The Vox V100 Amplifier 1970 - 1973
A Look "Under the Hood"


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© 1998 - 2017 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.
Vox product information for the V100 amp head heralded it as the successor to the legendary JMI Vox AC-100.
Let's spend a few moments and see how the V100 measures up.

Comparison of the AC-100 and V100 Preamp Circuits
The AC-100 preamp was powered by a 12AU7 (ECC82) and a 12AX7 (ECC83) tube. The V100 preamp utilized a single 12AX7. Either amp had just volume, bass and treble controls.

Summary of the AC-100 Preamp Circuit - The AC-100 was a hand wired ampliffier that used one half of a 12AU7 (V1) in the initial gain stage. The 12AU7 is a low gain tube with an amplification factor (mu) of only 20. It appears that Vox chose this low gain tube to restrict preamp gain to moderate levels. This aided in keeping the preamp signal free from distortion. After a .047 uf coupling capactor and the volume control, the signal path proceeded to V2, a 12AX7. The first triode of V2 provided an additional gain stage, the second triode powered the cathode follower circuit that provided gain recovery for the tone control stack.


Summary of the V100 Preamp Circuit - The V100 was constructed on a printed circuit board. The preamp used a single 12AX7. The first triode of the 12AX7 was used a gain stage. The second triode drove the cathode follower circuit that powered the tone control stack.

Comparison of the AC-100 and V100 Output Circuits
A 12AU7 served as the phase splitter in the AC-100, the V100 used a 12AX7. Both amps featured a quad of chassis mounted, fixed bias EL34 power tubes in the output stage. Neither amp provided controls to adjust the bias of the output tubes.

V100 Chassis
The V100 was built on a rectangular steel chassis pan that included a unique, chimney like duct to direct heat from the output tubes away from the chassis. The chassis rear photo near the top of the page shows the rear duct cover removed, the picture just above includes the cover.

Printed Circuit Construction
The V100 was the first Vox tube amplifier constructed on a printed circuit board (PCB). Successfully designing a tube amp on a printed circuit board presents a number of engineering challenges.

Problems occur when too much current is drawn through a circuit trace. A circuit trace can be vaporized by excessive current draw. Something as common as a malfunctioning tube can lead to this problem. Evidence of such a failure can be seen on the V100 circuit board shown at right. The orange wire was installed to replace a burned trace.

The photo at left shows yet another pitfall in the V100 printed circuit board design. Rather than hanging the screen resistors for the output tubes on the tube sockets, Vox included them on the circuit board. Even in normal operation, screen resistors run hot and heat can cause a printed circuit board to warp. Should an output tube develop a short, the screen resistor is likely to smolder and burn, scorching the circuit board under the resistor (see photo at left).

Photo Credits
The Vox Showroom sincerely thanks Espen Bergen Hansen of E.B. Hansen Engineering of Norway for the generous contribution of V100 chassis images shown on this page.






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The VOX Showroom!


Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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