The Vox TB18C1 "Tony Bruno Design" Amplifier
A Look "Under the Hood"

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The Vox/Bruno TB18C1 was introduced in 2012. Let's take
a look "under the hood" of this unconventional Vox amp.

Power Amp
The Vox TB18C1 used a pair of cathode biased 6V6 tubes (V1 and V2 in picture above) in its power amp section. The Vox/Bruno TB18C1, TB35C1 and TB35C2 were the first Vox amps to utilize this traditionally "US" output tube.

Preamp and Tone Control Circuits
A single 12AX7 tube powered the front end of the Vox TB18C1. One half of the first preamp tube (V4) was used as the first gain stage. Its output was directed to the tone stack.

The tone stack consisted of the treble, bass, and middle controls plus the "mid range boost" switch. The output from the tone stack was directed back to the second half of V4 for gain recovery.

Phase Inverter
A second 12AX7 tube (V3) served as the phase inverter.

Digital Reverb
Vox developed a modular digital reverb circuit board for the TB18C1. It was located over the right side of the main circuit board (see above). A close up can be seen at left. The black square in the center of the circuit board is the DSP (digital signal processing) chip.

I suspect that Vox will be incorporating this modular digital reverb board into future amps.

Dark Switch
Just like the rotary "Cut" control on an AC-30, the "Dark" switch combined out of phase high frequency signals coming from each side of the phase inverter. This caused a cancellation of treble frequencies when the "Dark" switch was engaged.

The Vox/Bruno utilized a single 12 inch, 16 ohm Celestion speaker.

The full rear panel of the amplifier cabinet interracted with the speaker to increase bass response. Three narrow horizontal slots were cut into the back panel. These slots would limit the the bass cancellation one would expect from an traditional open backed amplifier cabinet. When combined with the 6V6 output stage, this semi-closed back cabinet design gave the TB18C1 a significant boost in low frequency output. The slots also provided a source of cooling air for the chassis.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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