Table of Contents
- Getting Started
- Tube Sockets
- Final Assembly
- Setting the Bias
Some of the semiconductors used on this board require heat sinks for proper operation. Do not attempt to operate the board without the heat sinks in place. This will cause component damage. The heat sinks on the mosfets and CCS IC’s have the full B+ voltage on them DO NOT TOUCH THEM to see if they are hot when the amp is on! The tabs on the Hex FREDs are also carrying high voltage. This is one of the many reasons that the board should not be operated without a protective enclosure.
The photo above shows the semiconductors used on this board. From left to right, generic 3 amp diodes, Hex FREDs, constant current source IC’s, 2SK2700 mosfets. Lower right is the filament regulator. Upper right is the Schottky rectifier for the filaments.
The mosfets (Q1 and Q2) and the CCS IC’s (U2 and U3) can use the small anodized aluminum heat sinks that fit in the holes on the board. These are adequate up to 500 volts with reasonable ventilation, and they are OK in a closed box (the Lexan case shown on the Start Page) at voltages up to 350 volts. Most users can use the specified heat sinks for these parts. One of the heat sinks is shown below. The assembly instructions assume that this is the case.
The filament regulator, U1 (left) and the filament rectifier , D1 (right) also require a heat sink. The size of this heat sink is determined by the intended application, and the available ventilation. The situation is complicated by the fact that the components must be electrically insulated from the heat sink. The small anodized heat sink that fits into the holes on the board is usually adequate for 45 tubes using the small Allied transformer in a housing with good ventilation. For any other application a larger heat sink will be required. For these applications the small heat sink is still used as a support bracket to mount a larger heat sink.
In applications that use an aluminum chassis or mounting plate, an aluminum “L” bracket can be used to transfer the heat to the chassis. Since all of these cases use the basic heat sink (shown above) as a bracket, these instructions cover the installation of the basic heat sink. Some users may have their own mounting requirements for these components. In those cases it may be wise to leave them out at this time, and mount them during final assembly. Understand that this will make stand alone board testing impossible.
The Lexan amp, which has poor ventilation, required a fairly large heat sink that was removed from a Pentium chip out of an old computer. A spacer was fabricated from a piece of 1/8″ aluminum. Since the regulator IC dissipates most of the heat, it is the only part actually attached to the large heat sink. The heat from the rectifier is transferred through the small seat sink. This amp has been used with 45’s, 2A3’s and 300B’s. It has virtually no ventilation (there is a Lexan cover which is removed in this photo), and plays for hours without issue. This is probably the worst case that you would encounter, so this is the largest heat sink that you would ever need. I used it because it was free. If you have a heat sink similar to this, and the space to use it, by all means do so. The cooler these parts run, the longer they will last. 2A3’s seem to create the most heat since they pull 2.5 amps each.
If you look carefully you can see the Mylar insulators between the semiconductors and the heat sinks. There is a nylon shoulder washer on the rectifier since the screw passes through a metal tab.
This is the heat sink used in my 845SE amplifier. Here no spacer is needed since the heat sink sits on the edge of the board. There is no screw in the rectifier since the rectifier is not used in this application. The filament regulator is supplied from the external power supply. The rectifier is present because I build all of my boards as stand alone amplifiers, so I can test them completely, even if they are used as driver boards. This way I know that I have a working board. You may notice that this board is not green. This is a prototype board that I made myself, and it has no resist layer.
Installing the Semiconductors Using the Basic Heat Sinks
U1 and D1 will be mounted to the heat sink and then the assembly will be inserted into the board and soldered.
These are the components that you will use in this operation.
Begin by orienting the heat sink as shown. The solder tabs should be facing down. Place the Mylar insulators on the heat sinks as shown.
Place a #4 screw through the hole in the right side of the heat sink from behind. The screw should pass through the hole in the Mylar insulator. Then place the regulator IC over this screw. Thread a #4 lock nut on this screw. Lightly tighten the nut, but do not snug it. You should be able to move the chip from side to side.
Place the screw, and the rectifier diode on the left side in the same manner as before. Next insert the shoulder washer through the rectifier. Gently work the washer through the hole in the insulator, and the hole in the heat sink. When all of these line up the rectifier will fit flush against the insulator, which is flush against the heat sink. Finally thread the lock nut on the screw. Gently tighten the nut such that all of the slack is taken up and the washer will not pop out of its hole. You should still be able to move the rectifier.
The completed assembly should look like this.
Insert the assembly into the holes in the board. This will take a little manipulation. Make sure that all of the leads are in their proper holes, and none are bent. Note that the silk screen letters for D1 did not make it onto the board.
The leads for the regulator chip barely come through the holes in the board. This is normal.
Solder and clip the leads. Solder the heat sink mounting tabs. Now you should tighten the screws. They should be tight enough that the parts don’t move, but not tight enough to crack them when they get hot.
Next we will insert D2 and D3. The silk screen lettering may hard to read for these parts on some boards. Make sure that you observe their polarity.
Flip the board over onto the work surface. A book is useful here. You can position the board such that the heat sink is off the edge of the book and the diodes are holding up the board. Solder the diodes, and clip their leads.
When done your board should look like this.
Insert, solder and clip D4 and D5. They install like resistors, except that you must observe polarity. The banded end goes toward the edge of the board.
These are the components used on the last two heat sinks.
Assemble these in the same manner as before, except that there are no insulators, and no shoulder washer. One heat sink gets both 2SK2700 mosfets. The other gets both of the CCS IC’s.
Temporarily insert the Q1-Q2 (mosfets) assembly into its place in the board. This will assure proper alignment as you tighten the nuts. Tighten the nuts using a screwdriver and needle nosed pliers or a nut driver. Remove the assembly after you are finished. It will be in the way for the next step.
Insert the U2-U3 (CCS IC’s) assembly into its place in the board. This will assure proper alignment as you tighten the nuts. Tighten the nuts using a screwdriver and needle nosed pliers or a nut driver. Reinsert the Q1-Q2 (mosfets) assembly into its place in the board.
Now flip the board over and solder both of the assemblies in place. Clip the excess leads. Solder the heat sink mounting tabs.
The heat sinks should look like this when finished.
Your board should now look like this. If it does, proceed to the capacitors page.