A correspondent asked for a walk through of use of a couple of my online design tools for a 6m 350W single ended valve PA using three QQE06-40 valves. The request was perhaps inspired by a design he had seen, but I sound a caution about a large number of parallel valves (6 sections in this case).
Key design parameters:
- HV power supply fully loaded: 1200V;
- Power output: 350W;
- Class: AB1;
- Vak min: 180V (from datasheet anode curves);
- Pi output network, Q at least 12 (for reduction of harmonics on the FM broadcast band), select 15.
The datasheet gives max supply voltage at 450 for plate and screen AM, which implies max ‘instantaneous’ DC supply voltage in AB1 SSB telephony of 900V… so 1200V goes beyond the guaranteed ratings. Of more concern, it is probably close to 1400V lightly loaded, 56% greater than the maximum instantaneous supply implied by the AM specifications.
- output network efficiency 90%;
- valves load share equally.
An advantage of a high Q design is that it requires higher input C which makes accommodating the self capacitance of the 6 valve sections somewhat easier. A disadvantage is lower efficiency.
Above is a calculation using Calculate initial load line of valve RF amplifier. Note the anode dissipation, a total of over 190W is quite high for 120W total valve rating (though these are pretty robust valves). Total DC anode current will be almost 0.5A, so the HV power supply must deliver 1200V @ 0.5A. The 0.5A figure is within the absolute maximum of 720mA (for 6 sections).
Experience is that designers underestimate the sag in HV and it is the most common reason for failure to make the design output power… test the power supply under load before committing the design.
Above is the output pi network design from Calculate RF amplifier pi coupler. Hams tend to use low Q output networks, but as mentioned, medium Q is recommended where the FM broadcast band is on the second harmonic. The design 36pF C1 will easily accommodate the valve capacitance (6*3.2=20pF) and leave a practical value for the tuning capacitor.