Alembic F-2B preamp

Here’s some build info I’ve put together on the Alembic F-2B. I’m planning to build myself one of these sometime real soon.

The original…

My redrawn schematic is primarily based on R.G. Keen’s, except for the selectable hi cutoff switch SW1 + C2, which was not part of the original design. But, I saw it in someone else’s F-2B project and decided to include it.

The power supply shown in the GEOFEX scheme is just a tad cryptic. It has a voltage doubler, but no indications about the transformer except for 6.3VAC on the secondary that’s used for the filament supply. But, on the Alembic page, it indicates that the F-2B uses a 300 volt supply. So, I went with that and just started from scratch. I also wanted to be able to fit everything into a 1U rack enclosure, like the original. But, I had no luck finding a single low profile transformer with dual secondaries for both plate and fillament that would fit. So, the PSU I came up with uses a back-to-back pair of identical compact toriods with rectifiers. The low voltage side for the filaments is pretty much right out of the LM317 datasheet and supplies DC voltage to the filaments, instead of AC like the original. I also included the 5K trimmer to make it variable, so I’ll be able to see what different filament voltages might do to the sound. For the high voltage side I used PSUD2 and shot for around 300VDC.

Here’s a PCB layout for the audio section…


The PCB layout was made to utilize these cool horizontal tube sockets that I picked up from Angela Instruments.

And, here’s a PCB layout for the PSU…

More to come…

42 thoughts on “Alembic F-2B preamp

  1. Any chance you may be doing a pcb for this project charlie as i would be keen as hell to get one off you if you did. Cheers

      • Hey Charlie! Thanks for sharing your work and insight! I have copied your layouts to make a PCB but I am wondering if the photo of the layout should be reversed to make the proper PCB?
        Sorry for being a relative newbie. Thanks for any help
        Vaughan

        • The view of the layout is as if you are looking at the component side of the board, with an x-ray view of the traces on the underside (in blue). NOTE: there is one trace on the top of the board (in orange) that can be replaced with a jumper if you are etching a single-sided board. So, it depends on your method of making PCBs. If you are using photo-sensitive resist, then you should reverse it. If you are using “PnP Blue” or anything similar, then you do not need to reverse it. Have fun!

          • Hey Thanks for getting back to me Charlie!!

            I REALLY appreciate it. If you were to produce some boards for this I would definitely be interested. After researching around the net for the last week, I’m sure there would be lots and lots of other Muso’s who’d be interested in buying them too.
            Just a thought………have a great gig!

  2. I would most definitely be interested in purchasing a set of PCB’s as well, single channel preferred. I’ve got a nice tube power amp waiting for a tube pre such as this. Nice job! Pete

  3. Unfortunately, it doesn’t look like those tube sockets from Angela Instruments are available any more. Too bad. The horizontal PCB mount was really nice.

    Anyone know where to find something similar? If so, please post info here.

    Otherwise, I’ll just have to change the layout if I’m going to produce some PCBs for this project.

    Thanks
    ~ Charlie

      • Thanks for those links, Vincenzo! Good to know. I have been mulling over revising the layout with a vertically mounted PCB for the tube socket which would mount onto the main PCB. But those plastic sockets look like they might be a lot sturdier while still keeping a low profile. I might order a few and check them out. Thanks, again!

        • You can’t be serious thanking me, mate: what should I do, then?! :D Can’t wait for your trials with those sockets and for your PCB’s revisions, whish you all the best from a rainy Florence!

  4. Hallo!
    It might be a dumb question, but, as an European, where mains voltage is 220-240 V AC, I was wondering why not connect mains voltage directly to PSU bridge rectifier. Is there any problem with that? Thanks for the answer.

    • Good question! I don’t know for sure, but that just doesn’t seem very safe to me… And when it comes to high voltage, safety trumps everything else, IMO. But you might see if you can find any commercial European gear that handles the power like that. If so, that would be a good clue.

      • Well, I googled a little and I found out that it was a very common practice, so I guess I’ll try to do it.
        Another question: is there any chance to get silkscreen print and bottom copper layer print PDF or JPEG (in original size). I can recreate the pictures surely, but it is much more time consuming. Or at least tell me the dimensions of both the boards, I’m in the planing stage and just need to know how to lay things inside the chassis.

        • Cool. That’s nice to know.
          The audio and PSU boards are both 2.5″ x 1.9″. If you look closely, you might be able to make out a grid of small dots in both of the layouts. Those dots are spaced at 0.1″.

      • DON’T rectify the main directly. It’s possible, it works technically, but – especially for guitar (or anything you touch) – could be suicidal. You leave the possibility of a component failure leaving things at main potential. My advice again – DON’T do it. 12 volts is an easy enough voltage to get a transformer for.
        It once was common practice in double insulated radios and the like. Never for guitars or instrument work.

        • Great information! I actually agree. Thanks for posting this. When in doubt, it’s always smart to play it safe! Thanks, again!

    • FYI: Attaching mains voltage directly to the bridge rectifier or any other part of a circuit, even using a fuse and switch, is widely recognized as a dangerous practice in tube electronics. Using an isolation transformer is always recommended.

  5. Hi,

    Thanks for sharing this.

    I don’t know if it was deliberate or not but your schematic has VR3 as 250k while RG’s shows this as the normal Fender 10k. At 250k the mid-scoop is greatly reduced but can pretty much render the treble control useless (on the cut side) unless the mids are almost out.

    Cheers,
    Ian

    • Thanks for pointing that out, Ian. It must be a typo in my schematic. I looked back at my hand written notes and I do have the Mid control as a 10k pot. I’ll have to fix that when I get a chance.

      Thanks, again!
      ~ Charlie

      • That 250k mid pot is something I’ve used in a Fender amp before. It acts as a normal mid control but as you turn it up, it gradually removes the tonestack (by shunting it to gound?) from the circuit, giving you more gain and a “raw” sound. I think it’s a pretty cool and gives some tonal variation.

    • Nothing more than prototype of the layout, above. But I made another layout with axial caps so it could fit inside a 1ru enclosure. I might get some of those made sometime soon.

  6. Hello guys!
    I would like to build my own Alembic F-2B, so I start studying the schematics.
    But there is something I don’t understand, how the PSU and the preamp are linked (yes, i think it’s kind a dummy question ^^).
    Thanks by advance for your reply!

    • The high voltage (shown as 314V on the PSU schematic) connects to B+ on the preamp schematic. The low voltage (shown as 5.6, 6.3 or 12V on the PSU schematic) connects to the vacuum tube pins 4 & 5. That is not shown on the preamp schematic, but you can see it in the PCB layout. Finally, all the ground points are connected together. That’s it!

  7. Hi! I figured this might be the place to ask as you’re building an F-2B clone. What type of potentiometer are you using for the volume pot? I need to replace mine in an F-1X and I believe they are the same.

    Thanks for your time!

  8. Guys, can you help me? How are the caps in audio section rated? I mean volts. And the resistors in audio circuit, except the 100K 2W resistors, are the usuall 1/4W or 1/2W? Pots in tone section linear or log? Thanks!

  9. I can see some components have specific wattages and voltages against them. Are the values for the other components available please? From what I’ve googled, the other resistors are 1/2 watt. What voltage do the tone stack capacitors need to be? Thanks in advance.

    • 1/2 watt resistors should be fine. I think I might have used 1/4 watt for all the ones that do not have the voltage indicated in the schematic. The film caps I used in the tone stack were 400V, but anything above 100V is probably fine. I also used some ceramics, but I don’t know what their voltage rating was. I used 35V for the two cathode-to-ground electro caps.

      ~ Charlie

  10. Hi Charlie,thanks for posting your hard work.Question about the transformers,T2 is a 12/230v transformer otherwise the resulting voltage is lower than 300v but the caption on the schem states that T1 T2 are 115/12v toroids.
    Aharon

    • It’s been a while since I worked on this, so my recollection of how power supplies work is a little rusty. But, if I’m not mistaken, I think each of the two output coils in T2 put out 115V, so wiring them in series will give you 230V.
      ~ Charlie

  11. Here is my first F2B-like preamp that I made about 10 years ago.

    http://www.frontiernet.net/~jff/af2b/Dual_Channel_Blackface-style_Preamp_for_Bass_Guitar.html

    I used a Triad LB-1220 flat pack transformer for the PSU. It has two primary windings (120V) and two 10V secondary windings. I used one of the primary windings as a secondary feeding a FW voltage doubler. I think I got about 250 VDC, feeding both channels. I rectified and filtered the secondaries, and got between 11 & 12 VDC. Worked perfectly. Only using one primary for the mains halved the VA rating of the TX, but it was enough for 2 x 12AX7′s (~ 3mA). If I would have used Schottky diodes for the filament rectifier, the filament voltage would have been higher, but I didn’t know about stuff like that 10 years ago.

  12. Hi and thanks for sharing this project,

    I’d like to build this kind of preamp and I’m wondering if the F2-B can produce overdriven sounds.

    Thank you in advance for your answer.

    Regards,

    Mike.

    • It can but, personally, I didn’t particularly like the sound of overdriving mine when I was building it, so I decided to just keep it clean. I’m sure there are a number of different ways to do it. I was already really happy with the clean sound so I never bothered to do any more experimentation with it. I’ve got lots of pedals for that kind of stuff, anyway.

      • Thank you for your answer.

        My idea is to build a tube preamp delivering warm and clean tones but also some kind of little distorsion.

        I was wondering if it was possible simply using the gain knob.

        • I don’t recall getting much grit out of the gain knob. It’s possible… you might need to change some resistor values, or you could try clipping diodes (I never tried that). But I cannot tell you if you will like the sound of the distortion that you get. The F-2B will definitely give you those warm clean tones, tho.

          • OK; thanks for answering.

            I will take some time to work on a single channel version of this preamp and maybe a second modified channel…

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