Quote:
Originally Posted by Diabolical Artificer
Yes,the OPT's is designed for SE operation, came out of a Lowe radiogram driven by an ELL80 which was kaput.
Looking at my load lines you'll see we get about a 350v V swing, so with a 44:1 T ratio - 350/44 = 7.9v P-P, which is 2.79v RMS. Therefore the limiting factor in all this is the HT & high OPT T ratio, but HT most of all.
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Andy, have you measured the turns ratio of the OPT you are using? Was it 44:1 ? For a 6k primary that would be a 3 ohm secondary.
You show a 6k primary loadline, with a nominal 200V anode swing available to saturation end, and 150V swing available to cut-off end using the 250V 25mA idle point.
As indicated your idle operating point should be moved to 250V minus 9V (for cathode), and minus 17V for DCR (=700 ohm - did you measure that?) of OPT primary, so circa 220V.
Using an EL90 datasheet pentode curve with a 6kohm loadline, would suggest you need to have an idle up toward 50mA, given a 12W rating for anode. If you chose 40mA, and could maintain a 250V B+, then anode voltage idle point would be down around 210V. Anode voltage swing might then have limits of about 40V to 440V, so a swing limit of 170V and 230V. That may be a better starting point to see if you hit saturation clipping about the same time as cutoff distortion/rounding gets too noticeable. That's about 2.5W output capability.
To get a higher output power you likely need to increase B+, and push your idel operating point closer to the 12W max rated limit, or increase your speaker impedance to give a higher primary side impedance.