High Desert Sonic Institute

Mod Blog

Follow the process of converting a 1950’s Bell and Howell Filmosound 179 tube amp into a gnarly guitar amp, with pictures and detailed descriptions.

Leslie Type 25 Booster Amp Conversion

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LESLIE TYPE 25

Converted to Guitar

This little buddy has moved from Chicago to Minneapolis to the Mojave with me, always getting stored on a shelf in my new shop. I finally got the gumption up to make something out of it and ended up creating a sweet, versatile little amp that’s minimal in controls yet maximal in tones.

I no longer remember how it first came into my life, but it started it’s own life as a “Booster” amp inside a Leslie speaker cab. The amp is really just a simple 20W power amp with a 12AU7 paraphase inverter driving a couple of cathode-biased 6L6’s. Below you can see some gut shots of the original amp. Most of the caps in the amp measured way out of spec, so they were eventually replaced, along with the rectifier diodes. The wiring was largely redone as well.

Somebody clearly already tried to convert it to an instrument amplifier by adding a 1/4” jack, but without an additional gain stage in front of the phase inverter, you wouldn’t get much volume or tone from this puppy with just a guitar plugged straight in. To make this a useful instrument amplifier, I would need to add a preamp stage of some kind.

Plotting load lines for the 6AU6A

Plotting load lines for the 6AU6A

I decided to use a 6AU6A as my preamp stage. These small-signal pentodes have been slowly accumulating around my shop, and are a perfect high-gain single stage to throw in front of this power section. It’s rare in amp repair that you actually get to plot a load line, but to derive a good starting bias point for my pentode I dove into the graphs. The resulting circuit is a truly unique one in the world of guitar - there are very few amps out there with a 6AU6A input stage, or a paraphase inverter, let alone both infront of a pair of cathode-biased 6L6’s! Other than some 50’s Fenders and 60’s ampegs I couldn’t think of any off the top of my head that came close to this topology.

Measuring and computing the impedance ratio of the output transformer.

Measuring and computing the impedance ratio of the output transformer.

After designing the input stage, the next issue I had was to determine what output impedance the output transformer wanted to see. I ran a test in which I injected a known AC voltage into the secondary of the OT and measured the reflected primary voltage. From this you can calculate the impedance ratio of the transformer and determine what load speaker to hook up. It turns out that, for a pair of 6L6GC’s with around 450V on the plates, an 8 Ohm speaker will work with this OT just fine.

Next step was to finalize the circuit. Other than the new input pentode, the rest of the audio circuit (the phase inverter and power section) was largely lifted from the schematic on the unit itself, preserving the flavor of the original amp. The power supply was completely redesigned in order to provide the proper screen voltage and bias for the power tubes as well as to provide a supply node for the new preamp section.

I then largely gutted the wiring, measuring each component as it came out to see if I could reuse it. Turns out pretty much every single cap was bad in the amp, but I scrounged up some vintage coupling caps from my stash to keep things as era-correct as I could. I then fit the new components like front panel controls, can cap and tube socket.

Minor circuit tweaks begin at this stage to get the voicing just right. Then comes the custom cabinetry! I’ve been doing some remodel work on old homesteader cabins out here in the Mojave and have saved old wood when it is being trashed. I collected some knotty pine boards from a cabin here and have repurposed them into the housing for this amplifier. I really love using salvaged wood to house what is in effect a salvaged amp. The speaker baffle is Baltic Birch for its musical response, and the battens are solid oak for longevity. It gets a simple protective Poly finish that lets the wood shine for itself.

Annnnd voila!

I named it “the Preslie” because, as with the King himself, it’s a bit of an imposter: it has luscious, deep tones all its own, but it largely relies on the brilliance of those that came before it. In the case of this little amp it was the the Leslie organ that donated it’s booster amp to the cause, while the King used the music, style and sounds of Otis Blackwell as if they were his own.

This beauty is up for sale! Grab yourself a one-of-a-kind custom amp with vintage components and tones today!

Jacob Erwin