Modified Alembic F2-B

I'm going to be building a single channel bass preamp based on the F2-B. I think I have most of parts ready to go, just need to order the FET's and some caps so I can do a quick prototype. I happen to have one of the Stancor power transformers used in the F2-B, so the power supply will be identical to the original.

The buffered output should cover the main weak point of the F2-B, which is it's compromised ability to drive long cable lengths and the low impedance inputs of modern pro amplifiers.
Schematic attached.


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Cool build!
I'd up the value of the output capacitor though. If connected to e.g. a power amp with a 10k input impedance, which is quite commen, the -3dB point will be at about 160Hz(!). WAY too high for a bass amp. You want it to go down to at least around 30Hz, which is achieved with a 530nF cap, which doesn't exist, so you could use 1uF and be safe for even lower peripheral input impedances.
Updated the schematic, and placed an order for a Par-Metal 10-19082B 1U case.

I'm worried about the transformer fitting; even though the original Alembic is in a 1U case, and I have the same Stancor PS-8415 power transformer, the Stancor I have is 1.64" thick, which is the exact internal dimensions of the Par-metal case (1.74" - 0.1" for the top bottom panels)... We'll see how that goes.

Now I'll be working on the turret board layout.


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If you feel a little more ambitious and want to try another slighty more complex classic tube bass preamp, I have an extra unpopulated high-quality double-sided printed circuit board for a Trace Elliot V8 preamp, which includes a switchable additional stage for overdrive, and an equisite controllable tube compressor. The extra stage and on-board compressor make it much nicer than their V-series preamp. The V8 was the company's "statement" model. British Audio Service Inc. has some of the V8 printed circuit boards for sale.
Yes, I'm sticking with the F2B-ish circuit for this project.

Q1 is the output buffer. Just a simple source follower.

I am just sticking with a single channel for now. With that said, this project will probably have more in common with the F1-X, but they're all based on the old Fender preamp anyway.

If I feel the need for more gain, more tone shaping, or a different sound altogether I'll adjust the design later. This could easily just be a starting point.

Also, instead of posting several one line posts just edit your original post, please.
Layout finished. Haven't second checked it yet... The pdf should be 1:1; the board measures 7.25"/3.125". Plan is 1/8" Epoxy G10 material with Keystone 1540-4 turrets. Drawing shows the overall diameter of the turret base, not the hole diameter which is 0.063" for the 1540-4's.

I'm going to do a parts list later, it's all pretty standard. The power supply filter caps are Panasonic Radial 47uF/400V.


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Got some time off, and it was a nice spring day so decided it was time to get some real work done on this project out in the garage.

This is the first turret board I've done, so I made a DIY anvil and swage as per the tubedepot plans, with one exception. I had to use a 2" bolt for the anvil as most of my turrets were placed pretty close together. Also, not that the 1540-4 turrets require a 3/32" through hole for swage fitting. Use a #43 drill bit for press fitting.

It was a lot of fun to make, and I think it turned out pretty good.


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