VoCo Variable Voltage Control
This kit is an adjustable voltage control (up to 600V) which can fulfill many tasks. It is largely based on posts in the TT Forum, but with a few small adjustments and fine-tuning. It is built on a PCB for compact, safe, and simple usage.
As a G2 (screen) control
The output of the power tubes can be controlled by the voltage on the screen (G2). For this purpose, the TT-VoCo is insterted in the screen power supply.
G2 control is shown in red in the TT66 schematic below. The TT-VoCo can be fitted to other amps in a similar manner.
It is important to note that the TT-VoCo can be used for G2 control only in amplifiers in which the power tubes are cathode-biased, so that the grid voltage is automatically adjusted.
As voltage control for the preamp tube(s)
In some applications (e.g., harp amps), low plate voltage for the preamp tubes is desired; however, most amplifiers work with high voltages here. The schematic shows (in blue) a method of reducing the plate voltage for the preamp tubes. Fitting the VoCo to other amps is similar. In amps with more than one preamp tube, the VoCo can be used to reduce the voltage to any or all the tubes.
Construction
The heart of the VoCo is a BUZ90 (or equivalent). This power transistor has an I(Drain) greater than 4A and a VDS voltage of 600V – it can handle quite a bit of power.
In the examples above, only a few mA current flow. With such low current, the BUZ90 can be mounted directly on the PCB and cooled with the heat sink provided (Fig. 1). If more heat has to be dissipated; for example, if the plate voltage of the power tubes is to be regulated, then the BUZ90 could get quite hot, and the small heat sink
is insufficient. In this case, the BUZ90 can be mounted directly on the chassis (Fig. 2) to provide sufficient cooling.
When mounting the BUZ90 directly to the chassis, IT IS VERY IMPORTANT that it is ISOLATED from the chassis. A mica insulator and a nylon screw are included in the kit. The BUZ90 MAY NOT contact the chassis.
Our tests show that the most effective and sensible usage is G2 control, due to the low effort required for construction and assembly.
Security
Tube amps contain life-threatening voltage! Only people with
sufficient knowledge and experience should fit the VoCo to an
amp. Anything else is grossly negligent!
NO further documentation is included with the kit. This documentation is sufficient. The position of the components is marked on the PCB, as are the connections. If, after reading this documentation, it is not clear how the VoCo is installed and used, it indicates a level of knowledge insufficient to safely and correctly install the VoCo in an amplifier. In this case, we strongly recommend letting an experienced technician do it.
