Plate bypass capacitor location

[Zenabi]

From what I've gathered, there seems to be contradicting information as to whether there's a difference to where the plate bypass cap is placed.

Is there a difference between bypassing at the plate resistor vs shunting to ground right after the coupling capacitor?

[sluckey]

I'm not sure what you are asking.   

[Zenabi]

I'm not sure what you are asking.   

Please see attached pic.
I've marked the cap with a star.

Thanks.

[sluckey]

I would put it directly across the plate resistor.

[Dave]

Both of those caps shunt certain frequency content (depending on the value of the capacitor) to ground.


The top version will shunt through the respective filter capacitor.


The bottom one will shunt directly to ground.


Some people also shunt from plate to cathode which will ground through the cathode resistor/capacitor.


My mind doesn't see a lot of difference in any of the three unless there is no bypass cap on the cathode, in which case, the shunted frequencies will be shunted a little less because they have to go through a resistor to get there.

Dave

[Zenabi]

Yes. All of the variations are just shunting to AC ground, so theoretically there shouldn't be a difference.

However, some people claim there are differences and should stick with bypassing at plate.

Of course doing it after coupling cap has advantages in that I could use a lower voltage cap and also allows easier switching.

[tubeswell]

Is there a difference between bypassing at the plate resistor vs shunting to ground right after the coupling capacitor?

Yes. One is a plate bypass capacitor. But if you place a cap to ground after the coupling cap, its not a plate bypass cap anymore. It's just a capacitative divider to ground.

To keep it as a plate bypass cap, it needs to be either in parallel with the plate resistor between the plate and the HT supply node, or it needs to be between the plate and the ground (but before the coupling cap). It will work sonically the same either way, because both the HT rail and the ground 'look the same' when viewed from the plate (due to the shunting action of the filter caps - which shunt signal out of the HT rail to ground). Having noted all that, if you put the plate bypass cap directly from the plate to ground, it can reduce background noise compared to a bypass cap that is shunting to the HT rail (if there is otherwise any noise riding on an imperfectly shunted HT rail, that would get reverse-shunted through the cap to the plate).

Still confused?

[Zenabi]

Yes. One is a plate bypass capacitor. But if you place a cap to ground after the coupling cap, its not a plate bypass cap anymore. It's just a capacitative divider to ground.

To keep it as a plate bypass cap, it needs to be either in parallel with the plate resistor between the plate and the HT supply node, or it needs to be between the plate and the ground (but before the coupling cap)......

Still confused?

I'm not confused about the HT rail looking like ground.

I'm just not seeing how there would be sonic differences between having the capacitor before or after the coupling cap.

Yes, it creates a capacitive divider, but as far as the high freqs are concerned, it's seeing 2 capacitors in series towards ground. If the coupling cap is sufficiently larger than the 'bypass' cap, it shouldn't make much of a difference.

[pdf64]

Whichever way it's spun, it's a low pass filter.
As you're thinking about incorporating switching with it, after the coupling cap, keeping everything switched at 0V DC, looks the best way forward to me :)

[Zenabi]

Whichever way it's spun, it's a low pass filter.
As you're thinking about incorporating switching with it, after the coupling cap, keeping everything switched at 0V DC, looks the best way forward to me :)

Thanks.
Yes, that's what I see. It's simply a low pass filter.

I've always just bypassed at the plate resistor but saw a snippet in Merlin's book about doing it after the coupling caps and thought that I would implement it on a new build. However, when searching around the net, there seems to be pushback against placing it after the coupling cap. Some actually went as far as to say it was detrimental (not exact words) but never explained the reason.

Personally, I think it's obviously the same. But because of the almost universal pushback, I can't help but second guess myself and decided to ask here.

I could, I guess, test this out by switching it before vs after coupling cap. Just not sure about using a switch at high voltages.

[Dave]

Honestly, even if there is a difference, in my mind, I can't begin to see how it would be significant enough to even be audible.
I think you should pick the method that seems easiest in your particular build and just go with it.


Dave

[d95err]

My thoughts:

If you want the cap to always be there, it’s probably easiest to put it across the plate resistor.

If you want to make it switchable, it’s best to have the cap go to ground, to avoid DC on the switch.

Sonically, there should be no significant difference.

[Zenabi]

Thanks guys.
I will be placing it somewhere after the coupling cap.