Finishing up Amp #2

The rear faceplate came this week. Special Thanks goes to BNP Lasers. They look awesome and I can't say enough about their customer service. I would recommend them to anyone building an amp. This is the front faceplate. It took me about 2 weeks to create. If you notice they do go to eleven.

Here is the rear faceplate.

You plug the Tremolo Foot switch which is a stomp switch into this jack. It turns the tremolo on and off.

The line-out circuitry is post Output Transformer. So it contains all of the great sound of this amp. The line-out is controlled by the line-out switch. With the switch in the middle or off position the line out circuity is completely removed from the output signal path. With the line-out switch up the line out circuitry is activated. In this position you must have a speaker connected. In this mode you can be playing through this amp and take the line out into the input on a PA system or a bigger power amp.

Finally with the switch down, it adds a 25 watt 8 ohm resistor to the output so that you can use the line-out to feed into the PA or power amp without a speaker connected to this amp. The resistor prevents damage to the output transformer since it provides the same load as a speaker would.

Below is a line-out level control that will vary the level of the line-out circuit.

Here is the Tube Based (12au7) Active Effects Loop. On the left is the Send level control. The circuit basically is two preamp(s). The 12au7 tube has two channels so one half of the tube controls the SEND and the other half of the tube controls the RETURN. The send level and return level controls allow you to use any pedals. You can adjust the SEND level so that the Input of the pedal is just right. You can also vary the level coming out of the pedal train so that the amp sees the correct level. I used a patch cord from SEND to RETURN and the circuit actually adds more amplification to the 18 watt that the normal gain stage. Thus you get even more distortion at Eleven!

The downside with so much functionality on the back panel is the lack of real-estate. I had to sacrifice a speaker output. To compensate I'll add additional jacks to the speaker cabs to allow them to be daisy chained together. The Attenuate switch in the middle position eliminated the attenuation circuitry from the output signal path. There are three positions, Quiet, Off, and Medium. The Quiet and Medium positions use the attenuation modules on the top of the amp to reduce the power level of the amp (loudness). I'm still creating all of the modules (8 in all).

See the previous posts for more info on the power levels.

The Tube based effects circuitry can be completely removed from the signal path using a Push/Pull switch on the RETURN level knob. Pushed in the effects loop is removed/disabled.

Pulled out the loop is activated.

Pulled out (ON)

Pushed in (Off)

Full rear view

Attenuation Modules almost complete.

I was able to get 4 attenuation module turret boards created tonight. I then started working on adding the Wire-wound power resistors to those boards. Here's a pic showing the two modules in place. They have to be outside of the amp because they will generate heat. The Module on the left is wired to act as the Medium Level. The one on the right is wired to acts as the quiet level.
They are not finished. I still need to add the wires and the fork terminals to the ends of the wires to make it easy to change them out. I have to call it quits early tonight, I'm traveling to Wilmington to a Local HamFest. I'm hoping to find some tubes there.

Attenuation Modules

Tonight was spent working on the built-in Speaker Attenuation circuitry. Not pictured is a 3 position 4 pole rotary switch. I only needed to use three of the available 4 poles. Here's a diagram that explains how the circuit works. The Switch will have 3 positions. Quiet, Normal/Loud, and Medium. I'm pointed out where I tap into the circuit for my 3 poles.



Here's a diagram that shows how the switch works. It has 3 possible positions. There are 4 wipers that connect the various points together. The Poles are in the center.

I added some stand-offs on the amp. The Amp will have interchangeable Attenuation Modules.
This way any number of power output combinations will be possible.

The Attenuation Modules will have these Power Outputs. This way for example you can configure the amp to have 18.0 watts for large venues, 2.3 watts for practice sessions, and 0.3 watts while playing at home. Later if you decide 0.3 watts is still too loud you can remove that module and replace it with a 0.1 watt module.

• 18.0 watts
• 9.0 watts
• 4.5 watts
• 2.3 watts
• 1.1 watts
• 0.6 watts
• 0.3 watts
• 0.2 watts
• 0.1 watts
  
  

The Quiet Module attach to the three left most terminals. The Medium Module will attach to the three right most terminals. I plan on building the modules tomorrow.

Added the Active Effects Loop Circuitry tonight.

Last night I was able to get a rough prototype of an Active Effects Loop to function properly. It took all day to do it. Initially it didn't work, all I kept hearing was a loud tone shifting noise when I turned on the effects loop. I had to step away from it, and went to dinner. Upon my return I tried a few other things and finally was able to get it to work. Today I spent all day adding the circuit to the amp.

Here's an inside shot showing the added daughterboard with the effects loop stuff on it. I also was able to squeeze in a tube socket for the added 12au7 tube.


Here's the back panel. It will basically be totally removed from the amp until needed.
You then pull the Send Knob out. This adds the effect loop circuitry to the amp.
With the Send and Return volume knobs, you can then vary the input to the effects and vary the output from the effects back into the amp. I tested using a patch cord and It actually boosted the preamp output a little more than stock. It also gave the amp a little bit more crunch at max volume which was an unexpected side benefit. I've very happy with the end result as I was reading how a passive (with out an extra tube) effects loop cause a significant reduction in output. With the addition of another preamp tube we get the benefits of an effects loop without any loss in headroom.

Here's the 3 12ax7 preamp tubes with the added 12au7 Active Effects loop tube.

Getting into NOS Tubes Big Time!

This past weekend I had to go to the coast to check on the boat. On the way back I stopped at a small town HAMFEST (HAM Radio Festival/swap meet). I scored some NOS (New Old Stock) tubes from a guy who picks them up at estate sales and from some friends. Another guy had these NOS Capacitors which will be for future amp projects.

I was able to get the tubes for $1 each and the rest for $2, total spent $10.00.

The tubes are (6) NOS American Blackplate RCA's and (2) Raytheon 12au7's. Not a bad use of a half hour of time out of my weekend.

Also during the HAMFEST I talked to another guy who runs a Salvage business. I takes in equipment from all over and salvages all of the parts from said equipment. He invited me to his warehouse where he said there are bins and bins of vacuum tubes to be had. By bin I'm referring to the 4' x 4' x 4' pallet bin variety. Oh Boy, it looks like my dream of treasure hunting off the coast of North Carolina might actually involve sifting through a bin of vacuum tubes on an upcoming Saturday. Who knew!

To go about selling any tubes that I find I've been reading up on how tubes were tested in the past. I found a great web site devoted to Tube Testers http://www.tone-lizard.com/Tube_Testers.html. After absorbing this information I was able to snag a nice Hickok 600A Mutual Conductance Tube Tester.

It also turns out there there is one of the foremost authorities on Tube Tester Repair an Calibration nearby in Greensboro NC. My Tester was recently calibrated there.

Here's nice a pic of a Hickok 600a that I found on the web, click to zoom in.

You basically look up your tube on the roll chart, it tells you what to set the dials at then you press some buttons and turn some knobs. This is right in line with my Geek tendencies so i think it will make a nice addition to my tool collection.

Today Raleigh had their HAMFEST. Which is the largest in Raleigh and only once a year. The knowledge of the vendors was a little better as they weren't giving the stuff away. But I was able to sift though the countless boxes of old tubes to SCORE some sweet tasty little gems.

From left to right. A Raytheon EZ81 NOS Rectifier. That one is super rare but I'll put that in my friends Amp #2. The white boxes are NOS GE Blackplate 12au7's that are in the most pristine condition. Like new. Followed by an assortment of NOS American pre-amp tubes. The right most are the gems I found in a bin of about 200 unboxed tubes. I found 3, yes 3 Mullards!

What a SCORE!

The guy's jaw about dropped as he didn't know they were in there. I was able to walk away with them for $18 each. The rest of the tubes were sub $10. I'll have that tube tester paid for in about a week at this rate.

Here are the 2 mullards, I forgot one was still in the bag (not pictured) and a nice backplate

pre-amp NOS tube to the left of them.

On the Amp front I was able to change out the Tremolo Pot on AMP #2 from a 2meg to a 3meg. Doing so gives it a wider range of tremolo speed to choose from. After lunch I plan on diving into seeing If my design for the Active Tube based Effects Loop is going to work with this type of amp architecture.

Amp Head covering complete.

Tonight I came home from work and brushed on the DAP low VOC contact cement. Here's a pic of the Tolex (Vinyl) with 2 coats of contact cement applied. I let it dry about 1.5 hours and then applied a 2nd coat to make sure I didn't miss any spots.

I found that the Tolex (Vinyl) dried about 1/2 as slow as the wood cabinet. So I ended up having 3 coats on the head cabinet and 2 on the Tolex. Here's the head hanging in my garage.
The next step I used some wax paper to lay under the amp head and also on top. I then put the Tolex on top on the wax paper. The wax paper kept the two surfaces separated, until I was ready. I added some tape and drew reference marks at the middle (lengthwise) of the Tolex sheet. This is so that the seam on the bottom of the amp was somewhat in the middle.

I then measured the overhang on each side, at the front of the amp. This was to make sure that it wasn't on crooked. I needed it to be straight because I only had about 1/2" overlap on the bottom seam, so I was cutting it close.

Here is the bottom of the amp. This is the easy part. You just overlay the Tolex and using a sharp knife and a straight edge I cut down the middle of the two pieces where they overlapped each other. (2 thick). After I finished the cut you pull of the 2 scraps and you are left with a perfect seam where both pieces lie flat (no overlap), and butted up against each other.

Here I'm securing the inside of the head. At the bottom of the picture is the top of the head. (upside down)

Here's a corner detail, of the back side of the amp head. I read about applying Tolex, they say after your 100'th amp you get the hang of it. I got the hang of it during this one, but the next one will be better because I realized where I could improve the process. Especially when cutting corners.

Here is the completed Amp Head Cabinet. This is a view from the front, I'll most likely have a wire mesh under that slot (vent).

Here's the top of the head

A view from the other side.

This is a view of the back of the Amp Head.

Finally a view from the back of the Amp Head. I set the front of the cabinet face down on my cutting mat. I envision that the amp face plate will rest against the amp face boards.

Started to put the Tolex on

Prior to sending my order off to BNP Laser Engraving. http://www.bnplasers.com I had printed out the faceplate on some transparency film to make sure that all of my holes would match up. I had to remove all of the components and pull them inside the chassis, to make sure these measurements were right on. I was also able to finish rewiring Amp #2 this weekend. I must say that I was starting to get worried. After re-wiring it it had a nasty hum (60 cycle). I tried adding some shielded wires and even rerouted some wiring. I was stumped. I then realized that you can't have my el-cheapo guitar right next to the amp while you are messing with it. My guitar's pickups were picking up the magnetic fields from the power transformer. Duh! A hard lesson to learn. As a result I'm really happy with the wiring on the inside. Moving away from the amp about 2 feet, it's about dead silent, no hum!


This week I've been reading all about how to apply Tolex to the cabinets. I even had some scrap vinyl laying around that I was practicing with. This helped and If I had started doing it, it wouldn't have turned out very good. Tonight I started with adding Tolex to the Vent slot in the top of the Amp Head Cabinet. My strategy is to lay down a thin strip around the slot as a base layer so that when I have to cut the top covering around the holes, no wood will be exposed around the slot.
Here's the Red Tolex, that I got from Mojo Music Supply. The rotary cutter makes it easy.

After reading about contact cement, I went with DAP low VOC. I've used the regular DAP stuff and I must say that stuff takes years off your life. It's very potent. This stuff (green can) is water based which also makes cleanup a lot easier. The downside is that it takes longer to dry. I found it took about 1.5 hours to dry. Next time I'm going to do two thinner coats to speed the drying process up a bit.

Here a top view of the vent slot in the top of the head cabinet.

Here's an inside pic of the Tolex that I'm adding to the slot as a base layer. Yesterday I spray painted the inside flat back.

Here's a top view of that strip, added to the slot.

And a shot from inside. This low VOC contact cement is a little easier to work with. It allows you to move the Tolex around a little instead of grabbing it and not letting go, like the regular contact cement would do.

I'm letting this dry overnight.