T-Rex
Well-Known Member
Hello all, I have a new rebuild project on the bench. The original build way back when was a quad of 6v6's with a 1959 preamp. That changed several times over the years and was recently here on the forum in a thread called a "Single channel Fender AB763 Super". That build got ripped apart. Saved the chassis and PT, bought a new choke and had Matt at MPS wind me a custom OT.
The new build, other than a custom OT is a close clone to a Marshall JTM45, with the exception that the ClassicTone PT is a tad hotter than an original Marshall PT, voltage wise. Other than that, I followed Marshall's specs closely.
The new JTM 45 configuration is set up by the custom OT to be able to try several different tube types of power tubes in the amp without rewiring the OT. Of course each different type of power tube will need re-biasing. The OT primary values are either 3.3K or 4.2K and those two sets of primary wires are on a separate DPDT switch, so I can toggle between the 3.3K set of primaries or the 4.2K set of primaries. The secondary set of wires of the OT are wired as 2, 4, 8, 16 ohms and are wired to a 3PDT switch. By toggling that 3PDT switch, along with the DPDT primary switch I can go anywhere from 3.3K to 4.2k to 6.6K to 8.4K. Switching will not be "on the fly" but is rather there to prevent having to rewire the OT each time, while remaining in fixed bias.
The first set of power tubes that I plan to try out will be a pair of 6L6GC's. Then next will be a pair of EL34's, followed by a pair of KT 66's. The only set that I can't try is 6550's as they will draw too much current for this PT.
The amp is ready to power up, pics below:
The new build, other than a custom OT is a close clone to a Marshall JTM45, with the exception that the ClassicTone PT is a tad hotter than an original Marshall PT, voltage wise. Other than that, I followed Marshall's specs closely.
The new JTM 45 configuration is set up by the custom OT to be able to try several different tube types of power tubes in the amp without rewiring the OT. Of course each different type of power tube will need re-biasing. The OT primary values are either 3.3K or 4.2K and those two sets of primary wires are on a separate DPDT switch, so I can toggle between the 3.3K set of primaries or the 4.2K set of primaries. The secondary set of wires of the OT are wired as 2, 4, 8, 16 ohms and are wired to a 3PDT switch. By toggling that 3PDT switch, along with the DPDT primary switch I can go anywhere from 3.3K to 4.2k to 6.6K to 8.4K. Switching will not be "on the fly" but is rather there to prevent having to rewire the OT each time, while remaining in fixed bias.
The first set of power tubes that I plan to try out will be a pair of 6L6GC's. Then next will be a pair of EL34's, followed by a pair of KT 66's. The only set that I can't try is 6550's as they will draw too much current for this PT.
The amp is ready to power up, pics below:
