Plate to grid capacitance???

[Helmholtz]

Chuck H: What difference do you see with and without the 33k grid stopper in simulation?

[Chuck H]

R13 was changed to .01 ohms for the green trace.

[Helmholtz]

Thanks Chuck

The difference is even less than I expected.

[Chuck H]

Thanks Chuck

The difference is even less than I expected.

Shure thing. I like adding clear visuals for forum posterity It makes it easier for lurkers and guests to gather info here and just generally helps the forum.

FWIW that "test point" made sense to me because that is the pickup signal the amp "hears" as it were. Testing behind the grid stop resulted in about a 1dB difference in the resonant peak amplitude (for this pickup model) with no significant change in the resonant frequency. So almost no buffering from the grid stop.

[Helmholtz]

Testing behind the grid stop resulted in about a 1dB difference in the resonant peak amplitude (for this pickup model)

Looks more like 0.1dB from your sim curves.

[Helmholtz]

Just for curiosity:
How does the f-response look for the wrong PU model without cable capacitance and amp?
I'd expect to see just the notch filter effect but no resonant peak.
While the correct model should show a resonant peak at the same frequency (being the PUs self-resonance).

[Tom Phillips]

.

As Helmholtz already mentioned, the difference is far less than expected. I'm wondering if the results would be similar for the voltage response at the plate of the tube.

[Helmholtz]

I can't think of a reason why the response at the plate should be different.

Considering that the impedance of a 115p grid input capacitance at the PU resonance is around 450k, a series resistor of 33k can't make much difference.

[Chuck H]

Here is the pickup disconnected from any load following the volume control. The upper image is with the capacitor to ground (across the entire pickup model) and the lower is with the capacitor across the inductor and resistor only (not to ground.

[Chuck H]

Here is the response at the plate relative to whether the 33k grid stop is used or not.

NOTE: The keen observer may notice the resonant peak is at a higher frequency than the previous simulation. This is only because I had 600pF rather than 650pF plugged in as the cable capacitance because I forgot to change it and I had been running a different simulation.

[Helmholtz]

Great!

Exactly as expected.

Perfectly demonstrates how differently the similar looking models behave.
What a difference it makes just to connect the capacitance to the other end of the source!

[trobbins]

That makes modelling sense to me too. The driving emf generator is the coil itself. The bulk capacitance is just a simplified representation of capacitance distributed within the coil - same as the bulk series resistance. To that topic there may be some discrepancy between actual and simulated for different winding schemes, and measuring the bulk values may also be a bit tricky as the test point frequency would likely need to be less than 1kHz for the inductance, and above 10kHz for the capacitance.

[Helmholtz]

I can confirm that the measured f-response of a Strat PU loaded with a resistor looks just like the correct model sim.

Measurement was performed using a low wind exciter coil (magnetically coupled to the PU) fed from a swept current source having an 1/f response (based on suggestions by H.Lemme and M.Zollner).