Vox AC4 question. Need explanations.

[johngovan]

According to the schematic posted, I'm just confused what's the use of the 100nf  coupling cap and 1M grid leak (red circles) in the last triode stage after the voltage divider (yellow circle)?

Why not just connect the grid of the last triode stage after the voltage divider? (Yellow circle)

The first schematic is the original circuit and the second schematic is edited, i purposely removed the two components to show my thoughts on paper. Hopefully you can help me to clear my thoughts on why they put those two components.

[pdf64]

Yes, they seem kinda pointless.
I don’t like that the output valve grid leak path relies on a pot wiper though, I suggest to add a back up resistor.

[johngovan]

Yes, they seem kinda pointless.
I don’t like that the output valve grid leak path relies on a pot wiper though, I suggest to add a back up resistor.

Thank you for sharing your thoughts. Would 2.2M backup resistor enough to do the job?

[tubeswell]

What’s the green box? Op Amp?

[PRR]

If you turn MV way down, it is possible to overdrive V2b before the output. Using R-C coupling may grid-block but also may 'duck' with less ugly sound. It *may* be worth trying both ways, altho it may take a long time to find the pot settings where it makes a difference.

[HotBluePlates]

... what's the use of the 100nf  coupling cap ...

Why not just connect the grid of the last triode stage after the voltage divider? ...

What’s the green box? Op Amp?

AC4HW1 Schematic

The snipped-away Green Box is the Footswitch that can lift the tone stack for "Hot mode."

The capacitor (C15) is likely "just in case" some small amount of DC volts is present on the footswitch cabling, which would then alter the bias of V2, and be audible as a popping noise when switching the footswitch.

[johngovan]

What’s the green box? Op Amp?

It's the foot switch connection to bypass the tone stack.

[johngovan]

If you turn MV way down, it is possible to overdrive V2b before the output. Using R-C coupling may grid-block but also may 'duck' with less ugly sound. It *may* be worth trying both ways, altho it may take a long time to find the pot settings where it makes a difference.

Thanks PRR. I will try it both ways and decide from there.

[johngovan]

The capacitor (C15) is likely "just in case" some small amount of DC volts is present on the footswitch cabling, which would then alter the bias of V2, and be audible as a popping noise when switching the footswitch.

I see. It makes more sense now. Thank you Hot Blues.


EDIT: fixed missing quote tag --- HBP

[kagliostro]

Thank you Hot Blues

 

Mmmm I was thinking more to this



Thnan this

https://www.google.com/search?q=hot+blues&source=lmns&tbm=vid&bih=767&biw=412&client=ms-android-wiko-rev2&prmd=ivsn&hl=it&sa=X&ved=2ahUKEwjt9pP3qc_7AhX6yrsIHXlgBsIQ_AUoAnoECAAQBQ#fpstate=ive&vld=cid:1b514dd4,vid:LliNJcpYw8Y

 

Franco