Building the VCB variable voltage kit
barry (Administrator) 10-06-2015 at 02:41 AM.
Post: #1
IMPORTANT NOTE: Before unpacking the VCB kit, you must be aware that the MOSFET semiconductor has special handling requirements. You could accidentally destroy it with static electricity. Whereas all components in a conventional valve amplifier are able to withstand electrostatic shocks, the internals of the MOSFET cannot. This means that you should not handle it directly until you're ready to use it, and when you do handle it you MUST take the precautions mentioned below.

Kit contents
Check your package for the following items:
* Q1 - MOSFET (in its own bag)
* Q1p - insulating pad for MOSFET
* D1 - 12.1V 1W zener diode
* R1 - 100k/0.5W resistor
* R2 - 100k/0.5W resistor
* R3 - 10r/5W power resistor
* VR1 - 1M linear pot
* VR1k - control knob for pot
* VCB1 - printed circuit board
* 0.5m of each of red, black and pink wire

If there's anything missing, get in touch with me as soon as possible.

   
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barry (Administrator) 10-06-2015 at 02:48 AM.
Post: #2
Adding resistors and diode
In general, it's easiest to start by soldering the smaller components to the VCB1 board. The smallest is the zener diode, which fits more or less in the centre of the board. Check it's orientation - the end with the darker band is indicated on the white print on the board. Trim and solder the diode's leads.

   

Now add the three resistors. It's hard to mix these up, as the larger power resistor (R3) doesn't fit the other two positions.

   

Note that when you mount the R3 it's a good idea to leave a little space between the bottom of the resistor and the PCB. This allows some air to circulate and helps avoid any heat building up. (PCB longevity is better.)

   
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barry (Administrator) 10-06-2015 at 02:51 AM.
Post: #3
Adding the pot
The pot that controls the VCB's output voltage is VR1. There are five holes in the board for this component - the pot supplied with your kit will fit three of them. (The other holes are to allow for an alternative pot, in case of supply shortages. So you can ignore the two unused holes.)

   
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barry (Administrator) 10-06-2015 at 03:02 AM.
Post: #4
Add the wires
The VCB kit comes with plenty of wire to connect to your amplifier circuit. For convenience all three wiring points are brought out on the same side of the board. In most cases you will bundle/twist these together and run them over to the power supply part of the amplifier.

Connections are:
1) Black wire for Ground
2) Red wire for B+ in
3) Pink wire for B+ out

   
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barry (Administrator) 10-06-2015 at 03:16 AM.
Post: #5
Soldering the MOSFET
The MOSFET handling is covered above. Basically, we want to handle it was little as possible in order to provide as little risk of damaging it with any static charge we build up. Of course, you do have to handle it to:
1) solder it to the VCB
2) stick the SILPAD insulator to the tab of the MOSFET

So bear the above in mind and take proper care. When I'm working I always have a grounded amplifier chassis (switched off, but plugged into my workbench power supply) close by. Before I handle the MOSFET, I make sure to touch that chassis.

You need to experiement with the pot-board-MOSFET mounting before deciding how and where to bend the MOSFET pins. Use needle-nose pliers to bend the pins and align them with the holes in the VCB (the three holes labelled Q1).

In this example, I've put the MOSFET at a right-angle so that the MOSFET back will mount on the bottom surface of the chassis, and the VCB board is vertical. Depending on the space available inside your chassis, you may be able to have the board parallel to the chassis.

   

In any case, make sure that none of the PCB's solder points will short against the chassis surface or a nearby component.
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