5842 (417A) "Spud" Amp
This amp is based on the designs of Lucas Cant of Black Art Audio in Victoria, Australia. Starting on the next page are his comments and schematics, taken from his website (with his permission), which is located at:
I attended both prior NY Noise events, and came away both times very inspired by what I saw. The depth of innovation and knowledge of the exhibitors is very impressive, and a bit intimidating to someone as inexperienced and untrained as me. With each aspect of this little amp that I experiment with, I learn a lot - much more than I do by just reading of the diy
exploits of others. My purpose in exhibiting this amp is not to impress anyone, or even to teach anyone anything. Rather, I hope that it will inspire or encourage others to get off the fence and start building.
As an inexperienced Joelist lurker, I started with kits from Bottlehead, and lots of reading and questioning. Finally, I saw the spud amp schematic, and got brave enough to try to breadboard the amp. I used some Allen Bradley carbon composition resistors I had and some other resistors and caps I got from Radio Shack. Based on the admonitions from the Joelist and other sources, I carefully applied 100 ohm resistors to each of the four grid pins on each tube. (These are from Radio Shack too.) I used a pair of Tango U808 output transformers, set up for a 5Kohm primary.
Due to fear and ignorance, I chose to avoid trying to put together a power supply, and instead used a Hickock tube lab power supply I bought on eBay. When I hooked up the 6.3 volt filament supply, I got too high a voltage. I had the same problem with a separate filament transformer I had, so I put some big resistors (from Radio Shack, of course) in line until I got in the right area. Remnants of this effort have been left for discovery by future audio archeologists.
At some point, I spotted the batteries used in my daughter's Barbie Jeep, one of those little Power Wheels vehicles for preschoolers. I charged one up, slapped it in there, and found it read exactly 6.3 volts! It also eliminated the filament hum, so I kept it. It seems to last a long time without discharging, so it's not so bad that I have to disconnect it and hook it up to the Barbie charger.
Next, while visiting the local boom-car stereo and electronics parts emporium, I finally found the type of 2 volt lead acid batteries that Lucas mentions on his website. Although they had the "D" size version, they also had bigger ones, and so I figured more is better, and I took the bigger ones. I hooked them up with some clipleads, and was amazed that the amp still worked! This has temporarily exorcised my desire for expensive cathode bypass caps, but I guess at some point I'd like to compare both bias configurations.
Of course the next step is a power supply. As I type this I've recently experimented with using a 115V isolation transformer and a diode bridge, but I must have miswired it, so it needs more work. I also will try something along the lines of Lucas' schematic with a higher voltage transformer I have on hand and a
Variac, but my knowledge of my ignorance makes me want to take it very slow.
Ultimately, my goal is to cobble together a power supply, try some better quality parts, and then duplicate the whole thing in a package that looks like an amplifier, can fit on a shelf, and is not a danger to family members. When I do so, this breadboard will likely remain as a first stage of an amp using other triodes as output tubes. Better look for some more plywood!