Was your 20 watt Plexi build based on EL34s or 6v6s? Curious how you were able to reduce the wattage. At low volume settings with a PPIMV, do you hear a difference?
It may be worth noting that a JTM45 has far more negative feedback than a 1959.
My 20 watt Plexi is based on a 6V6GT output - it resembles something like a Fender Deluxe reverb output section. I went by "Sluckey"s plan on his site. He has closed his site but the contents are still available online with a search. After nearly a year of having this amp,, I used the amp distortion for the first time last weekend at a gig.. I had been using Run-off-Grooves Thor "Tube-to-FET" conversion pedals for Marshall and Vox type overdrives. So, the amp has been used as primarily as a pedal platform and clean all this time. I did find that the PPIMV does give you the opportunity to get things grinding fairly well at a reasonable stage volume. Sure, its not going to have quite the balls that you'd get from master full up and preamp full up. Ha, I'll never get to do that at a gig every again, unless I'm trying to make enemies with my band and the venue - even with a a pair of 6V6's. But, earlier that day, at home while trying some different ANOS 6V6's, I tried different recipes of PPIMV and pre phase inverter Master volume and found that there was a definite difference in the texture's of distortion and response that one could get out of different blends of MV amounts. I'll need to think about and listen for how the PPIMV affects NFB. Obviously, it would because the NFB goes into the PI directly from the OT secondary tap, and with the use of the PPIMV, there would be less signal getting sent back in to the PI to be negatively fed back. Hmm. I did install an adjustable NFB pot on this amp so, It would be worth my while to go back and see if that can be used to compensate for the lesser NFB caused by using the PPIMV. I'm glad you brought that up because even though it makes total sense, it just never occurred to me. Yup, this amp building game is always a process of continual never ending knowledge acquisition.!! Good luck in your ventures! - Phil D.
Sounds like you have almost the exact same amp I have! I built a Hoffman Plexi 6v6, which is based on Sluckey's 6v6 Plexi. It has a standard pre-phase inverter master, but I made a couple of small mods recommended by George Metropolous - I added a 220K resistor in series prior to the MV pot, then a small 7p cap across the lugs. It really helps preserve bass and treble response at lower volumes.
Gene -
I forgot about rectification, mine is SS and I think Sluckey's is tube. I have not compared the two schematics in depth, but I think they are very similar.
Check post 28, the link to Sluckey's schematic is there. Other than tube rectification, I don't know specific differences between the Sluckey and Hoffman design. I am very happy with my Hoffman build.
I"m using a 5V4 tube rectifier which is giving me around 440vdc on the plates of the 6V6's. The 5V4's can output up to 170mA which is quite enough for a pair of 6V6's plus they will all be NOS and arguablly more robust than a current production rect tube. Maybe, mabye not but, its doing a fine job in my 6V6 plexi as far as I can tell.
Sluckery is or was on Doug Hoffmann’s amp forum on EL34World.com.
