want to make a preamp for recording

[cminor9]

I am interested in building a tube preamp for recording mainly guitar.

I had the idea of building a preamp section and tone stack from a fender amp which I think should sound pretty good. Could I then take the output from the tone stack recovery and instead of running to a phase inverter, run it out to a mixer or recording device? Is that what's called line level?

Would it make sense to put an isolation transformer right before the output?

Is this a stupid idea or did I just describe like every preamp ever made?

[glebert]

I would take a look at a schematic for an amplifier whose sound you like, preferably with an effects loop or preamp out, and see how they do the transition from tubes and tone to the external world. Some fender amps (e.g. Hot Rod Deluxe or Deville) actually put an op amp at that point IIRC.

[The Dude]

I agree with that ^^^^^, but I'll add: Using the preamp out of an amp, or the head end of an amp designed to be a preamp, won't sound like the amp itself. Much of the sound of an amp comes from the power amp, speaker(s), room, and microphone. That's why modelers have cabinet simulation, mike placement and type emulation, etc. If you're really after a true amp sound, just use an amp and mic it. If it's a convenience thing or some other problem with micing a real amp, you might as well use a plug in or modeling box of your choice. Just my $.02.

[nickb]

The choice of a known preamp seems perfectly reasonable way forward. There is a problem with your suggestion in that tone stack has a high output impedance so you are going to to have to add a buffer of some kind, cathode follower. Or, if you are not averse to solid state, a source follower using an LND150 would be a cheaper option.

Am isolating transformer is not esssential but will help keep the hum down. A balanced output would be an inexpensive but more complex alternative.

[Bloomfield]

The Alembic F-2B is essentially a Fender preamp; there's an R.G. Keen schematic of it out there.

[cminor9]

These are really helpful suggestions everyone! Thank you!

I just so happen to have a Fender DeVille and a Mesa Boogie VTwin pedal and both do what I want. I checked both schematics and I'll admit they are a little beyond my comprehension. I could just keep using the VTwin or the DeVille, but honestly I just want a winter project to tinker with. I'll check out the Alembic F-2B.

I realize that it's not going to sound like a full amp, but I don't have a quiet room to record in. I guess I could also build an isolation box or something. I have made some recordings going directly in and to my ears it has sounded surprisingly good, though the type of stuff I'm currently making isn't really guitar driven.

The choice of a known preamp seems perfectly reasonable way forward. There is a problem with your suggestion in that tone stack has a high output impedance so you are going to to have to add a buffer of some kind, cathode follower. Or, if you are not averse to solid state, a source follower using an LND150 would be a cheaper option.

Am isolating transformer is not esssential but will help keep the hum down. A balanced output would be an inexpensive but more complex alternative.

Output impedance is one of the things I was wondering about. I don't understand the problem well enough to even ask the question properly, which is why I kept things more general in the original post.

I do not completely understand output impedance from the various stages of an amp. I have successfully built several amps, but this isn't something you need to worry about when you're just building from a known schematic. It's just fed into the next stage and it just works.

I read that mixers and recorders are looking for line level inputs but don't know what that actually means in technical terms. Preamps seem to work fine in that context, so I just assumed that preamps output stuff at line level. Is that a low impedance, low voltage, high current signal? Or am I just conflating two things?

I guess it's time to read up on cathode followers after a tone stack.

[cminor9]

The Alembic F-2B is essentially a Fender preamp; there's an R.G. Keen schematic of it out there.

Checked out that schematic (found both the RG Keen version and one that looks like it's from Alembic), and man that's almost exactly what I want!

[Helmholtz]

Line level/impedance: https://en.wikipedia.org/wiki/Line_level

[glebert]

Maybe look at something like a BK Butler Tube Driver schematic. I don't know if it gets the output up to line level, but that can always be fixed with an op amp stage.

[eschertron]

I am interested in building a tube preamp for recording mainly guitar.

Good on ya, and the fundamental issues have been addressed. Don't over-think it, just study to understand the impedance and signal level expected by any line-level input gizmo. There's two standards, but most current gear can be selected for either -10dB or +4dB, so it may not even be an issue if you find one peak voltage is easier to produce than the other.

..and welcome back from the future, again

[cminor9]

Good on ya, and the fundamental issues have been addressed. Don't over-think it, just study to understand the impedance and signal level expected by any line-level input gizmo. There's two standards, but most current gear can be selected for either -10dB or +4dB, so it may not even be an issue if you find one peak voltage is easier to produce than the other.

..and welcome back from the future, again

Thanks friend! It's good to be back here, and I am humbled as always by the friendliness and knowledge of this community.