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

[fazeka]

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...

[sluckey]

Not the first time a schematic does not exactly match the actual amp.

[fazeka]

Not the first time a schematic does not exactly match the actual amp.

Yup. Which I am fine with. Just trying to understand the difference between the two from a theory perspective.

[sluckey]

I am wondering what the "off" position sounds like in each scenario?

Why? Is this just a brain exercise for stuff you will never use? 

I think about stuff like this a lot. But I just have another IceHouse and soon it's all better.   

[fazeka]

Why? Is this just a brain exercise for stuff you will never use? 

Never use? Well, I'm contemplating a build/clone. Am good at copying circuits but less good at understanding and would like to learn.

[PRR]

> there is a slight difference

No. These are exactly equivalent.

[fazeka]

No. These are exactly equivalent.

Thanks PRR. After some thought, I understand now. 

[fazeka]

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?

[pdf64]

As it’s a volume control bypass (for higher frequencies), it’s not going to have much effect unless the volume is set low.

[fazeka]

Hi there,

Understand. However, even with the volume down low, I can't detect any difference.

Which is odd given the value of the cap (2200pF), I was expecting to hear a drastic difference with it on.

[PRR]

> (2200pF), I was expecting to hear a drastic difference

Yeahbut. The resistances around here are quite low. A fuzzy-head calculation suggests a few dB boost starts at 7.5k, which is high for g-strings and past what most g-speakers do.

Tack a 0.02 or 0.01 across and listen.

[pdf64]

Try running https://www.ampbooks.com/mobile/amplifier-calculators/bright-boost/calculator/
The bright / boost switch doesn’t seem to do much, 2 - 3dB, depending on the vol setting, up to 50%.

[fazeka]

Yeahbut. The resistances around here are quite low. A fuzzy-head calculation suggests a few dB boost starts at 7.5k, which is high for g-strings and past what most g-speakers do.

Tack a 0.02 or 0.01 across and listen.

PRR,

You nailed it, thank you! I strapped a 0.022 across that 0.0022 cap and I could hear a subtle change. So that tells me it's wired up correctly.

I didn't realize it till you made mention of it wrt to 7.5k, but I'm using a EA Wizzy 12 cabinet with a Walter Woods MI-100-8 as a power amp. This setup is probably less optimized for guitar than something like a F/M/V power amp section with appropriate speakers.

Try running https://www.ampbooks.com/mobile/amplifier-calculators/bright-boost/calculator/
The bright / boost switch doesn’t seem to do much, 2 - 3dB, depending on the vol setting, up to 50%.

Thanks to pdf64 as well, this calculator helped me with a rough understanding of the circuit, playing with the volume setting and the cap value, despite not knowing the output impedance.