Boost switch question, cap/resistor order (Vox UL/Hybrid 7-series preamp)

[seven]

Hi there,

I've been bored as of late so I decided to do some analysis on the (in?)famous Vox 7-series (you know, same as the Beatles used during the Revolver and Pepper sessions).

I've been studying actual amps with the schematic and notice something subtle with the boost switch wiring on the "normal" channel.

What I am finding is that there is a slight difference between what the schematic shows and the way (at least some of) the amps are wired (most if not all of the ones I am seeing, anyway).



For reference:



It seems that the way the amps are wired work as per the schematic (i.e., when the switch is in the "on" (down) position, R48 is shorted), we have essentially (as I understand it anyway) an HPF (i.e., LF is attenuated = "boosting" HF). So I get that (I think).

However, I am not sure what happens between the two when the switch is in the off position. I am wondering what the "off" position sounds like in each scenario?

Per the schematic, C28 comes after R48 in the off position. Is it rolling off less LF due to R48 before it?

Per an actual amp (if my analysis/studies are accurate), R48 comes after C28 in the off position. What is the purpose of R48 in this case? Is it doing anything? Reducing the signal? Does the boost even happen in this case because the cap affects the signal first? If so, it wouldn't be the first time an amp was wired differently from the schematic (e.g., 6G6-B Bassman, IIRC).

And no, I don't have an actual amp to hear the difference/experiment with the cap/resistor order...

[Dave H]

And no, I don't have an actual amp to hear the difference/experiment with the cap/resistor order...

They will sound the same. The order of the components doesn't matter in a series circuit.

[seven]

They will sound the same. The order of the components doesn't matter in a series circuit.

OK. Even in the "off" position?

[Helmholtz]

OK. Even in the "off" position?

Yes. No difference between wirings.

[seven]

Yes. No difference between wirings.

OK. Thank you. Was hoping to get an education but your confirmation is welcomed, too.

[Helmholtz]

OK. Thank you. Was hoping to get an education but your confirmation is welcomed, too.

In series wiring all components see the same current, so order doesn't matter.

[seven]

In series wiring all components see the same current, so order doesn't matter.

Ah yes, you mentioned this earlier. Thank you again!

[seven]

UPDATE: Since I posted last, I did a little proof-of-concept of sorts, built the preamp based on the schematic.

Everything seemingly works fine with the exception of the boost switch. I can't tell when it's engaged. Absolutely no difference.

Tried wiring it up both ways, per the schematic and per actual amps. No difference.

Any ideas?

[g1]

I think it should only affect high frequencies, so not what I expect from 'boost', but Vox does that, like 'top boost'.
C28 is a bright cap across the volume pot. Switching R48 in and out just tweaks the highs getting around the volume pot.
Are you listening closely for differences in high frequencies?

[seven]

I think it should only affect high frequencies, so not what I expect from 'boost', but Vox does that, like 'top boost'.

Yes, I think I understand. More like a bright switch, a la Fender blackface amps, correct?

C28 is a bright cap across the volume pot. Switching R48 in and out just tweaks the highs getting around the volume pot.

Understand on the cap's effect, I was wondering what that resistor was doing, thank you.

Are you listening closely for differences in high frequencies?

Oh yes, I am trying to. I mean, I am getting older but my hearing's not COMPLETELY shot.

With the value of that cap (2200pF), I thought it was going to be OBVIOUSLY bright.

And, yes, I am listening for this with the volume down lower as I understand at higher volumes the effect is ultimately nil.

[Helmholtz]

The "boost" effect will depend on volume setting.

The series resistor mainly keeps the cap charged, so no pop.

[g1]

The series resistor mainly keeps the cap charged, so no pop.

I guess what you mean is that with the resistor in circuit, it is the equivalent of not having the C28 there at all, so pretty much a standard bright switch, with anti-pop?

[Helmholtz]

I guess what you mean is that with the resistor in circuit, it is the equivalent of not having the C28 there at all, so pretty much a standard bright switch, with anti-pop?

Yes, considering the low resistance from vol pot wiper to ground I think so.
Lifting one leg of the resistor with the switch open would show.

[seven]

The "boost" effect will depend on volume setting.

Yes, understand. At lower volumes, still can't tell if it's on.

[Helmholtz]

Are you testing with a guitar or a sine signal?