Paralleling PI output coupling caps with a switch

[fazeka]

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

[d95err]

0.05 or 0.15 would probably make no audible difference there.

If you want to switch those caps, I’d try much lower values, for example switching between  0.001 and 0.05.

Tip: find an online RC filter calculator and experiment with cap values to see how they affect the high-pass cutoff frequency. Use 220k for the resistor.

A question - there are only two PI output caps. Why would you need a four-pole switch?

[pdf64]

… Use 220k for the resistor.
 …

Just to note that the output resistive impedance of the LTP anode should be added to the R term. That’ll probably be about 40 - 50k, so about 270k should be suitable for the R term in the calculation.
Ok in this case it’s not a big deal, but there are scenarios where it can make a significant difference.

[fazeka]

A question - there are only two PI output caps. Why would you need a four-pole switch?

Based on my very limited knowledge/expertise: one pole per leg of the cap, times two caps.

[sluckey]

A question - there are only two PI output caps. Why would you need a four-pole switch?

Based on my very limited knowledge/expertise: one pole per leg of the cap, times two caps.

one pole for each cap, times two caps.

[d95err]

A question - there are only two PI output caps. Why would you need a four-pole switch?

Based on my very limited knowledge/expertise: one pole per leg of the cap, times two caps.

You only need to switch one end of each cap. The other end can be permamently connected.

[stratomaster]

The effect of the switch would be very subtle.  The cutoff for both filters is well below the fundamental of the low E or a drop D.  Both are effectively "full frequency" for the range of the guitar.

You're better off switching a coupling cap in the preamp for a noticable tone change. 

There's a video of Headfirst Amplification doing a PI coupling cap comparison in a Marshall circuit.  I think his was 0.1μ vs 0.022μ.  It's noticeable in that comparison, but more likely because of time constant changes in the overdriven condition more so than frequency.  And even then it's very subtle. 

According to the amp books calculator for PI coupling caps, the difference between the two caps you're proposing is 0.1dB at the low E. 

[PRR]

When you drive a guitar amp into HEAVY clipping, it un-biases itself, "blocks". The recovery time "un-block" depends on the coupling cap.

That's theory. In practice only Neil Young can hold his amp that close to crap-out that the audience would hear any difference.

[WimWalther]

When you drive a guitar amp into HEAVY clipping, it un-biases itself, "blocks".

Hey PRR,

I'm not quite understanding your meaning of "blocks" in the phrase.. ".. it un-biases itself, 'blocks'".

[fazeka]

You only need to switch one end of each cap. The other end can be permamently connected.

Thank you for this.

[fazeka]

According to the amp books calculator for PI coupling caps, the difference between the two caps you're proposing is 0.1dB at the low E.

Thanks for this. You're right.



What do you all think of this solution?

[d95err]

Putting the caps in series rather than in parallell works equally well (as long as you do the appropriate math…).

Switching series caps can sometimes be easier to layout. You can mount the switched cap directly on the switch lugs.

[stratomaster]

According to the amp books calculator for PI coupling caps, the difference between the two caps you're proposing is 0.1dB at the low E.

Thanks for this. You're right.



What do you all think of this solution?

I don't like that solution, and I think we've established it's not worth doing with the two values you've proposed.  Pick one value and use the switch elsewhere for a more substantive tonal change. 

[fazeka]

I don't like that solution, and I think we've established it's not worth doing with the two values you've proposed.  Pick one value and use the switch elsewhere for a more substantive tonal change.

Cool. Thanks for your thoughts.

[HotBluePlates]

When you drive a guitar amp into HEAVY clipping, it un-biases itself, "blocks".

Hey PRR,

I'm not quite understanding your meaning of "blocks" in the phrase.. ".. it un-biases itself, 'blocks'".

Blocking Distortion