Paralleling PI output coupling caps with a switch

[seven]

Hi there,

I am slowly planning out a Vox AC30/6 top boost build and am thinking about putting in a few switches to change the voicing of the amp.

This may be crazy but I was thinking about putting in a switch (4 pole) to parallel the 0.047 PI output coupling caps C6 and C9 (as found in the "Treble" model) with 0.1 caps for a total of 0.15 (as in the "Normal" model).

Any reason why this would be a bad idea?

I also wanted to do a few more switches, these seem (?) less problematic:

1. Split cathode (seen in "Treble" models) switch for Brilliant channel (1k5/0.1 cap)
2. Bright switch for Normal channel (220pF)
3. Switch for "Normal" or "Treble" model voicing for Cut control (paralleling C10 as a 0.0022 cap with same)

I'm a newb and still learning a lot so if it all sounds ridiculous, you'll know why...

For reference, schematics for AC30 "Normal" and "Treble" versions

[Chuck H]

You won't hear the difference between .15u and .047u PI coupling caps. The change is too small and the frequency is too low. But more important would be that there is high voltage at the plates of the PI so it isn't safe to switch the caps parallel. That would mean the switch would have high voltage on it. What you could do instead would be to use two caps in series and use a switch to short across the DC isolated cap. So no high voltage on the switch. Use .15 caps off the PI plates and follow those with .068 caps. Use the switching to short the .068 caps. Resulting value of .04678 (close enough). But as I said, you won't hear it. And further it just makes for more length on signal leads increasing the possibility of instability.

Nothing wrong with the other proposed mods that I can think of except to say that I've found such mods to wind up one way or the other. That is, when I've done fiddly mods like that I always end up leaving the switch in one position that works best for the amp and my ears. So I might suggest that instead you do the mods on non permanent switches. Pick the circuit values you like best and then hard wire them in.

[seven]

Hi ChuckH,

You won't hear the difference between .15u and .047u PI coupling caps. The change is too small and the frequency is too low. But more important would be that there is high voltage at the plates of the PI so it isn't safe to switch the caps parallel. That would mean the switch would have high voltage on it. What you could do instead would be to use two caps in series and use a switch to short across the DC isolated cap. So no high voltage on the switch. Use .15 caps off the PI plates and follow those with .068 caps. Use the switching to short the .068 caps. Resulting value of .04678 (close enough). But as I said, you won't hear it. And further it just makes for more length on signal leads increasing the possibility of instability.

You're right:





Doesn't make sense to see that mod through. Fair enough.

Nothing wrong with the other proposed mods that I can think of except to say that I've found such mods to wind up one way or the other. That is, when I've done fiddly mods like that I always end up leaving the switch in one position that works best for the amp and my ears. So I might suggest that instead you do the mods on non permanent switches. Pick the circuit values you like best and then hard wire them in.

I was suspecting this could very well be the case. However, with different guitars and differing preferences over time, I could see these others as perhaps having some utility. As long as these mods don't harm the amp/circuitry, I'd be OK with doing them. Though I suspect I would still get pops during the switching? If so, would a shorting type switch mitigate that? Or perhaps a resistor over the switch?

[g1]

What you could do instead would be to use two caps in series and use a switch to short across the DC isolated cap. So no high voltage on the switch.

Can you draw this?

[Tom Phillips]

Seems like, however you configures the switch, a bleeder resistor would be needed to keep the caps charged if you wanted to avoid a loud pop when flipping the switch to the position that puts the cap back in the circuit.

[g1]

Yes, but however you do it, I think it leaves you back with high voltage on the switch.
edit: incorrect, see drawing in post #17

[Helmholtz]

To keep the second (to-be-switched) cap discharged it must be bypassed by a large value resistor.
This bypass resistor must stay in circuit when the cap is switched in and out.

[The Dude]

I'm not sure what the proposed point of the mod is (more signal or altering frequency?). That said, I think (whichever is the objective) it would be better to do switching in the NFB circuit- either switching in/out caps for frequency changes or switching in/out resistors for level changes. IMO, more effective and also no B+.

[Helmholtz]

An AC30 doesn't employ global NFB.

[The Dude]

Correct. But he's looking to mod the amp, so it could.

[Helmholtz]

Correct. But he's looking to mod the amp, so it could.

Adding global NFB to an AC30 wouldn't be a straightforward mod, as both PI inputs are occupied by different channels.

[The Dude]

I found this schematic of someone employing it (hand drawn in). I confess, I haven't investigated the circuit yet. I haven't the time at the moment.

https://www.voxac30.org.uk/images/sc...rge_os_065.jpg

Also, apologies for steering the thread off course.

[seven]

I'm not sure what the proposed point of the mod is (more signal or altering frequency?).

I'm trying to get the different voicings i.e., "Normal" and "Treble" models (see https://www.voxac30.org.uk/vox_ac30_treble_circuit.html and https://www.voxshowroom.com/uk/amp/ac30brn.html as a reference/for more info) within one amp, mostly for the Normal channel.

That said, I think (whichever is the objective) it would be better to do switching in the NFB circuit- either switching in/out caps for frequency changes or switching in/out resistors for level changes. IMO, more effective and also no B+.

I'd rather not switch the NFB, thank you; both "Normal" and "Treble" models had no NFB.

[seven]

Correct. But he's looking to mod the amp, so it could.

I found this schematic of someone employing it (hand drawn in). I confess, I haven't investigated the circuit yet. I haven't the time at the moment.

To be clear, I'm doing a new build, I would never dream of modding an original vintage JMI AC30. That would be blasphemy.

And I don't want to add NFB to the build.

[Helmholtz]

I found this schematic of someone employing it (hand drawn in).
https://www.voxac30.org.uk/images/sc...rge_os_065.jpg

NFB factor will vary with the bright channel vol. setting.