After a lot of grief with Excel trying to open and fix some 10 year old spreadsheets… finally…
I was recently asked about FT-8 on the Colling 30-L1 linear amplifier considering my article Collins 30L-1 and AM.
The first thing to note is the Colling 30-L1 manual cautions against AM and FSK:
That said, what are the reasons for such a prohibition?
The RF Power Amplifier Tube Performance Computer gives us a tool for exploring different operating conditions without risking damage to the valves.
Using the previously constructed model for the 30-L1, key parameters were tweaked for the following conditions:
- output circuit adjusted for rated input (1000W) under key down CW conditions and Vdc=1600V.
So the amplifier is operating essentially tuned for rated key down CW. That is different to SSB because the power supply sags more for key down CW.
The model extract above shows Idc should be 625mA, and Pout 674W, and anode dissipation is 251W, just within the ICAS limit of 260W. So long as the power supply can sustain that current, it should be possible to safely operate the 30-L1 on FT-8 or similar FM like modes… but Collins warns of the power supply limitation.
Of course it is prudent to monitor plate dissipation in high duty cycle modes, the 811A plates should not colour up.
A question that arises in the context of power supply limitation, is there a lower power level that can be achieved efficiently for a specified Idc? Calculation of an optimal configuration for say Idc=250mA is not too difficult, but it requires a significantly higher resonant load impedance which might not be achievable on all or any bands… because the output circuit is designed for rated power rather than reduced output.
All in all, the 30-L1 is not well suited to ‘high duty cycle’ modes because of the power supply.
A relevant question with FT-8 is whether operation at high power in a shared channel is sociable.