ODS Overdrive question

[FREYES_7]

Hello All!

What's the finction of the 500pF that's shunted between plate and cathode?

What's the effect of sending hi freq signal to the cathode?

Thanks for the insight!

[scstill]

Here is a good "article" from Gemini when posed with this question...

"In vacuum tube amplifiers designed by Ken Fischer of Trainwreck Circuits, a very small capacitor is sometimes connected between the plate and cathode to create a special type of high-frequency negative feedback. This technique is used to shape the tone, reduce harshness, and manage how the tube distorts.

How it works
This small capacitor creates a frequency-dependent negative feedback loop:
Capacitors and frequency: The impedance of a capacitor is inversely proportional to frequency. A larger capacitance value presents a low impedance to low-frequency signals and a high impedance to high-frequency signals. Conversely, a very small capacitance value, typically in picofarads (pF), presents a high impedance to most audio frequencies but a low impedance to very high frequencies.

Negative feedback: The plate and cathode of a vacuum tube are 180° out of phase with each other. By connecting a capacitor between them, a portion of the out-of-phase high-frequency signal from the plate is fed back to the cathode.

High-frequency signal cancellation: When this high-frequency signal is fed back to the cathode, it cancels out some of the high-frequency content present in the tube's input signal. This effectively rolls off or "shunts" the highest frequencies.

Tonal effects
This design choice is part of the "wizardry" that contributes to the unique sound of amplifiers like those from Trainwreck. The specific tonal effects include:

Smoothing out high frequencies: The feedback tames unwanted, harsh high frequencies that can occur when the tube is overdriven. This results in a smoother, less piercing high end.

Shaping distortion: Since the feedback is only applied to the highest frequencies, it allows the lower and mid-range frequencies to distort naturally, while managing the upper harmonics. This contributes to a rich, complex, and musically pleasing distortion tone.

Tightening up the gain: By reducing gain at very high frequencies, the overall sound becomes more controlled and less "fizzy," which is a hallmark of many high-gain amplifiers.

This technique is just one of several methods used to shape the frequency response and manage negative feedback in vacuum tube circuits. Other common strategies include cathode bypass capacitors (from cathode to ground) and treble-peaking networks (capacitors connected from plate to grid). The specific implementation depends on the desired tonal outcome. "

[FREYES_7]

Thank you very much, beautiful explanation indeed

Here is a good "article" from Gemini when posed with this question...

"In vacuum tube amplifiers designed by Ken Fischer of Trainwreck Circuits, a very small capacitor is sometimes connected between the plate and cathode to create a special type of high-frequency negative feedback. This technique is used to shape the tone, reduce harshness, and manage how the tube distorts.

How it works
This small capacitor creates a frequency-dependent negative feedback loop:
Capacitors and frequency: The impedance of a capacitor is inversely proportional to frequency. A larger capacitance value presents a low impedance to low-frequency signals and a high impedance to high-frequency signals. Conversely, a very small capacitance value, typically in picofarads (pF), presents a high impedance to most audio frequencies but a low impedance to very high frequencies.

Negative feedback: The plate and cathode of a vacuum tube are 180° out of phase with each other. By connecting a capacitor between them, a portion of the out-of-phase high-frequency signal from the plate is fed back to the cathode.

High-frequency signal cancellation: When this high-frequency signal is fed back to the cathode, it cancels out some of the high-frequency content present in the tube's input signal. This effectively rolls off or "shunts" the highest frequencies.

Tonal effects
This design choice is part of the "wizardry" that contributes to the unique sound of amplifiers like those from Trainwreck. The specific tonal effects include:

Smoothing out high frequencies: The feedback tames unwanted, harsh high frequencies that can occur when the tube is overdriven. This results in a smoother, less piercing high end.

Shaping distortion: Since the feedback is only applied to the highest frequencies, it allows the lower and mid-range frequencies to distort naturally, while managing the upper harmonics. This contributes to a rich, complex, and musically pleasing distortion tone.

Tightening up the gain: By reducing gain at very high frequencies, the overall sound becomes more controlled and less "fizzy," which is a hallmark of many high-gain amplifiers.

This technique is just one of several methods used to shape the frequency response and manage negative feedback in vacuum tube circuits. Other common strategies include cathode bypass capacitors (from cathode to ground) and treble-peaking networks (capacitors connected from plate to grid). The specific implementation depends on the desired tonal outcome. "

[pdf64]

Here is a good "article" from Gemini when posed with this question...

"In vacuum tube amplifiers designed by Ken Fischer of Trainwreck Circuits, a very small capacitor is sometimes connected between the plate and cathode to create a special type of high-frequency negative feedback. This technique is used to shape the tone, reduce harshness, and manage how the tube distorts.

How it works
This small capacitor creates a frequency-dependent negative feedback loop:
Capacitors and frequency: The impedance of a capacitor is inversely proportional to frequency. A larger capacitance value presents a low impedance to low-frequency signals and a high impedance to high-frequency signals. Conversely, a very small capacitance value, typically in picofarads (pF), presents a high impedance to most audio frequencies but a low impedance to very high frequencies.

Negative feedback: The plate and cathode of a vacuum tube are 180° out of phase with each other. By connecting a capacitor between them, a portion of the out-of-phase high-frequency signal from the plate is fed back to the cathode.

High-frequency signal cancellation: When this high-frequency signal is fed back to the cathode, it cancels out some of the high-frequency content present in the tube's input signal.  ... "

Note that from schematic, the cathode is fully bypassed, certainly over the upper part of audio bandwidth relevant to this scenario.
So no NFB effect applies.
It's purely a low pass filter.

[scstill]

Note that from schematic, the cathode is fully bypassed, certainly over the upper part of audio bandwidth relevant to this scenario.
So no NFB effect applies.
It's purely a low pass filter.

Bypassed with the 5uf cap? So because of this it cancels the negative feedback effect

What would be the effect on tone with NFB (removing the 5uf) vs no NFB (leaving the 5uf)
And effect on the gain without the 5uf?

[pdf64]

Note that from schematic, the cathode is fully bypassed, certainly over the upper part of audio bandwidth relevant to this scenario.
So no NFB effect applies.
It's purely a low pass filter.

Bypassed with the 5uf cap? So because of this it cancels the negative feedback effect

There's no AC signal there to cancel. For there to be any AC feedback, the cathode bypass would need removing.
Connecting that 500pF anode to cathode is equivalent to connecting it between anode and 0V common ground. Because the cathode is held at AC 0V by the 5uF decoupling cap.

What would be the effect on tone with NFB (removing the 5uf) vs no NFB (leaving the 5uf)
And effect on the gain without the 5uf?

I suspect that the possibility for much of a negative feedback effect to occur is low. Because the anode has a 60k output impedance and the cathode has an 800 ohm input impedance. Hence signal voltage at the anode will collapse with an 800 ohm load.
So whether the cathode is bypassed or not, my guess is that a single pole low pass filter will be the predominant effect.
The anode impedance being the R term, 500pF being the C term.
Anode impedance will be about 40k with its cathode bypassed, about 60k with it unbypassed.
https://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers

If a designer intended to implement negative feedback on a triode stage, between anode and grid would seem to be the more obvious arrangement.
https://www.aikenamps.com/index.php/designing-single-stage-inverting-feedback-amplifiers

[acheld]

Not often that I get to quote Merlin, but this is what he wrote when I asked about the role of these capacitors.

"Oh they just shunt treble, like any shunt capacitor. They create an RC filter with the tube's output impedance (about 40k). I have added this to Chapter 2!"

When put in basic terms like that, it makes sense.   And matches what I hear . . . 

[FREYES_7]

I have added this to Chapter 2!"

Chapter 2 of which of his books?? Cool advice from the wizard himself!

[acheld]

I have added this to Chapter 2!"

Chapter 2 of which of his books?? Cool advice from the wizard himself!

The forthcoming one -- ie, not published yet.   There is a live thread which includes the topics . . .

[pdf64]

Not often that I get to quote Merlin, but this is what he wrote when I asked about the role of these capacitors.

"Oh they just shunt treble, like any shunt capacitor. They create an RC filter with the tube's output impedance (about 40k). I have added this to Chapter 2!"

When put in basic terms like that, it makes sense.   And matches what I hear . . .

The 40k output impedance might imply Merlin is referring specifically to the bypassed cathode scenario.
Because the output impedance of an unbypassed cathode stage would be higher, more like 60k.
Hopefully he'll chip in and clarify