Here's the 411:
In a bit of downtime thinking about a new amp design request by a customer and musician friend of mine, I had an idea to create a sort of prototype which would satisfy several things: to build another amp for myself (been a while since lately building for others) while testing an idea that will be designed into a Hot Rod Deluxe after I gut that circuit out. This will be similar to what I've done here on a smaller scale. I had a Vox Pathfinder 15 which served as the donor cab, chassis, and speaker. I got it for about $60 delivered in need of repair. I fixed it and sold the working transformer, board, and pots for appx $30. I also had a Hammond AO-44 that reportedly didn't work right for about the same $60 with tubes. This provided the iron, tubes, and some sockets. I had all the basics for under $100.
I wanted this one to be a simple, clean, and good sounding amp & circuit and only adding single tube due to the space available and not overloading the Hammond's PT. I wanted to once again use an EF86 for it's great tone and excellent range and versatility operating from clean to overdrive.
The name: well since there's only 4 tubes total and three of them have an "86" on them...and one of my favorite TV shows as a kid that still makes me laugh to this day - Get Smart. The main character is Maxwell Smart code named Agent 86 (for anyone not familiar?)
I spent a number of hours on the tone stack program coming up with something there that has a similar notch or V-shape and in the same frequency range as Fender & Vox's - yet it has a better range and not as much of a loading effect (to help with the EF86 being loaded as much). That's the idea anyway and it worked out as intended. Another idea came for the NFB switching via a lot of almost 20K vintage SPST push-pull pots that I've had for a while. They were originally wanted to be used for a mid control and to operate a tone stack lift but after testing they are normally open pull to close. I could still use them in this way but it would be the opposite of how I normally do it. So for this build it then introduces NFB when pulled for a range of different tones and feel plus higher headroom if wanted. For economy and space saving I went with a cathodyne pi and finally wanted a larmar type ppimv.
There was very little tweaking when it was up and running. This involved removing a bright cap I thought it would need using 1M vol pots early in the circuit. I lowered the gain of the pentode since there was plenty of tap anyways. I lowered the pi anode and cathode load resistors to lower the output swing since ECL86s are rather sensitive like EL84s it turns out (I've not used these before), then at full throttle there was bit of unbalance that increasing their grid stoppers resolved with no loss of high end due to the cathodyne's low Miller capacitance.
I don't put very many of my amp designs on here mainly because there's no ground breaking circuit ideas here, no reinventing of the wheel as we say. I mainly like to color outside the lines creating whatever I can think up and tweaking things to become better than they are (same with my effects pedal line as it evolves too). I am humbled by the generosity of sluckey and tubenit (geezer too going way back) as to how much they share of their circuit ideas along with dummyload too. A big thanks to PRR and HotBluePlates sharing their great wisdom! Shout outs to Willabe, Mike S, Ed C, SG, Jimbob, S&P, PT, Chip, Josh, and foreign friends tubeswell, Timbo, Kagliostro, etc... and last but certainly not the least DOUG! Sorry I can't name everyone but it can't be said too much - thanks again all!
Topic: Agent 86 build (Read 10404 times)