VIBRO-KING CONVERSION


This project converted a Marshall JCM900 series amp into a Fender style Vibro-King amp, by gutting the entire amp, keeping only the chassis, transformers and tube sockets.

The amp gained a GZ34 valve rectifier, a 6V6 tube to drive an Accutronics long tank, and two more 12AX7s.  The power tubes were changed to Soviet military 6L6GTs.  Two small heater transformers were added, one for the rectifier heater (5 volts), and one for the two extra signal tubes.  Changing from EL34s to 6L6s freed up a bit of heater capacity in the existing power transformer, which was used for the 6V6.

The Vibro-King architecture was used, which has a reverb main channel (with Dwell, Tone and Mix and Channel Gain), followed by the Fender Bassman style tone stack (Treble ,Middle, Bass, and Volume).  A tremolo was also added, with Speed and Intensity, and power stage injection (via the bias circuitry).

Parts of the orginal Marshall board were kept - the power end with fuses, and the original Line In/Line Out circuitry was maintained.

In keeping with the Soviet military theme, all of the new electronics were wired on 18-tag turrets, sourced from the Ukraine.


Converted amp, with new front panel fitted.  Not quite the same colour, but I didn't want to rework the power and standby switches.  I suppose the original bit on the right could be called the "legacy panel".













Here is the chassis, with the rectifier heater transformer just to the right of the existing blue power caps.  The chassis was designed for a quad of output tubes, which was handy, because one hole could be used for the GZ34 rectifier (next to the power caps) and another hole used for the 6V6 reverb driver tube (the black one in the middle).  The two new 12AX7s are in front of the 6L6GT power tubes.


Here's a guts shot, showing the use of the original Marshall board for the line in/line out circuitry (left rear) and the power input and fuse section (right rear).

The 18-way turrets make re-wiring circuitry very easy, and keeps signal paths very short.




Here's the reverse pic of the circuitry, which shows the turrets and the control wiring. 

The new bias control board is right up the top left of this pic - much easier to access for setting bias.