Doug's 5f6a layout has an 820r resistor for V1a. The Fender 5f6a layout and schematic as well as the reissue show an 820r resistor shared by both sides of V1. Doug's schematic is consistent with the Marshall style of using an 820r resistor on V1a (Plexi 50) and a separate resistor for the b side cathode. I presume Doug tried a 1.5K resistor and an 820r resistor and like Marshall went with the unshared 820r resistor. Hopefully this is the case and could be a difference between the original Bassman and the reissue for that matter and an amp made to Doug's specifications. Has anyone tested both the 820r and 1.5K resistor and preferred one over the other?
As I have mentioned before the amp in its first build status sounded just like my Fender '59 Bassman LTD. While I very much like my LTD I want my build to sound a little more like a '59 Bassman I heard on the internet. It growled. I want this amp to growl. In addition to the 820r resistor change mentioned above I am going to replace the plate resistors with 110K two watt metal film resistors. Read somewhere that some repair technician said Fender amps with 110K plate resistors sounded better than 91K plate resistors and since the 110K is within the 20% tolerance limit listed on the Fender schematic it seems to be worth a try.
Another guy had original '59 and '60 Bassman amps. He liked the '60s tone best. Known differences were a 6.8K PI tail resistor on the '60 versus 10K listed on the schematic for the 5f6a and on the '59. The reissue amp, which was supposedly based on a good sounding '59 shows 6.8K on its schematic. The '59 had a 250K bass pot instead of the 1M pot it should have had. Both amps had .1uF bass tone caps and 100K slope resistors instead of the .02 and 56K combination shown in the 5f6a and Doug's layout. The reissue schematic has the .1uF/100K combination consistent with both amps of the era. The PI plate load resistors have a 47pF cap across them on all schematics except for Doug's who leaves it out. The 5f6a has no grid stopper resistor on its schematic which led to oscillations in my first build when I left them out entirely. I corrected the oscillation problem with 1.5K grid stopper resistors. The reissue has 47r grid stopper resistors and Doug uses 1.5K which was the standard for the blackfaced era amps. Has anyone experimented with any of these things and created an amp that growls?
Another thing I am thinking of trying is putting a 10K pot to balance the PI plate resistors. All schematics I have ever seen show one as 100K and the other as 82K. Seemed to remember this being something Fender techs learned years ago and that some amps sounded better than others because of it. Dumble put the 10K balancing pots in his amps so he could tune the PI. Made a little difference in feel as I recall so might be worth doing here as I have space on the board for adding one and have one on hand. Anyone have any opinions concerning this?
Thanks
Mike
Topic: Doug's 5f6a layout for V1a cathode. (Read 1764 times)