Modified Alembic F2-B

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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.
The preamp from the V8 is 14 tube stages, so it's a lot more complicated than even the F2B with both channels. It's just that those two (F2B and V8) are my favorites, and quite possibly the two most 'classic' and desirable bass preamps. I assume you know what you wanted, I just thought I'd offer...
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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