The "Bedrock" Vox AC15TBR Prototype Amp - A Look "Under the Hood"

The AC15TBR prototype amp from Bedrock amplification featured an eight tube circuit.

The following chart details the function of each tube.

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V1a V1b

12AX7

Tremolo Oscillator Reverb Receive Circuit

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V2a V2b

12AX7

First preamp gain stage Not used

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V3a V3b

12AX7

Second preamp gain stage Top Boost Tone Controls

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V4a V4b

12AX7

Reverb Drive Circuit

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V5a V5b

12AX7

Phase Inverter

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V6

EL84

Output tube

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V7

EL84

Output tube

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V8

5Y3

Rectifier

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Printed Circuit Boards and Chassis
The chassis was assembled into a rectangular steel pan that was fastened to the sides of the cabinet by four screws.

The circuitry was assembled on two printed circuit boards; one for the preamp and a second for the power amp. The tube sockets and controls were mounted directly to the printed circuit boards.

"Top Boost" Preamp
While designing the AC15TBR, Korg requested that Bedrock utilize the circuit from the"Top Boost" channel of the Marshall/Vox AC30TB(X) without modification. A close examination of the schematics confirmed that both amps had the same "Top Boost" preamp circuit.

Reverb Circuit
The reverb circuit for the Bedrock designed AC15TBR utilized a drive transformer. This transformer was not included in the reverb circuit of the Steve Grindrod designed AC15TBR from Marshall.

Power Amp Circuit
The power amp stage of the Bedrock designed AC15TBR used two EL84 tubes in the classic cathode biased, no negative feedback design from the original JMI AC-15.

"Bedrock" AC15TBR Specifications

Year	1995
Production Numbers	A few prototypes
RMS Power Rating	15 watts RMS
Channels	One with two inputs
Controls	Volume, Treble, Bass, Cut, Reverb, Tremolo Speed, Tremolo Depth
Tube Complement	Five 12AX7, two EL84, one 5Y3
Dimensions	23¼" wide, 16¾" tall, 10" deep