Donald Chester wrote:
What does 1500 watt pep have to do with it?
Well... you know. I certainly don't want to Advertise ;-)
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.
My rig is a pair of 250THs in the final, modulated by a pair in the modulator. Like what you prefer, the final has plug-in coils for the grid input tank, as well as the final tank. The problem I have with that rig however, is that there seems to be some interaction between the grid circuit and the final circuit, even though the plug-in coil and link are 'underneath' the chassis, that the final coil sits on top of. When the rig was built back in the mid 50's, most everything available was steel.
A suggestion of raising the final coil further up to get out of the field of the grid tank could be realized, if i wanted to give up the front-panel control of the Faraday shielding link. As a result, it's as close as possible, but probably not close enough.
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.
I've got plug-in coils for that rig, from 80m thru 10m, but I'm more than a little leary of running a pair of 250TH's in Class C on 10m ;-)
I wonder though Don, while I've got your .. uhm 'ear' (eyes?) The link could use a little tuning to take a bit more load in the final. I've modified one of my 80m B&W HDVL plug-in coils and removed two turns on each side (4 turns, total), to raise the plate current a bit, because the Q is a bit sharp in that circuit. A breif excursion off-resonance, w/1.5kWDC on the plates, draws around 550mA. At Resonance (with 125mA of grid drive) the thing dips to around 200mA. That said, what value of series capacitance would you suggest/recommend to tune the link?
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.
In my rig, the neutralizing caps come from the crossed grid-input lines, and big silver disks mounted on screws on the back end of the B&W bread-slicer butterfly tuning capacitor.
Looking forward to your reply and info. -- 73 = Best Regards, -Geoff/W5OMR
