F2a Operating Point

Have been looking at using an European power tube to build a SE amp. Trying to find a tube different to the usual pentodes (KT88, El34, 6550, etc), and steering away from the very expensive triodes (AD1, PX4, PX25, DA30, etc), and came across this tube, the Siemens F2a. Touted at a high quality tube lasting 10,000 hours, build only by Siemens, and used by some high end audio amps then, and now. The Shindo Cortese. There are actually 2 tubes, the F2a and F2a11. From reading through various datasheets and info, both are identical except that the F2a11 has a different basing. Having bought 4 samples of this tube, calculating that I listen to around 250 hours of music a year at present, maybe growing to 500 hours in the future, 2 sets would last me around 20 to 40 years! But I'll probably invest in another 2 pairs, just in case...

Looking at a basic design for the amp, I started with the F2a operating points. See the curves below. The loadline is at 7k5, operating point at 375V on the plate, 70mA bias current and grid bias at -16V. The approximate power is around 3W. This requires a driver stage that can swing 32 V peak to peak. Not a bad set of parameters.

The interesting item with this setup is if you use the estimation tools at Steve Bench's site, with the following parameters:

Va = 120V, Vb = 375V, Vc = 600V

Ia = 95mA, Ib = 85mA, Ic = 70mA, Id = 60mA, Ie = 45mA

Power is: 3.84W

2nd Harmonic Distortion is: 75*(95+ 45- 2*70)/(95+ 85- 60- 45) = 0%

3rd Harmonix Distortion is: 50*(95- 170+ 120- 45)/(95+ 85- 60- 45)= 0%
 
Now, I wouldn't read too much into this, as the formulas are only approximations and the curves are only approximations and the determination of the various points are only approximations. But, it does give some confirmation that this is a good starting point.

One might be thinking that it is a pretty expensive way to get 3Wrms of power with the plate disspation at 26W. Perhaps a different load line is required, this might warrant a 5k or 3.5k loadline to get more juice out of this tube. More to follow.