MagneQuest/Peerless Forum

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RE: TFA-204 300B Operating Points

Posted by jdrouin on March 10, 2017 at 13:39:17:

I've made a rudimentary model of the circuit in LTSpice, which I'm just starting to learn. This first attempt ignores the power supply from the schematic above, using DC voltage sources to drive the signal section only.

The plate and cathode voltages of the 6SN7s are about 10V below those measured in the schematic. The OPT and 300B are a different story altogether.

According to an LTSpice video tutorial on transformers, you model the shared inductance across the transformer by creating separate inductors for primary and secondary, and connecting them via a K statement, which I've done. You need to set the inductance values to reflect the square of the turns ratio.

I'm not sure if this was the correct way to do it, but since Mike says that the turns ratio of the primary to the 8ohm secondary is 18.26 to 1, if we round down to 18, the square is 324, so the primary and secondary H values (L1 and L2 in the schematic) are set to 324H and 1H.

If I set the B+ supply at 250VDC and enter 3K as the series resistance for the OPT primary, the B+ becomes way too low at 80V.

However, an analysis graph reveals that the current in the OPT primary and the plate of the 300B are acceptable, between 50-60mA.

So, it would appear I need to figure out how to model OPTs better, but it seems like this is getting somewhere. Any pointers appreciated.

Thanks,

Jeff