This photo journal will cover the installation of a “Top Boost” module into a vintage Vox AC30. The Top Boost design modification was introduced in about 1961 and for the first year or two of it’s existence, it was added on to existing AC30s as a separate module mounted inside the back of the amplifier. A rectangular cutout was then routed into the back panel in order to provide access to the new bass and treble controls. Later on, the new circuitry was incorporated into the tag board design and the new controls were integrated into the control panel itself, thus eliminating the need to cut into the back panel.
One of my customers recently brought in an early 60s Vox non top boost amp, and he’d also purchased the required module to convert it into a Top Boost model from these good people:
Vintage Pedal Workshop » AC30 Top Boost Unit
Before we get into the actual install, let’s first take a closer look at just what the Top Boost module is and what it does. Here’s the schematic:
Basically, what we have here is a standard full-range gain stage into a cathode follower circuit that then leads to the tone stack. The cathode follower changes the high impedance signal into a lower impedance signal that is strong enough to drive the tone stack with minimal signal degradation. This is why the cathode follower is used here. The tone stack itself is a bit odd in construction as you can see, and rumor has it that the Vox designers simply stole this circuit intact from a Gibson amp design. The bass and treble controls are EXTREMELY interactive. This is to say that adjusting the “treble” control affects the sweep of the “bass” control and vice versa. You’ve really got to play with the controls a bit to get the hang of how they operate, but they do sound great together!
To install this new circuit, we need to:
1. Physically mount the bottom of the module to the existing bolt underneath the center of the chassis between said chassis and the slider board.
2. As the module contains a tube, we need to connect it’s heater into the existing heater string.
3. Connect the module’s B+ into the existing HT rail at the junction of voltage dropping resistors R10 and R11.
4. Connect the module’s ground wire to the grounding tab next to V1.
5. Insert the module into the bright channel’s signal path between the wiper of the volume pot and R9, which carries the final signal to the phase inverter.
6. Add a 100pf “Bright Cap” to the aforementioned volume pot.
7. Drill a hole into the chassis for the module’s support screw.
Here’s a schematic I’ve doctored showing all of these insertion points:
Here’s a picture looking at the back of the amp that shows all of the insertion points for the grounding wire, the B+ wire, and where the module’s heater wires will be attached.
Now, let’s do it!
First, I soldered the B+ wire and the heaters to their respective terminals. Note that the heater wires on the module are blue and yellow, which match the color code of the amp’s existing heater string. Thus we simply solder the blue wire to pins 4/5 on V2, and the yellow wire goes to pin 9. Here’s what this step looks like completed:
Note that I’ve dressed the heater wires out and away from the tag board in order to keep induced noise from the heater string to a minimum.
Now we can attach the black ground wire to the aforementioned grounding tab at V1. I ran this wire UNDER the tag board for cleanliness. You can see how it turned out here (note that there are now TWO black wires attached to the bottom solder lug underneath the nut):
Now that the module has it’s B+, grounding and heater wiring straightened out, we can jump it into the signal path. As this module is inserted between R9 and the volume pot wiper, first we unsolder R9 from the brilliant channel volume pot. Here’s the pot pre-mod – it’s the one on the LEFT in this picture:
In the following photo, I’ve connected the “hot” of the shielded cable going to the module’s pin 2 input grid to the volume pot wiper, and the cable’s ground connects to the pots’ grounding buss, as you can clearly see here:
Also note that I’ve also installed the silver mica 100pf bright cap across the two “hot” terminals of the pot. Actually, the above picture is of the fully completed mod (output wire to R9 is on the right), but here's how I did it:
To connect the other shielded cable from the module’s “treble” pot directly to R9, I stripped away the end’s shield and covered it with heat shrink tubing for cleanliness and to quell any accidental shorts due to exposed shielding wires. Also note that I’ve slid the final piece of shrink wrap over R9’s lead to insulate the final joint in the next step:
Finally, here’s the joint with it’s shrink wrap fastened:
Now, we simply drill (and counter-sink) the hole for the top mounting screw and attach it. Here’s the completed modification:
Normally, the next step would be to rout out the rectangular hole in the back panel for control access. However, since this is a vintage amp we didn’t want to do any permanent alterations outside of drilling the one tiny hole for the top screw (which is covered by the back panel anyway). This client has ordered a reproduction cabinet and back panel in a vintage correct “fawn” color, and it already has the cutout in the back panel done for us. This way, we can keep the original cabinet fully intact and at the same time preserve it from further wear and tear. He’s brining it with him when he picks up the amp, and I'll post pics once I get the amplifier fully re-assembled.
This amp sounds AMAZING, and the addition of this module really made it’s “Brilliant” channel sing. I hope this has helped you understand both what’s going on in the circuit as well as how to physically mount the module. As you can see, it’s not TOO hard, and the amp really roars now.
Cheers, folks, and as usual feel free to post if you have any questions, comments or rants!
P.S. - Here's a GREAT reference link for all of you Voxophiles out there!