Different 45 Operating Points

Working out the operating points using the traditional ruler method and the RCA charts has been fun, but using TubeCAD's SE Amp CAD was not only much faster but produced far more information. The parameters are using a Tamura F5003 (Amorphous core US$600+ each transformer) with the plat at 202V biased at 35.8mA, results in an output of 1.3W and a distortion profile of; 2nd @ 1.7% and 3rd @ 0.1%). The input voltage swing required is 60Vp-p. If we wanted to maximise power without going into Class A2 (positive grid current), then the input swing would be 66V, and this results are 1.6W and a distortion profile of; 2nd @ 2.1% and 3rd @ 0.2%. According to SE CAD, %I max is 100%, %V max is 73% and %W max is 91%. Running the 45 reasonably hard. This will also be run with the Lundahl Amorphous core LL1620 or LL1623 output transformers. See the curves below.

Working on the assumption that we keep the 45 in Class A1 (i.e., no positive grid current), then the driver stage will need to deliver 23Vrms or 66Vp-p. If we assume that a 2Vrms signal is capable of being delivered by the preamp, the driver stage will need to have a gain of say 12x. This brings into contention a large range of tubes beyond the higher mu and high gm types (presently been considering C3g, D3a, 5842, etc.). Using a 1:2 transformer, either in the grid position (i.e., Lundahl LL1670) or as the interstage, would mean that the gain required by the driver itself is lowered to 6x. Opening up a range of DHT tubes. On the other hand, if a 2:1 interstage transformer was being used, then the driver stage will need a gain of 24x, BUT the impedance driving the 45 would lowered.

In one thought exercise, I might be able to get away with a 26 Tube into a 1:1 interstage transformer, and using the 1:2 step-up function of the Lundahl grid choke to provide a gain of around 15~16x. This would necessitate another Tentlabs Filament heater (to reduce hum) and probably far more attention to reducing microphonic and hum effects on the 26. Considering I already have starting a small stash of 26 tubes, this may be an interesting option. With various reports about the sound of the 26, this may be worth the effort.

At present, if using a 26 driver, the B+ for the 26 would be less then 200V, which means I can use a pair of 0C3/VR105 regulator tubes in series to provide a stable voltage for the 26. Perhaps a pair of 0D3/VR150 might be better if we were willingly to drop some voltage over a divider network. Consideration will be made to regulating the B+ for the 45 tube.