Help on DIY F2B with mods

[Mike DL]

Hello! Lurking here a while, trying my first post.

I want to build an Alembic F2B (http://www.alembic.com/support/F2B_manual.pdf) as a guitar preamp. I've drawn two schematics that I'd like to ask for comments/suggestions on. I've developed them by trying to learn from other people's variants.

The first is a fairly true-to-the-original F2B circuit, except for two things: (1) power supply and (2) output capacitor. With the power supply I am trying to make it easy to experiment with different B+ values. The capacitor change is from a suggestion to limit some of the low end.


The point of the second schematic is to add a cathode follower (The Valve Wizard -Cathode Follower) so I can have the preamp drive two separate power amps. With the power supply changes in this version, I want to elevate the heater supply to stay within heater-to-cathode voltage limits.


Comments, suggestions, etc. welcome! Analog stuff like this is new to me, but I'm trying to learn.

Thanks

[dwmorrin]

On schematic 2, I would question the need for R11, the 12AU7 plate resistor. With a cathode follower, you should be able to connect the plate directly to the B+. The purpose of a plate resistor is to convert the alternating current into an alternating voltage. Since we're not interested in the plate circuit at this time, we can skip it. We, instead, stick an appropriate resistor in the cathode circuit to get our voltage output there.

Another difference I spot from the valve wizard example is your output coupling cap C7 is significantly smaller (47n vs 470n)... and your R13, 1M, is significantly larger than valve wizard's 22k.

The larger capacitor is a good idea because of the low impedance of the cathode circuit. A smaller cap may introduce a low cut effect. And since you've paid the price of a whole extra triode stage and additional power considerations, etc, for this cathode follower, you might as well make use of its power by changing R13 to a smaller value. Valve wizard's 22k is a typical value. In general, when possible, lower resistances are more desirable due to the lower Johnson noise (inherent in all resistors), and the higher currents make the circuit less susceptible to interference. The problem is that lower resistances take power to drive them. In your first circuit, the driving impedance is the plate resistor, 100k, so 1M is needed to not load it down. After the cathode follower, you are free to go much lower. A lower value of R13 will also discharge the C7 cap quicker, reducing the chances of a POP when you plug in.

[Dave H]

Your bridge rectifier will short out the transformer. Reverse the polarity of D3 and D4.

[Mike DL]

Wow, thank you both so much. Especially thanks for explaining the whys! Hopefully, I will start building it next week.

mike.

[Bodtker]

I would be highly interested to hear if you successfully built either of these.
Also, I'm looking for a good schematic/Layout for an all tube bass preamp.
Thanks!

[Mike DL]

Yes, I successfully built it. I've changed the design a few times, and the current version sounds great even with a long cord to the amp! I'm finishing up the final build now and will post a final schematic when finished.

[nashvillebill]

Please do, I've been thinking about building the F2B with a cathode follower as well.