Yeah, there's another Alembic transistor preamp with an effects loop, but it's no FB-2.
I've owned several FB-2 preamps...always a personal dilemma deciding whether to modify a classic.
Here's some of what I've picked up along the way as a roadie and builder...I highly recommend building two channels, unless you plug the one into another subsequent guitar amp with its preamp...or if you only play relatively clean orchestral guitar or clean bass.
1) Don't think hi-fi. A lot of the tone can be rather radical.
2) Remember sometime around 1975 when a lot of high-end and even mid-line Japanese phono preamps started coming with switches to select input capacitance to match the inductance of the phono cartridge? Well, IMHO on a guitar preamp the match to the guitar's pickups is even more critical (assuming you're not using powered pickups). The low inductance of an old strat pickup versus the high inductance of a 1/4 pounder humbucker makes a huge difference in the amount of treble loss from inductance, given they are driving the same input impedance. Conveniently, just messing with the input impedance and sensitivity makes huge differences in treble response and signal to noise ratio. First, like a Fender, for strange reasons each channel of a real FB-2 had 2 input jacks. As if anyone would plug 4 things into an FB-2. Some of the Alembic FB-2's had switching input jacks, so that when you plugged into only the first jack of only the first channel (and not anything plugged into the second jack of the same first channel), the litle resistor network between the two input jacks was bypassed. This made the input much more sensitive and improved signal to noise for the whole amp...right at the first series and parallel resistor. There was a note about this on some of the pink Alembic schematics, and not on others, and when I called they could not come up with the details again. The result was probably like the simplified input schematic moosapotamus has (or was R1 much much smaller when only 1 input was used??)...there's usually really no good reason to have two input jacks for one channel. So...if you have an FB2 without the switching jacks or are making your own, just have one input jack for a channel and DO mess around with the values of R1 and R2 to suit your particular guitar; it makes a difference. Might even try adding some trim pots to vary them. If your guitar makes more noise when you turn the volume on the guitar from full to 2/3, consider adding a 500K trim pot from the preamp input to ground (or change the value of your pot in the guitar or add a resistor across the pot). Better yet, take the volume knob out of the guitar, wire it full, and use a volume pedal after the preamp. For that matter, I install my preamps on the floor, with pedals on the pots. Or consider phantom-power to install a compact tube preamp in your guitar. I had my Travis-Bean running that way for a short while, with real tube buffers and a mixer for the pickups, no controls on the guitar.
3) LiIke a real Fender, the Bright switch is really just a cap across the volume control pot. You can think of this many ways, but be aware that it only has any effect when the volume control is low! If you drive into distortion at higher volumes, the reduction in treble is then often appreciated, which is the practical reason it's done this way, as the distortion harmonics generated are all higher-frequencies and Fender doesn't want to irritate you when you crank it. But it's far less than ideal, since you'd really rather do any "smoothing" (or "creaming" as the guitarists might say) of the treble due to distortion AFTER the distorting stage. More on that later.
3) Jerry (rest his gentle soul) often used to plug into channel 1 then route that channel's output (the real FB-2 had seperate outputs for each channel and a summed mono output) into the input of the second channel...then into his McIntosh 2300. With the two channels in series is a WONDERFUL way to use an FB-2, though you might want to snip out the resistors for that summing output circuit. Jerry sometimes just ran another short cord with 1/4" plugs around his FB-2 or wired it internally so they were always in series, but I usually installed a switched volume pot instead one of the input jacks for the second channel, so that when that volume control was switched off, it bypassed a channel but when it was on it controlled the signal level from first channel output into second channel input. Use a high-power-handling wirewond pot like the Alembic's Allen-Bradley pots. In fact, the circuit is really hard on all the pots, so though we usually don't use wirewounds I'd recommend them for all pots on an FB-2. Using both channels in series had some very interesting results. First of all, for a really phenomenal clean tone (especially for a strat pickup), if you optimized that R1 and R2 network that connnects your guitar pickups almost directly to the first grid, this preamp is so clean you can use a lot of gain and gain stages for unusual and extreme tone tricks...like REALLY using that bright switch (in fact I installed three mini-toggle bright switches in parallel on the first channel, each with a different size cap, so that I had 9 different knee points for the bright switch on the first channel; nice for bass too). Like I said, the bright switch only does anything if the volume knob is low, so for a crystalline shimmery incredibly bright treble build it clean and keep the first volume low and use the second channel in series for some additional gain to make up for that first low volume control. Fact is, you can't add "bright" you can just allow some thru and take away everything else, sacrificing overall gain...so for "super bright" you NEED that second channel for more gain. Try both channel's volume controls very low with the inter-channel high (or plugged direct) and both bright switches on! Yes, IMHO for the BEST clean tone you still need a lot of stages and a bunch of "Bright" treble. Of course the real FB-2 had no high-cut switch like moosapotamus' schematic. I used the volume knob I added between channels to determine whether to overdrive the second channel. Sometimes two such pots with a relay operated by a footswitch. The truth is that I never wanted just one channel once I got used to controlling both in series, but sometimes I wanted that inter-chanel pot on a footpedal (whereas Jerry always had it full). The second channel's tone controls can be used to control the harshness of the distortion of the second channel's first stage if you're driving it hard. Here in the second channel moosapotamus' hi-cut might come in really handy to tame the harmonics from distortion; maybe even multiple mini-toggle switches again. So for clean IMHO you want super-bright which requires extra gain, and for dirty IMHO you want super-bright (or at least lots of treble) in the early stages, then you can remove the "extra" treble and the distortion treble in the later stages. For a pleasant diry very rich in lower harmonics, just like you apply the RIAA curve before you record a record and then apply the reverse curve after playback, you should have control of the tone curve before you inject distortion, then be able to choose the "reversing" curve later...this pre-post tone curve control gives you the ability to control the "tone" of the distortion independently of the "tone" of the original guitar signal. You can also get similar phenomenal control of your transistor distortion by putting your stomp-box distortion devices between the FB-2 channels in series and using the tone controls of the two channels for "pre" and "post" processing. Between the channels is also a nice place to put a tube dynamic range compressor or expander. I don't particularly like starved-plate "blues" devices so I won't recommend any...
One more nice trick...if you use a trigger-filter (like an auto-wah etc.). These devices convert the volume envelope into a control signal for a subsequent filter (usually controlling the frequency of a low-pass but quite configurable on some models). Sounds like a wah (usually with no pedal) that responds quickly to each pick stroke, or can do a reverse wah with sound with each pick stroke, etc. Now some trigger filters like the lovetone meatball or some kits have an effects loop. The stomp box samples the volume envelope before the effects loop, then applies the filter after the effects loop. This gives you the capapbility to seperate the trigger signal from the filter input. With others, it's not difficlut to open it up and figure out how to seperate the envelope demodulator input from the filter input. With your 2-channel in-series FB2, you want to sample the volume envelope before any distortion clips or compresses that volume envelope (or you can end up with on/off wah instead of nice full control) and it's even best to take the signal before tone controls...so just add a big cap and a volume pot to make an additional "early output" on the first channel before the tone controls and plug that into the trigger-wah input, engage the effects loop on the triger-wah, and then plug the second channel's main output into the effects return on the trigger wah. The trigger-wah will be very responsive and controllable and make consistent use of its sensitivity and range settings regardless of how you mess with your preamp or amp knobs or distortion or any controls, yet the trigger-wah won't add any noise like it would if it were inserted between guitar and amp. And you can use a switched volume pedal with an oscillator (or sometimes even just a battery) to turn the atuo-wah into a manual wah pedal.
Or you can modify a Mesa-Boogie tube footpedal chassis into a 2-channel FB-2 preamp.
I also have two Japanese tube foot "boxes", each of which really is again one Fender preamp channel with a teensie speaker and transformer dummy load. I picked them up for about $30 each. With very little work they can be put in series and sound much like an FB-2 with its channels in series. The work would be converting to a better external power supply and maybe better pots.
Some people claim a case for adding a third channel in series. The idea is to apply distortion, change the phase 90 degress, and apply distortion again, to a different part of the waveform. It does look different on a scope, more harmonics without more compression. It may have its uses.
I've owned several FB-2 preamps...always a personal dilemma deciding whether to modify a classic.
Here's some of what I've picked up along the way as a roadie and builder...I highly recommend building two channels, unless you plug the one into another subsequent guitar amp with its preamp...or if you only play relatively clean orchestral guitar or clean bass.
1) Don't think hi-fi. A lot of the tone can be rather radical.
2) Remember sometime around 1975 when a lot of high-end and even mid-line Japanese phono preamps started coming with switches to select input capacitance to match the inductance of the phono cartridge? Well, IMHO on a guitar preamp the match to the guitar's pickups is even more critical (assuming you're not using powered pickups). The low inductance of an old strat pickup versus the high inductance of a 1/4 pounder humbucker makes a huge difference in the amount of treble loss from inductance, given they are driving the same input impedance. Conveniently, just messing with the input impedance and sensitivity makes huge differences in treble response and signal to noise ratio. First, like a Fender, for strange reasons each channel of a real FB-2 had 2 input jacks. As if anyone would plug 4 things into an FB-2. Some of the Alembic FB-2's had switching input jacks, so that when you plugged into only the first jack of only the first channel (and not anything plugged into the second jack of the same first channel), the litle resistor network between the two input jacks was bypassed. This made the input much more sensitive and improved signal to noise for the whole amp...right at the first series and parallel resistor. There was a note about this on some of the pink Alembic schematics, and not on others, and when I called they could not come up with the details again. The result was probably like the simplified input schematic moosapotamus has (or was R1 much much smaller when only 1 input was used??)...there's usually really no good reason to have two input jacks for one channel. So...if you have an FB2 without the switching jacks or are making your own, just have one input jack for a channel and DO mess around with the values of R1 and R2 to suit your particular guitar; it makes a difference. Might even try adding some trim pots to vary them. If your guitar makes more noise when you turn the volume on the guitar from full to 2/3, consider adding a 500K trim pot from the preamp input to ground (or change the value of your pot in the guitar or add a resistor across the pot). Better yet, take the volume knob out of the guitar, wire it full, and use a volume pedal after the preamp. For that matter, I install my preamps on the floor, with pedals on the pots. Or consider phantom-power to install a compact tube preamp in your guitar. I had my Travis-Bean running that way for a short while, with real tube buffers and a mixer for the pickups, no controls on the guitar.
3) LiIke a real Fender, the Bright switch is really just a cap across the volume control pot. You can think of this many ways, but be aware that it only has any effect when the volume control is low! If you drive into distortion at higher volumes, the reduction in treble is then often appreciated, which is the practical reason it's done this way, as the distortion harmonics generated are all higher-frequencies and Fender doesn't want to irritate you when you crank it. But it's far less than ideal, since you'd really rather do any "smoothing" (or "creaming" as the guitarists might say) of the treble due to distortion AFTER the distorting stage. More on that later.
3) Jerry (rest his gentle soul) often used to plug into channel 1 then route that channel's output (the real FB-2 had seperate outputs for each channel and a summed mono output) into the input of the second channel...then into his McIntosh 2300. With the two channels in series is a WONDERFUL way to use an FB-2, though you might want to snip out the resistors for that summing output circuit. Jerry sometimes just ran another short cord with 1/4" plugs around his FB-2 or wired it internally so they were always in series, but I usually installed a switched volume pot instead one of the input jacks for the second channel, so that when that volume control was switched off, it bypassed a channel but when it was on it controlled the signal level from first channel output into second channel input. Use a high-power-handling wirewond pot like the Alembic's Allen-Bradley pots. In fact, the circuit is really hard on all the pots, so though we usually don't use wirewounds I'd recommend them for all pots on an FB-2. Using both channels in series had some very interesting results. First of all, for a really phenomenal clean tone (especially for a strat pickup), if you optimized that R1 and R2 network that connnects your guitar pickups almost directly to the first grid, this preamp is so clean you can use a lot of gain and gain stages for unusual and extreme tone tricks...like REALLY using that bright switch (in fact I installed three mini-toggle bright switches in parallel on the first channel, each with a different size cap, so that I had 9 different knee points for the bright switch on the first channel; nice for bass too). Like I said, the bright switch only does anything if the volume knob is low, so for a crystalline shimmery incredibly bright treble build it clean and keep the first volume low and use the second channel in series for some additional gain to make up for that first low volume control. Fact is, you can't add "bright" you can just allow some thru and take away everything else, sacrificing overall gain...so for "super bright" you NEED that second channel for more gain. Try both channel's volume controls very low with the inter-channel high (or plugged direct) and both bright switches on! Yes, IMHO for the BEST clean tone you still need a lot of stages and a bunch of "Bright" treble. Of course the real FB-2 had no high-cut switch like moosapotamus' schematic. I used the volume knob I added between channels to determine whether to overdrive the second channel. Sometimes two such pots with a relay operated by a footswitch. The truth is that I never wanted just one channel once I got used to controlling both in series, but sometimes I wanted that inter-chanel pot on a footpedal (whereas Jerry always had it full). The second channel's tone controls can be used to control the harshness of the distortion of the second channel's first stage if you're driving it hard. Here in the second channel moosapotamus' hi-cut might come in really handy to tame the harmonics from distortion; maybe even multiple mini-toggle switches again. So for clean IMHO you want super-bright which requires extra gain, and for dirty IMHO you want super-bright (or at least lots of treble) in the early stages, then you can remove the "extra" treble and the distortion treble in the later stages. For a pleasant diry very rich in lower harmonics, just like you apply the RIAA curve before you record a record and then apply the reverse curve after playback, you should have control of the tone curve before you inject distortion, then be able to choose the "reversing" curve later...this pre-post tone curve control gives you the ability to control the "tone" of the distortion independently of the "tone" of the original guitar signal. You can also get similar phenomenal control of your transistor distortion by putting your stomp-box distortion devices between the FB-2 channels in series and using the tone controls of the two channels for "pre" and "post" processing. Between the channels is also a nice place to put a tube dynamic range compressor or expander. I don't particularly like starved-plate "blues" devices so I won't recommend any...
One more nice trick...if you use a trigger-filter (like an auto-wah etc.). These devices convert the volume envelope into a control signal for a subsequent filter (usually controlling the frequency of a low-pass but quite configurable on some models). Sounds like a wah (usually with no pedal) that responds quickly to each pick stroke, or can do a reverse wah with sound with each pick stroke, etc. Now some trigger filters like the lovetone meatball or some kits have an effects loop. The stomp box samples the volume envelope before the effects loop, then applies the filter after the effects loop. This gives you the capapbility to seperate the trigger signal from the filter input. With others, it's not difficlut to open it up and figure out how to seperate the envelope demodulator input from the filter input. With your 2-channel in-series FB2, you want to sample the volume envelope before any distortion clips or compresses that volume envelope (or you can end up with on/off wah instead of nice full control) and it's even best to take the signal before tone controls...so just add a big cap and a volume pot to make an additional "early output" on the first channel before the tone controls and plug that into the trigger-wah input, engage the effects loop on the triger-wah, and then plug the second channel's main output into the effects return on the trigger wah. The trigger-wah will be very responsive and controllable and make consistent use of its sensitivity and range settings regardless of how you mess with your preamp or amp knobs or distortion or any controls, yet the trigger-wah won't add any noise like it would if it were inserted between guitar and amp. And you can use a switched volume pedal with an oscillator (or sometimes even just a battery) to turn the atuo-wah into a manual wah pedal.
Or you can modify a Mesa-Boogie tube footpedal chassis into a 2-channel FB-2 preamp.
I also have two Japanese tube foot "boxes", each of which really is again one Fender preamp channel with a teensie speaker and transformer dummy load. I picked them up for about $30 each. With very little work they can be put in series and sound much like an FB-2 with its channels in series. The work would be converting to a better external power supply and maybe better pots.
Some people claim a case for adding a third channel in series. The idea is to apply distortion, change the phase 90 degress, and apply distortion again, to a different part of the waveform. It does look different on a scope, more harmonics without more compression. It may have its uses.
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There are a number of projects which are based on tube amps, but replace the tubes with JFETs. I think the most famous one is the flipster, a solid state remake of an Ampeg Fliptop, sounds great.
In the same line of thought, someone made a layout for a solid state F2b, it's called the FET2b, not sure if this was on the diystompboxes.com forum or freestompboxes.org forum.
I made one and liked it (although I gave it away recently), it's pretty clean, but doesn't have the very large headroom of the valve version. Not sure which JFETs I used, but some widely used ones have a limited clean range because of their pinch-off voltage. MPF102 is not too bad, and J310 is even better, but you'll tweak the circuit to accommodate the FET replacements, and if you buy the same J310's I did, there's quite a bit of current flowing if you bias it right.
But for guitar there's plenty of clean headroom.
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