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EL84 Tube Amp Revisited

…and the next tube amp build on the horizon of completion.
I have been playing this amp quite a bit lately, and I keep checking the parts against my schematic. Today I noticed that the chassis wasn’t grounded, and the heaters were referenced to the output tube’s cathode voltage. I figured I would make a few changes.

I referenced the heaters to ground using two 180 ohm resistors. I also connected the ridiculous looking star ground to the chassis to keep someone from accidental death. I’d rather pop a fuse than pop a human being should something come loose in there.

This amp has a little hum to it, and I’ll check that out in Duncan’s power supply design program. Single ended amps need some stiff HT supplies to keep the 120Hz hum from moving the speaker. It won’t be anytime soon that I mess with that though.

I would like to point out that I dated the turret board “April 23, 2010.” I have learned some since then. One thing for sure is this: that thing is a wiring mess.


This is my latest project amp that hasn’t quite made its debut on I call this “Upcycle” because it is made from quite a few pieces of trash. A chassis and toroid power transformer from a Backline 250 bass amp, some capacitors from a desktop PC power supply, tube sockets and transformers from the original 6DG6GT tube amp that I recently redesigned, feet and handle from some amp that was scrapped, and pots left over from some other experiments. Some of the caps were also from the old build. Many parts came from my inventory (which isn’t much), and I purchased a few items new to fill in the gaps.

I believe that I have only spent about $120 on this amp. The speaker was $60; I bought a 12″ Jensen speaker. I wish I could afford a Weber.

I’m getting better at this. Once I get some paint of the cabinet and get my logo embroidered on the black burlap, I’ll put a design blog post explaining how I built this thing.
This is the dry fit of the chassis of “Upcycle” in the cabinet. This amp has a few more steps to go before it is complete.


This is the revised 6DG6GT amp. It is a single tube output. You can see my logo on the speaker cabinet. This amp has a Weber Blue Pup 8″ speaker.

Modifying VHT Special 6 Footswitch

Adding an LED Indicator

VHT Special 6 FootswitchI enjoy my VHT Special 6, and I can’t find many things that I would change about it. The one most annoying thing isn’t exactly related to the amp except that the footswitch that was sent with it is just a switch in a metal cone. Occasionally, I get caught trying to play a guitar solo without the boost enabled. I found that the switch of the original footswitch pedal is already modifiable to make it work with just a battery, an LED, a current limiting resistor, and a 1/4″ stereo audio jack.

VHT_Special_6_switchOriginally, I assumed that when the switch was closed the boost was enabled. I built a whole circuit and enclosed it in the enclosure. The 1/4″ stereo jack was used to connect the switch to the amp, and it also turned on the power supply which powered a transistor to invert the LED. Once assembled I found that I didn’t even need the transistor inverter. I already had the hole drilled for the jack so I kept it in. Here me out on this one…You could simply connect the switch to the original cable and opt not to install the jack, but if the button is pushed in this manner, the LED will illuminate. The stereo jack serves two purposes: it connects the switch to the amp, and it completes the negative path from the battery to ground to enable to LED to illuminate.

VHT_Special_6_original_switchThe original switch is a single pole dual throw (SPDT) switch with the ground soldered to the outside tab of the switch. The common is from the amp’s circuitry that, when grounded, disables the boost. In order for an LED indicator to work, the common connection has to be the ground. The image in the second paragraph shows this connection. I just desoldered the switch and removed it from the original footswitch housing because I chose to install it in a small box from Radio Shack. The enclosure is a 3″ x 2″ x 1″ plastic box. You might want to consider a larger box. It’s possible to fit it all in there, but it’s a tight squeeze, and the final layout is asymmetrical.

VHT_Special_6_modified_footpedal_schematicThe image to the left shows the schematic of how the new switch will work. When a mono guitar plug is inserted into the stereo jack, it completes the ground path for the battery. When the footswitch is pressed, it toggles between the amp circuit being grounded (boost off, LED off) and the LED being grounded (boost on, LED on). When the boost is off, the LED is not connected; It is left floating, which like like having an infinite sized resistor that completely block current flow through the resistor.

Bill of Materials (Radio Shack)

modified VHT Special 6 footswitchYou’ll want to do a dry layout before drilling holes in your box. I placed my 9v battery between the two screw posts on one end of the box. Then I laid out the jack, switch, and LED holder to make sure nothing touched. By following the wiring diagram, you’ll make each connection as shown. I used a bottom made of metal (not the original supplied one), so I hot glued the switch contacts. I also hot glued the battery wires down to ensure that they don’t get pulled out on accident. I drilled pilot holes using a 1/8″ drill bit, then I used a stepper bit and a whirly gig to take each hole to its final size. I decided to solder all the parts together after I installed them into the box.

This was an easy and cool project. It took about an hour to make the whole thing once I decided on the design. Once completed, you’ll never have to guess or audibly check to see if your boost is on. The LED will tell you!

VHT_Special_6_footpedal_wiringVHT Special 6 footswitch wiring diagram

Where did go?

I just received an email where someone was looking for Lumenlab CNC micRo config files. I had to check it out, and apparently Lumenlab has flown the coop. This is a bit disheartening to see.

Many months ago I logged into the forum to see many people asking where the owners of LL had been, and there were no replies from them with an answer. There were only suspicions and guesses as to where they could be. Had they ditched the production at that point?

I know that the micRo router existed because I have a version of it, but apparently there may be others that never received their finished CNC router. I’d really like to know what happened.

If anyone knows more to the story, I’d like you to comment. To be sure, I am clearly in the unknown on this situation other than does not display. I haven’t contacted Lumenlab in a long time since receiving my router. One thing is for sure, I am on my own with my router if something ever goes wrong.

Further reading: (to include comments)




Installing Eagle and PCB-GCODE in Ubuntu

This article explains the steps to install Eagle cad and PCB-GCODE to my computer. Installing Eagle is not very difficult.

  1. Go to Right click the link for Linux and select save link as. Save it to the Desktop.
  2. Once downloaded, you’ll follow the instructions from Cadsoft. Open up the terminal and navigate to /home/user-name/Desktop and execute the command “sh”. The program will begin.

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Review of the Low Voltage 12au7 Tube Headphone Amp

12au7 Headphone amplifier
This is the link that I found on Makezine’s website a few weeks ago. Of course, the actual link is on the page I linked to. It is definitely worth checking out. I pretty much had everything available with the exception of a 12au7 tube. I just happened to win an eBay auction with about eight 12au7 tubes. I paid $25 dollars for 14 tubes.
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