Board Layout - Tube Pin Connections

Worked on layout - drawing wire connections to be made from board to tube socket pins. Found a clever way to draw a smooth curved line in Photoshop (http://www.agavegroup.com/?p=68).

8:00PM-10:00PM (2HRS)

Board Component Overlay

Confirmed board layout before cleaning board/installing components. Created layout on PC to use a guide.

To do:

• Get extra 100k resistors (missing a few)
• Draw up wiring guide/confirm connections
• Decide on bias circuitry
• Clean up used tube sockets (remove old solder)
• Extend wiring for O/P XFMR

(2.5HRS)

Making the Solid State preamp stage PCBs

Pulled out the dremel, made room on the kitchen table, got the bits ready...

Tonight I drilled, tinned, and soldered in the components for the small solid state preamp section that is intended to go into the tube amp, as its own channel.

I designed and drew up the PCB layout myself in Protel a while back. The first revision I had (which was posted earlier in the blog) was incorrect. I had the pinout for the J201 junction field-effect transistor (jfet) wrong.

The PCB was made using a precoated, positive photoresist type blank board and a transparent layout of the circuit traces. The transparency was placed on the blank pcb and light was exposed onto the transparency. Any space on the PCB that had light hitting it would be a section of copper that would later be removed. Because the circuit traces were printed in black, no light hit the PCB at those places. After exposing the board, it was placed into a developing solution to remove the coating the resists the etchant later used to remove the unwanted copper. After developing the board it was placed into yet another solution of diluted hydrochloric acid, made from muriatic acid and hydrogen peroxide. I shall not go into the specifics for making this solution. I do not wish to be held responsible for someone burning themselves or others. Google it, if you're interested.

With the etched board it was time to drill the holes. I used a Dremel rotary tool sitting in a press specifically made for it, and some cheap Harbor Freight miniature drill bits. For most of the holes I used a 0.0256in drill bit, and bored out some to 0.0330in as needed. This setup is not perfect. I noticed some heavy pivoting action going on at the drill bit, probably because of the weak tolerances of every part I am using, from the Dremel to the aforementioned "cheap" Harbor Freight bits.

After drilling and tinning the pcb, I began populating the board. Everything went smoothly. The time I had spent so tediously measuring the component leg spacing paid off. I'll have to admit as well, the photography is not half bad...

Here are some pics of the process, beginning from the drilling stage:

A bit of acetone to remove the resist coating of the pcb. And a bit of magic tinning solution. This stuff is awesome!

7:40PM - 11:30PM (3.83HRS)

Solid State Gain

Another thought - I might need an extra gain stage between the output of the solid state preamp and the phase inverter. I plugged a "pedal" version of the anticipated preamp into a tube power amp and noticed somewhat low volume, as well as noise, showing up as hum on the output. Perhaps some kind of impedance matching is necessary. I will consider this while preparing the amplifier, and hopefully be ready to implement something if necessary.

(0.5HRS)

Mounting turrets

Today I went to school and used the heavy duty drill press they have in the lab and drilled all the holes (1/8" diameter) for the turrets, and holes for wire routing. The fiberglass board that I drilled into was quite nice to work with, albeit a bit of a dust maker. **Always wear eye and respiratory protection when cutting fiberglass** I think I was expecting similar results to PCB material, but then I believe that those are more densely packed fiberglass and a different type of epoxy to hold it all together.

I covered the board with tape and marked lines for the cutouts, center-punched the soon-to-be holes and drilled away.

Once the holes were drilled I removed the tape and cleaned up the board. Now came the task of mounting the turrets. I was missing the bit that sits in the press and pushes the top of the turret against the board, while the conical section flares the bottom. I had a standoff that I bored out with a 5/32" bit. That fit the turret head pretty good and I did the same thing to a longer metal hex standoff and pressed in the turrets.

 

Next step - mounting the components. I'll have to find some chassis mount terminal strips to mount the other components (diode rectifiers, bias circuit, etc...)

3 HRS CLOCKED - 1 HR @ HOME

Building the boards, PT 1

Today I began the task of building the turret populated boards that will hold the amplifier circuitry. I have decided to omit the relay-switching of channels that I had planned and opt for the simpler "always-on" mixing circuit that the original Fenders had. Using this method I can still have reverb on the solid-state channel by jumping out of 1 of the unused inputs into the tube Fender channel, and be able to mix it in as necessary.

Turrets - what are they?

They are used as junction points for circuit connections. The device is installed onto a board - fiberglass, in this case, by a special turret staking tool. Once installed, components can be soldered directly to the turret, thus making an electrical connection. It can be considered one method of point-to-point wiring.

In the old Fender amps, they used solder "eyelets" which were essentially contact points like the turrets, but had a profile that was nearly even with the board level.

I chose turrets because I have some, and because a mechanical connection can also be achieved with them, strengthening the connection of the components.

I began the design of the turret board layout by using a CAD program called CADStd. The designer of the software offers a freeware version of the program than is very user friendly and has a quick learning curve.

Using CADStd, I drew up several of the components I had that would be going into the build of the tube amp - then after drawing them up, stopped. It became clear to me that things would go much quicker if I simply worked with the actual components, on the actual fiberglass board, and draw directly on the board.

This decision to head away from the computer was a good one. I took my schematics and layouts and laid everything I had out on the kitchen table and began setting things up. Within a few hours I had completed the layout for the turret board.

Now that I have this done, I need to drill the board (including holes for wires), install the turrets and begin populating the board.

I also have a self-etched PCB of the solid state preamp that needs preparation.

Regarding the turret staking tools, I may have to build the part that presses against the top of the turret, as I do not have this part, and none of the Keystone staking tools do not seem to match the size of the turrets that I have.

2:50PM-7PM; 11PM-12AM (5.17 HRS)

Back on the project. Parts ordering.

Probably the most boring part of the project, but someone's got to do it. I have a method of compromise between price, quality, and stocked items from a distributor. I've gotten pretty good at getting what I believe to be the lowest price for parts (at this quantity level).

The hard part is trying to remember if I need any parts for other projects. Now's the time to buy, so I can save on shipping.

Another somewhat time consuming part is finding parts that fit dimensionally within the scope of my project. If you search for 22uF on Mouser's site, you see that there are literally thousands of 22uF capacitor types that can be purchased. So it's a game of switch back and forth between schematic, website, price guide, dimensions, and repeat.

It's really worth it to me in the end since I want to spend the least amount of money on parts (while getting a quality product). Doesn't everyone want this?

10PM-12AM (2 HRS)