Plexi; yeah there are A TON of different sounds you can squeeze out of that preamp. It's totally not user friendly, it's too complicated and not practical at all, but if you like messing with knobs and getting different sounds, it's FUN!!!
Silverfox. The biggest differences are in the -V grid bias supply. On the drawing it has a 10K on either side of the bias pot. That didn't get me anywhere near -51VDC for 6L6, so I had to change them. I can't remember exactly but it might be 5K on the diode side and 30K on the ground side of the bias pot. Now I can go far enough to get bias for El34 or 6L6. For a dual purpose el34 or 6L6 amp pins 1 and 8 need to be joined together on the tube sockets (not on the drawing). It is an internal connection for a 6L6 tube but not on an EL34. if you wire 1 and 8 on the sockets together, all you need to do is change the -VDC grid bias for each tube type. (I can't comment on a cathode bias setup, that would be different).
If I were to set the bias for 6L6 and measure the R, then install EL34's and set the bias for that and measure the R of the bias circuit, I could eliminate the bias pot and put a switch in there, switching different bias resistors, for quick bias setting, but taking the amp out of the rack and taking the lid off and yadda yadda yadda to change tubes, I might as well take the time to adjust the bias pot, ha ha ha.
Also not shown on the poweramp drawing is the cooling fans. There are 2- 12VDC brushless computer fans In there. The one on the back pulls air in, and the one on the front pushes air out. I tore apart a wall wart and zip-tied it inside the chassis. it is switched on/off by the main power switch. The wall wart powers the fans. Easy!
other than that, the ??? cathode resistor of the phase inverter is 1K, and across the 82K phase inverter anode resistor I installed a 180pF cap.
The presence control pretty much stays turned down all the time (full NFB), it gets way to nasal and too much high frequencies with presence turned up.
I was running a QSC 500 Watt poweramp before this was done. I had the "500 Watt" amp cranked all the way up and still didn't really have enough volume with the band. Now, with my "100 Watt" 6L6 poweramp, it is turned up just a little bit under half way!!!
I think I got lucky with the poweramp. I built several incarnations of preamps before I got them sounding even half way good. After the green preamp I moved on to the poweramp and it sounded great once I got the bias figured out. It didn't need much tweaking after the initial "design" and build.
This particular power transformer is a bit hotter than I expected. It is the Weber transformer for "marshall 100 Watt" amps. I have 512 VDC before the choke, and then down from there. The numbers shown on my drawing were pre-build guesses on what I was going to get. The OT is a single 4 Ohm output from Weber.
Topic: Preamp and Poweramp up and running! (Read 2178 times)