[R-390] Role of the 3TF7 in PTO frequency changes

[Shoppa, Tim]

> the measurements prove that the observed power line voltage induced
> PTO frequency changes result from poor regulation of filament current
> by the 3TF7 current regulator.

I'm sure that filament current changes can make the PTO and BFO shift frequency. In fact Dallas wrote some recommendation on picking 6BA6's which would be less sensitive to any such changes.

IMHO the 3TF7 is more than good enough in the real world. Meaning it's overkill already, especially if you aren't using the 390A in a truck run from unregulated generators in the middle of nowhere. You were measuring +/- 6 Hz changes around nominal line voltage and those aren't really audible to me (I am not a particularly musical person) in my usage.

For me, the shift in the crystal oscillator frequencies due to AGC action is much more noticeable and annoying (I'm a big CW junkie). I've made some mods and measurements but haven't written them up, mostly because I'm not satisfied with the results.

Tim N3QE