Quote:
Originally Posted by Peter.N.
I remember those, the frequency of the inverter changed as the PA valve warmed up - which wasn't long as it was a special quick heat valve.
Peter
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Yes, and the quick-heat valve's filaments were powered from the inverter!
This was actually quite a cunning scheme: at initial power-on the filaments were cold and provided a serious load on the inverter - which, because the filaments weren't hot enough to be emissive also meant that there was at the time no load on the HT-side of the inverter.
So most of the inverter's power got applied to heating-the-filaments-as-fast-as-possible rather than charging the HT-side's smoothing capacitors to a crazy voltage.....
As the filaments warmed, their resistance rose so their load on the inverter fell - but at the same time the filaments were also becoming cathode-emissive so the load on the inverter's 'HT' side increased towards full RF output.
So at all times the inverter was being worked to its fullest capacity even as the balance between LT and HT demand shifted.
See
https://frank.pocnet.net/other/Phili...k-heat_VHF.pdf
for the Mullard/Philips range of these quick-heat valves. Somewhere out there is also a series of design-notes by Mullard/Philips on suitable transistor inverters - I remember the big ADZ11-style "Top Hat" transistors -
https://www.radiomuseum.org/tubes/tube_adz11.html
being the preferred devices for such tasks.