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Is the Frog Fx1 Tube Preamp a Clone of the Alembic F2b?

Frog Fx1 Tube Preamp
Fender Dual Showman Tube Amp
Alembic F2b Tube Preamp

Some may ask if the Frog Fx1 Tube Preamp for guitar and bass guitar is a clone of the venerable Alembic F2b tube preamp.  The answer is, yes, no, or maybe.  Here is a little history of the tube preamp circuit that is used in the Alembic F2b tube preamp and the Frog Fx1 tube preamp that may help you decide:

The basic circuit that both the Frog and Alembic use was originally found in the RCA receiving tube manual. When Leo Fender needed a circuit for his first amplifiers (the K&F model amps) in about 1948, he went to RCA receiving tube manual and found the basic requirements/schematics for building preamplifiers and amplifiers.

In the late 1960’s the circuit was incorporated into the Fender Dual Showman amplifier.   In the early 1970’s the Grateful Dead were using Fender Dual Showman amps with the internal preamp output rewired to McIntosh power tube amplifiers courtesy of their primary PA provider, Alembic.  They liked the preamp, but not the power amp of the Fender.
Because the Dual Showman was too big for a rack, they built a 19 inch rack mount tube preamp with 2 channels based on the Dual Showman amplifier.  The Alembic F2b was born.

Jump ahead 40 years or so to 2015 and Mark Price decides he wants to experience the Alembic F2b magic and build one himself, because the Alembic F2b costs around $1,300.00USD.  After 3 printed circuit board re-designs, I designed what became the Frog Tube Preamp PCB 2.0 and other DIY’ers became interested in building the tube preamp themselves.  I have sent about 60 of the DIY PCBs around the world from the USA, Canada, almost every country in Europe as well as Austrailia and Japan. In 2016, I decided that there were probably a lot more people that would prefer to purchase a completed tube preamp than make one themselves, so I began to make prototypes of what would be my first complete single channel tube preamp for sale, the Fx1. I designed it to fit on your pedal board and runs off of a 9VDC wall transformer while maintaining 200vdc voltages for the tube. The Frog Fx1 started selling in February of 2017. If I would have known, I could have saved a lot of money by just purchasing a brand new Alembic F2b, because the development and FCC certifications cost for the Frog Fx1 was a lot more than the $1,300.00USD for the Alembic! Thats ok, it has been fun and the Fx1 is starting to catch on.  So far I have sold completed preamps in the USA, Canada and Germany.

Now, at the current price you can experience the “Alembic F2b sound” that is so popular for less than 1/3 of the Alembic F2b cost. I am now developing a more ruggedized version of the same preamp that has the tube protected inside the pedal enclosure. Check on Facebook @ frogpedals for ongoing updates of all of Frogpedals developments.

There are a few differences or shall I say updates for the better, so here is a list of features of the 3 products:

Features               Fender   Alembic  Frog Fx1

 All tube circuit        Yes      Yes      Yes
 Bright Switch           Yes      Yes      Yes
 Mid Boost Switch        No       No       Yes
 Diode Clipping          No       No       Yes
 Dual Channel            Yes      Yes      No
 Grid leak protection    No       No       Yes
 Master Volume           No       No       Yes
 6 and 12 volt tubes     No       No       Yes
 Class A tube operation  Yes      Yes      Yes
 Power amp output        Yes      No       No
 Cost                    ouch     ouch     ahhhh

Some future Frog plans:

  • Ruggedized version with internal tube, true bypass stomp and master volume sized for pedalboard
  • Ruggedized version with internal tube, true bypass stomp and master volume sized for pedalboard with 1/4 inch and DI XLR output
  • Dual Channel pedalboard sized
  • Dual Channel rack mounted with 1/4 inch outputs and DI XLR outputs
  • Stand-alone High Quality DI unit

Note: DI will be a high quality Jensen, OEP, Cinemag or Crimson Audio transformers, your choice!

So is the Frog Fx1 a clone? I would say no. I would call it a natural progression of a great idea started when Leo Fender first picked up that RCA Receiving tubes manual.

What do you think? Feel free to leave comments below. Thank you!


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Alembic F2b meets KLON Centaur = Frog Fx1 Tube Preamp

I want to point out something that I very intentionally did, but really haven’t emphasized much at all and that is in regard to the 2 diode clipping options on the Frog Fx1 Tube preamp pictured below.  The clipping options were not in the Alembic F2b preamp or in the original Fender circuit.  I had some room available on the printed circuit board (PCB) when I designed it and decided I would add some diode clipping options to give a little bit of fuzz or overdrive tone to the preamp.  I was familiar with the Klon Centaur that was designed in the 1990’s and had built a clone of the circuit and liked the mild overdrive tones it gave me with guitar.  So, I researched the circuit diagram of the Klon Centaur (it is not tube based) and found where Bill Finnegan placed the pair of clipping diodes in the circuit.  They were right after the first opamp buffer stage and Bill only included one clipping setting.    So having found where they were placed, I tried placing the clipping diodes right after the first preamp stage of the Frog Fx1 (there are 2 preamp stages in the Frog Fx1) and found that they worked great in that position.  There were a couple different types of diodes I liked that gave distinctly different clipping characteristics so I added 2 different clipping options.  One is more of a mild overdrive and the other is a mild fuzz/distortion tone.  They are very touch sensitive, so their influence on the sound is related directly to the output/string attack of the instrument used.  I am sure there are many more that would sound great, but I only had so much room on the PCB.

Just so you know the street price of the Alembic F2b is about 1300.00 and the KLON Centaur has truly become mythological and runs between $2000.00 and $3500.00 when you find used ones online.  Makes the price of the Frog Fx1 downright reasonable!

Alembic, Fender, and KLON are the property of their respective owners.

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Frog F1a Tube Preamplifier PCB for DIY


Class A vacuum Tube preamp based on the Fender Dual Showman preamplifier section which inspired the Alembic F2b preamp. It is high quality 1.6mm board with 2 ounce copper traces (double the normal printed circuit boards) with Gold plated solder pads.