alembic f-2b suitable for guitar preamp?

[armstrom]

I ordered a set of Real McTube PCBs from Tonepad and after studying the schematic for the Alembic F2B I noticed that could build either circuit with just a few component changes. The alembic seems to get rave reviews as a bass preamp (and some good reviews for guitar as well). Since the McTube doesn't have a tone control I'll just build a separate board for the tone stack components and tap in to the signal using the pads for the gain control pot.

So, would this preamp be worth while for "regular" electric guitar? I'm not interested in any kinds of effects in the preamp (distortion, clipping, etc...) since there are plenty of pedals for that. I just want a nice, warm, clean tone to work with. I'll be pairing the output with a digital effects board (reverb, rotary speaker, chrous, blah blah...) and adding a master volume control before running the whole shebang into an LM3886 power amp.

So, any thoughts? Is the schematic found here: http://www.freeinfosociety.com/electronics/schemview.php?id=1191 suficient to recreate the much acclaimed tone of the F2B or is there some other black magic "mojo" involved that I'll be missing?

Regards,
-Matt

[bent]

Gilmour is using the alembic f-2b for is guitar

http://www.gilmourish.com/?page_id=71

bent

[Jered]

  What's the 250K pot on the heater supply do?

[armstrom]

Hmm... Good catch..I have no idea. I plan to use the power supply from the McTube but just increase the +B to 300V or so. Obviously I'll have to bump-up the caps and diodes to handle the increased voltage. Here's another schematic with a different tone stack and power supply. I'm not sure which represents the real tone stack used by Alembic though. Since the F2B  is basically a direct rip of the Fender showman clean channel preamp I will probably stick with the standard fender tone control shown in the first schematic I linked.
-Matt

[Ben N]

As noted, the F2B is basically a Fender clean preamp, and is used by Gilmour (with mods for more gain, reportedly) to drive his Hiwatts (preamps bypassed). BUT running this into a chip amp may give different results than where you are driving 4 6L6s or 12 EL34s--whatever saturation occurs in those setups happens in the power amp, or at least in the phase inverter and beyond, where it sounds (really) good, and contributes to that warm, clean sound. (The distortion that might otherwise occur in a RMcT will likely disappear due to signal loss in the tonestack and higher plate voltages.) OTOH, you probably don't want any saturation happening in your 3886 power amp, so, without adding something--compression or limiting, EQ--you may be facing a choice between ugly power amp chip distortion and sterile cleanliness. This was the rap on the '70s Fenders, that had the same preamp topology feeding ultralinear power amp sections that just refused to give it up. At that point there is no real advantage over a well designed SS preamp.

[armstrom]

thanks for the reply. I know I will be giving up the tube distortion by not having a tube power amp stage. I'm not looking for any distortion though. What I'm after is a warm clean sound. I could always build a simple opamp preamp that would provide lots of clean headroom... But that would likely lack the character of a tube preamp. At least that's my goal. Can't hurt to try

[Ben N]

I got ya. But my point was not about a real dirty sound. What makes tube amps sound good "clean" IMHO is what happens when you are just hitting saturation with the peaks of the notes. The little bit of power tube saturation gives a slight emphasis to sweet harmonics, and also a bit of compression. You don't hear that as "distortion," as if you were running through a cranked amp, a distortion channel or a dirtbox. It just sounds warm and sweet, with a bit of added sustain. That is the tube sweet spot, IMHO. That is what I don't think you will get from just running a clean tube preamp, which probably will sound not much different than a similarly configured SS preamp.

But--I could very well be wrong, and at the very least you will get great experience building it and end up with a nice preamp, so WTH?

[snoof]

be aware on that linked schem the pinout on the second half of the 12ax7 are wrong.  pin 6 is plate and 8 is cathode.

[armstrom]

thanks for all the tips guys. I'll just see how things go. You can be sure I will post sound clips when it's all done. This may not be the preamp I end up using. I'll build a few and see which one sounds best.
As for the error in the schematic... I will be using a pre-made pcb from tonepad so I shouldn't have to worry about that error. I'm simply going to change some component values to match the alembic schematic.