of Leo's

Apr 25, 2003
The AC signal bypasses the cathode resistor, the resistor is now not part of the AC equivalent circuit so it is removed.

11. petebFriend of Leo's

Apr 25, 2003
The addition of the bypass capacitor has this affect on the signal voltage at the cathode compared to post 8.

12. petebFriend of Leo's

Apr 25, 2003
In phase, clockwise flow

Out of phase, counter clockwise flow

13. petebFriend of Leo's

Apr 25, 2003
One model for the AC signal passing thru the amp is clusters of electrons, similar to cluster of photons, similar to the compression and rarefactions in sound waves.

Apparently this is not the case.

Last edited: Jun 6, 2018
14. petebFriend of Leo's

Apr 25, 2003
By studying the cathode phase inverter, the standing wave on the series cathode and plate resistors makes the standing wave the leading option. The shape of the wave is debatable but a standing wave definitely exists on the load resistors.

Is the standing wave the way the amp works and what is used to transmit the signal thru the amp?

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15. petebFriend of Leo's

Apr 25, 2003
Before we answer that question, it's interesting to note that only half of a sine wave forms over the load resistance.

It takes both the in phase wave and the out of phase wave to make one full cycle.

16. petebFriend of Leo's

Apr 25, 2003
Since it's a standing wave how is magnitude and frequency shown?

Magnitude or volume is the height of the wave.

How does frequency show up? It isn't more waves in the same space. It's always just the one wave.

How does frequency show up when the signal is scoped?

Is it a standing wave when the signal is scoped?

Back to that later.

17. petebFriend of Leo's

Apr 25, 2003
The frequency of the wave on the tube's load resistors would have to be shown not by tighter closer packed waves but the only way it can, it just cycles up and down faster.

18. petebFriend of Leo's

Apr 25, 2003
The signal on the oscilliscope appears to be a standing wave, like the wave on the plate load.

Hold it, I'm not sure. It doesn't travel but is it inverting like the standing wave? I'm not sure.

The signal on the scope shows frequency like would be expected. Tighter waves and wider waves.

19. petebFriend of Leo's

Apr 25, 2003
It has been suggested that the signal level is linear along the plate and cathode resistors due to the linear voltage drop in resistors. That would make the signal level the same as a see saw.

How would the current flow be modeled?

Electrons would always be attracted to the uphill end. True current, true to gravity would always flow downward. Kind of for the same reason that they defined true current in the first place, it is supposed to flow to the lower potential.

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Last edited: Jun 6, 2018

Apr 25, 2003