Vox Top Boost Preamp pedal

[JimSoprano]

Hi,

I want to build a vox top boost preamp pedal with two 12ax7. Following a 'standard' top boost schematic with a 500k pot for volume after the 220k resistor to output, but also thinking more of matchless or wreck style...
I'll use a 12v voltage booster for high voltage supply, 12v for paralel heaters.

A few questions...

What will be the tonal difference between a single first gain stage and a paralel stage, as I have a spare half.

Any opinions of the filtering of the preamp, as I like some push an firm sound.

Thinking about an output buffer, maybe fet or that half 12ax7, any ideas?

If it works out good, I want to make a project of it and share all information.

Thank you!

[pdf64]

If you want to avoid the unknown capacitance of the cable connected to a pedal's output from from defining it's upper frequency response, then a reasonably low impedance output is required (eg <20k); may or may not need a buffer to do that.
Best to connect the heaters in series (rather than parallel) if supplying them with 12V
For anything further to be meaningful, you really need to provide a schematic of the project and the options you're asking about.

What are you planning to connect this 'preamp in a pedal' to? eg a dedicated power amp line input, the regular instrument input of a regular guitar amp? If the latter, have you ever tried such a thing, ie cascading a full preamp into another full preamp? The result may be way too trebly to be any use.

[JimSoprano]

Hi,

I want to use below schematic as basis. Top boost Preamp only with a 500k volume pot.

Is it possible to load another tone stack and volume to the cathode follower? and switch 'channels' at the volume pot taper only?

Thank you,

Jim

schematic: https://el34world.com/Hoffman/files/Hoffman_AC30.pdf

[pompeiisneaks]

Your question seems to have nothing to do with a preamp pedal and everything to do with an amp.

It's usually better to start your own thread instead of tacking an unrelated question onto a completely different one.

That being said, if you're asking if you can have a 'switchable dual tone stack' yes there was another thread about exactly that same thing just yesterday if I recall.

You'll just need a DPDT switch that switches the entrance and exit of the tone stack simultaneously so that you can ensure you completely lift the other stack out of the circuit.

~Phil

[pdf64]

...Is it possible to load another tone stack and volume to the cathode follower? and switch 'channels' at the volume pot taper only?...

Yes, it's possible, and I've seen some dual channel Marshalls that do that. But it's not a transparent addition, the cathode follower's output is reduced, and its frequency response changes, by adding the load of a 2nd tone stack.
Hence, from the info provided, Phil's DPDT switch suggestion (to select one or the other) seems a better way forward.