There’s not a lot going on this week. I’m finishing up the second tube amp build. This evening, and probably at lunch, too, I’m going to finish the V1.3 changes. I intermittently changed some things that were not noted in the schematics prior to V1.3, and I need to change them back. I am going to make a video or two to post to Youtube to demonstrate the amp. Nothing beats seeing. Seeing is believing. I want to install a switch to change from pentode to triode mode like the AX84 project did, but I’m still debating whether or not it’s worth doing. I’ll probably do it, and if it doesn’t sound different, I’ll just be stuck with it.
I also had to add a DC cooling fan to blow in the cabinet because the LM317 cases get fiery hot. The temperature of the heat sink was over 180° F after an hour of playing. The chassis was around 150° F. That’s just too hot. The EL84 runs at about 350° F, but I’m pretty sure that temperature is acceptable for a power tube. I suspended a fan to blow on the heat sink to see how much noise it imposes on the amp, and there is none. I also checked to see how direct the air needs to be to keep it under 125° F. Direct it stays under 100° F, but I’d wind up sticking my hand in the fan every time I went to turn it off. I made a makeshift bracket from a scrap piece of 1/8″ aluminum sheet. I put a piece of vinyl to keep the fan from rattling on the bracket. A picture of this is certainly worth a thousand words.
BTW…I played the amp this morning at 7, and it sounds much better now that I changed the first preamp stage’s cathode bypass resistor to 2.7k. This makes me happy.
I spoke with Grayson Sigler about the micRo CNC router delivery, and they were running a little behind due to a family illness. I expect within another week or so I’ll be pulling hair out learning a new skill. I’m glad that Grayson and Lumenlab are happy to help. I have also sought help from Pcb-gcode.org and probably others. I want to gain as much knowledge and experience so I may also be of help in the future. You can expect that the CNC routing experiences will be heavily blogged and video logged.
Other than waiting for amp parts to arrive, everything is calm on the tech front.
After a little research I’ve discovered that most effects pedals have a negative tip configuration. The first few cigar box amps that I built and sold with a power supply jack were positive tip. I’ve found that a positive tipped power supply is much harder to come across. Continue reading →
This is a quick post to update any followers of the site. I spent the beginning of the week installing laminate floors in my house. I neglected most of my college course work, and now I’m trying to catch up.
I’m working on a 4 speaker Cigar Box Amp. It’ll make it to eBay sometime next week. It’s a working prototype. The electronics are basically the same except that I’m adding an external power jack, a hard battery clip, and a front cover that may or may not look good. I covered most of the front with fabric to cover the speaker holes. I’ll have pictures up next week in the Cigar Box Photo album. I’m also removing the power switch and using a stereo input jack as the switch, and I’ve added an LED to signify that the power is on.
The suitcase amp is still under construction. I do not like version 1.1, and I’m probably going to scrap the 2 watt version for now. It has a horrible “hummmm” when used with wall-warts, and I probably need to add some smoothing capabilities to the incoming power. I am going to test it out with the LM386 circuit and see how that sounds.
That’s about it for Tech-tut.com’s latest updates. I haven’t said this in a while but remember that “it’s more than just a project.”
<August 6, 2010> This power supply configuration should only be used if you can find a positive tip power supply. Negative tip is more common in the music scene (effects pedals). General Guitar Gadgets sells a negative tipped power supply for $11. Check out this post for more info…Adding a power supply to a Cigar Box Amp (REVISED) Continue reading →
This is a quick demonstration of how to mod a 1k ohm potentiometer to be able to work in a region between 0 and 25-30 ohms. The Make schematic for the cigar box amp calls for a 25 ohm rheostat, which is either hard to find or much more costly than it needs to be.
The amplifier circuit from the pages of Make is a very simple one, but I had some issues finding the 25 ohm rheostat that the schematic calls for. This is the only modification to the circuit that I made. I used a 1k ohm pot for the volume, but I modified it with a 33 ohm resistor to get it as close to the schematic as I wanted to. The volume doesn’t completely cut out, but who wants to use an amp at zero volume anyways?
modified amp schematic
The trick to this mod was easy. I just used the parallel resistance formula where total resistance equals the inverse of the sum of the inverse resistances (or the inverse of the sum of conductances). Either way, I know that when the volume pot is at 0 ohms, most of the current will flow through the pot as if it were just a wire. If the pot were to go to infinite resistance, the current path would be through the shunt resistor.
I used my graphing calculator to graph y = 1\((1/1000)+(1/x)). Then I traced the line to find the Y-value (my shunt resistor). I chose the point where x = 25. Y was so close to 25 that I just rounded up. Well, I didn’t have a pile of 25 ohm resistors so I used a 33 ohm resistor.
The volume pot now works a little better. It’s not perfect, but it provides a better range than just a 1k pot.
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