BML4

BML4 is a 100/50-Watt Guitar Amplifier: A new direction in amp technology and tone

Background: In the summer of 2014, I developed the first successful Active Output Transformer (AOT) circuit after many iterations. The AOT circuit is a hybrid tube-transistor output stage capable of ~50-100w RMS without a traditional iron-core output transformer, saving cost and weight. This idea evolved into a prototype in a Princeton chassis/cab. Several amps followed, two versions of the Bassman-Lite, and the Sugarbee; all incorporating evolving versions of the AOT output circuit. The photos below show two of these amps. Both produced 50w into 4 ohms (or ~30w into 8 ohms).

In 2019 I began development of a single-pcb-board AOT amp aimed at simplicity of use and low cost. Features include a dual-channel preamp (clean/OD), optional spring reverb (PCB stuffed or not) and an AOT stage that could be configured for 50w or 100w (into 4 ohms).

A second iteration of this design is complete (June 2020) and described below. A 50w head version was built first and a 100w combo was created in a Bassman (5f6a) chassis format. The 100w Bassman version also represents the 4^th generation of Bassman-Lite designs – thus the BML4 designation.

The 50w head prototype is shown below. The PCB is 12” x 5”. The clean channel has Volume, Tone, and Presence controls. A mini-toggle provides three tone range options (Blackface, Blackface bass-cut, and Tweed).

The clean channel sound is killer at bedroom levels and gets richer as room volume increases. The OD channel has Gain, PreQ, Tone, and Volume controls. PreQ controls the tone of the signal before OD, and Tone alters the signal after OD. The dual tone controls provide a wide range of OD tones and there is ample gain for sustain.

All the pots and jacks and connectors are soldered on the circuit board. The two mini-toggles (spst and spdt) require (short 2-3”) wires to the board. The boards with switches can be completely assembled and tested outside of the chassis. This approach is also used in the Solo 100 amp described in my Guitar Amplifier Design book.

Switch-mode power supplies are used exclusively, meaning there are no heavy power transformers. The large (blue) supply is a 24v 5a supply. The small blue one is a dc down converter, and the medium (orange) one is a high-voltage dc step-up converter.

If a traditional B+ supply is desired, the board provides connections for a B+ transformer and associated rectifier and filter caps, but they’re not used in this configuration. Two AC inlet options are on board as well. The one shown is a combination inlet, switch and fuse module. The other option is for separate inlet, fuse and switch components (as shown in the 100w prototype farther below).

Two front panel LEDS indicate power (24v and B+). A two-color LED shows channel select state (green for Clean, amber for OD). The 50w prototype includes reverb and the pan is mounted to the wood cabinet top. The front panel Reverb knob controls wet/dry mix for both channels. The pan connects to the board with RCA jacks/cables. The complete head cabinet (with pan) as pictured weighs in at 7 Lbs 3 ozs. (About half the weight is the wood cabinet.)

The 100w amp uses a Bassman chassis format, as shown below. A colored face plate is added to the aluminum U-shaped chassis. A perforated top plate provides ventilation and adds foam support that gently hold the tubes in their sockets.

Screw-terminals (blue) provide PCB connections to power supplies and the power switch, so no soldering is needed to assemble (or disassemble) the chassis. The back panel has a stereo FX jack, two speaker jacks, and a stereo pedal jack.

The pedal jack switches channels and turns reverb on/off. LEDs on the pedal show switch states. Plugging in a pedal disables the front panel channel-select switch.

The 3-page PCB circuit schematics are below.

I have some spare PCBs. Contact me if you are interested in them.