I know of a guy who wants to build a rig using a single 450TL in the final,
modulated by a pair. He >wants to pi-net the output, but I've heard that's
a bad idea.
I want to build a rig using a medium powered tride, perhaps a 250TH,
modulated by a pair of >811's. Pi-Net, or Link Couple?
What does 1500 watt pep have to do with it?
If he wants to use pi network, he will have to use a balanced grid tank to
produce the out-of-phase rf voltage necessary for neutralisation. A tube
with as much grid-plate capacitance as the 450TL or 250TH must be
neutralised for proper operation, unless it is grounded grid. Grounded grid
is not recommended for plate modulated finals.
My Gates BC1-T uses a pair of 833A triodes in parallel, with a pi-network
followed by a T network and another L netork, to couple the final to the
antenna. The grid tank uses a tapped coil, with the tap grounded to produce
the out of phase voltage. Both the adjustment of the tap and the
neutralising cap will affect neutralisation.
The problem with grid neutralisation, sometimes called Rice neutralisation,
is that it doesn't hold very well over a wide frequency range, especially if
you attempt to switch over several amateur bands. My Gates stays
neutralised over the 160m band from 1.8 to 2.0, but I never have tried to
use it on any other band.
Plate neutralisation, using the same kind of tank circuit as a pushpull
final, and single ended grid tank, works better over a wider freq range,
because the plate-to-ground capacitance is usually much less than the
grid-to-ground capacitance, and capacitance across one side of the tank
circuit upsets the balance of the circuit. Also, grid loading effects
cause some additional unbalance, even if the capacitance is perfectly
balanced out with additional fixed capacitors. The pushpull circuit works
best of all, since it is inherently a balanced bridge circuit, and
theoretically works equally well over an extremely wide frequency range.
Limitations lie in the precision of the balance of the split stator tank
capacitors from minimum to maximum capacitance.
I have alway preferred the pushpull circuit with plug in coils and link
coupling. It is more foolproof and works better. If everything is working
properly, there should be negligible difference in the output power with
either circuit. I have always used a link-coupled tuner into open line wire
with mine, and have never received any harmonic complaints on any band. The
biggest inconvenience is the necessity of changing coils to go from band to
band. In any practical setup there are tradeoffs subject to the preferences
of the operator.
Don k4kyv
