Don K4KYV said "Even a slight change in line voltage, < 1v, will cause the PTO to drift frequency, due to the change in filament voltage."
Don, I was experiencing similar filament voltage induced oscillator drift in the Command Transmitter VFO (BC-458) for my Central Electronics 20A which uses a 1626 with unregulated DC on the filament. I found it interesting that for a given change in voltage, say 10-12 volts, different 1626's shift frequency of varying amounts. Then after trying 3 or 4 tubes, I tried another that was dead stable when varying the filament between 9-14 volts. Apparently the oxide cathode was very stable with this tube. I didn't need a Sola here. The point here is that if you drift when going key down due to a drop in the line voltage, just maybe a different oscillator tube will perform better. I once took a Heath VF-1 (Varying Frequency number 1)and resorted to DC on the filament with a 7805 five volt 3 terminal regulator, and a silicon diode on the common pin to ground to provide a regulated 5.6 volts DC. That took out the 60 HZ hum, and line voltage induced changes in filament voltage. I later experienced band switch contact issues with that VF-1, and it was more like a spread spectrum exciter. Off to the dump that POS went..... Jim JKO -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.1.392 / Virus Database: 268.5.6/336 - Release Date: 5/10/2006
