Reverb Mixing Cap vs. Bright Switch Cap vs. Treble Control

[MarkF786]

On the typical blackface/silverface amp, how does the affect of the cap placed across the 3M3 reverb mixing resistor compare to the affect of a bright switch (or the very similar bright cap)? Are the approximate corner frequencies and slope similar?

Also, does the treble control on the tone stack cover a similar range?

The reason I ask is I have an older Allen Sweet Spot that does not have the cap across the reverb mixing resistor; I love the sound of the amp and my ears have grown accustomed to it - also, it's extremely pedal friendly. The amp does have a bright switch to give it more "sparkle" when needed, but I rarely use it. I also built an Allen Chihuahua which does have the cap across the reverb mixing resistor and I don't like the brightness of the amp when using fuzz, OD, or distortion. Now the obvious thing is to just remove the cap, but I've been thinking over the options.

- I could add a push-pull knob to switch the cap in/out.
- This lead me to thinking maybe a bright switch might overall have a very similar effect - which is possibly why David Allen removed the cap in the Sweet Spot (and several other of his amps) but added a bright switch.
- For ultimate flexibility, I could add a push-pull for the reverb mixing cap AND a push-pull for a bright switch - but in the end, they might sound near identical.
- Also, I found turning the treble control up a few numbers sounded just about the same, so maybe neither the reverb mix cap or the bright switch is necessary; instead, just turn up the treble when needed.

On a side note, what I find shocking are the amps that have both a bright cap and the reverb mix cap - like the vibrato channel on a Deluxe Reverb. Ugg, who wants that much treble boost?! Talk about "ice pick" to the ears.

I'm going to do some experimentation, but I thought I'd ask for feedback from anyone who might have already been down this road.

Thanks!

Mark

[Enzo]

So DO the experiments and report back. That is far more educational than us guessing.

I think you are over-assuming that bypass caps like that all act as brighteners. It may be the case, but to compare the tone of two different amps based solely on the reverb resistor having a cap across it or not ignores all the many other differences between the amps.

So you have an amp you like without the reverb bypass cap. So open it up and just tack solder a cap in parallel with the resistor. Now play. How much does it alter the sound?

And any amp you have that DOES have the cap, well, unsolder one end of the cap and lift it away from the part board. Now play. How much change do you hear?

[MarkF786]

Thanks for the feedback. One thing I forgot to mention is the preamps of both amps are near identical; the main difference between the amps is the power amp (the Sweet Spot is push-pull while the Chihuahua is single-ended). Though certainly the power amp will affect the sound, I have little doubt the reverb mix bypass cap accounts for the greatest difference in brightness of the amp, which I verified through some simple experimentation (like trying the same reverb mix bypass cap on the Sweet Spot with some test leads).

At this point, I'm putting together ideas - and getting all the necessary parts - so when I next open the Chihuahua amp for mods, I can go through the various options to determine what works best.

From an amp design point of view, I was interested in hearing other people's experience with these approaches to increasing an amps brightness. Some amp designs seem to use one approach over another (or none of them or a combination of them).

[Pedro Vecino]

The effect of the bypass capacitor in the set 3M3/10pf. is different from the generated by a 82pf. capacitor across the volume pot. (lighter than in a Twin Reverb 120pF) with the volume adjusted to 4, for example.
The 10pf capacitor have a more focused effect in high mids/treble. Away of the extreme brightness (very high treble frequencies emphasis) of the other system. More associated with the "plane" of the instrument that with the extreme brightness.
Itīs useful to adjust in Fender models (3 to 20pF, for example) to accommodate different speakers responses. At least I use it for that purpose