You want a VFO challenge? Try PSK31.
I attempted to use a vintage Hallicrafters SR-150 on PSK31 about a week ago to QSO with a buddy in Massachusetts and caused quite a disturbance. Of course I let her warm up for about 30 minutes before trying. I knew to keep my drive down so I was seeing about 10 watts on the meter so as not to get into ALC. The waterfall display was fairly stable but when I went to TX and then came back to receive, the station was far out of the passband and it took about 10 seconds for the decoder to lock even with AFC. It was worse for the poor soul on the other end. He had to basically attempt to find and track me till I settled in. This is how not to make friends on the band. Now to make a signal the Halli has a VFO and two crystal oscillators in a classic single conversion scheme.
Measurement with my counter revealed that the VFO was slewing more than 100 Hz between TX and RX for the first 10 seconds. Now 100 Hz might not be an issue for AM it is and is probably not a big deal for SSB, but for PSK31 where the signal is only 30 or 40 Hz wide, it is a show stopper.
The VFO is the pentode section of a 6EA8 and it is regulated off a 150V gas tube.
The Buffer is the Triode section and it is on the unregulated 250VDC.
Most of my problem was traced to the VFO. It was not a pushing problem - varying the load was not the culprit.
All of my mods are ridiculous but reversible and they are all good VFO stuff:
First the Hallicrafters SR-150 was some kind of a milestone rig because it is the first rig to sport a RIT function with a proper varactor diode and pot control. It also uses a diode for frequency correction. I deemed these to be worthy of disconnecting and i got some immediate improvement. Don't hang stuff off the VFO like relays and switches and such.
Then I decided to try regulation on the plates of the buffer and the two XTAL oscillators. This was a couple of zeners and a mosfet for 240VDC. Again, improvement but still I was getting 50 Hz or so of shift.
Next I did what I should have done in the first place - changed tubes. I saw everything from 150Hz shift to 10 Hz shift! Depending on the tube!
I also put a diode across the relay coil which is a 250V job! This kicked back directly into the medium voltage line.
Checking the fillaments on the VFO I noticed that they put two tubes in series to make up the 12.6VAC. This sagged 2 tenths of a volts 12.6 to 12.4VAC on TX. I did a simple half wave rectifier to a low drop our regulator and set it so I had exactly 6VDC on the fill of the VFO.
BINGO - that was the culprit. All became stable. No shift RX to TX.
Now the 1962 Hallicrafters SR-150 is as stable as those synthesized jobs
Well maybe not quite...
Mike WU2D