My Small Box Plexi: A re-creation
These pages describe a personal journey - my attempt to recreate the Marshall 50 watt 1987 head I owned in the early seventies. I was a tube geek even back then, and carefully hand-drew a schematic while I owned it.
I was unable to resist fiddling a little bit -- I added two pull switches on the two volume controls, one is a "Bright" switch which disconnects the unpopular 5nF cap across the high treble volume control, and one is a "Boost" switch, which adds a .68 uF cap across the V2 cathode resistor. I also added a "half-power" switch, which converts the output tubes to "pseudo-triode" mode, as is done in later JCM900s (it turns out I never use this!). Here are the schematic (PDF) (GIF) and parts list.
Also, I used KT66 output tubes instead of the 6550s in my original, although I could stuff 6550s or EL34s in there (with a bias tweak) if I wanted to. I also put in an octal rectifier socket so I could put in a JTM45-style GZ34 for fun. I ended up using Weber VST "Copper Cap" rectifiers, either a "WZ34" which emulates the "sag" of a GZ34, or a WS-1, which has no sag, only current inrush limiting. Currently, I'm liking the WS-1 more.
Unlike all of my other tube amp projects to date, I went for "maximum mojo" vs. science in my selection of parts for this project. I used the Mallory 150-style yellow tubular caps, carbon film resistors (except for some select MFs in a very high resistance value positions, e.g., the 1M input resistors.)
I used all ceramic tube sockets, a lay-down power transformer meant for a Fender Bassman, because it had the right ratings and fit the chassis exactly, and the OEI 50 watt plexi output transformer. The aluminum chassis and plexi panels are from Weber.
How much did it cost? I don't really know! I really never kept a running tally of how much it cost, so I don't really have a good feel for it. The expensive items were the head cabinet (~$300 from Swanson), the chassis and panels (~$110 from Weber), the OT (~$120 from OEI), the PT and choke (~$108 from Hoffman). All the other parts do add up as well, but don't cost much individually. I got the tubes from Lord Valve, knobs, fuseholders and jacks from Hoffman, the tube sockets and wire from AES, and most of the other parts from Mouser or Digikey. I did not buy anything locally, mostly because there were no good stores near my home.
The only part I would not use again is the PT from Hoffman, which was a New Sensor "for-Fender" part, and runs way too hot for my preference. He may sell a different and better part now, but I haven't checked lately. Heyboer makes great plexi tranny sets for reasonable cost, I would recommend looking into them.