|[AMRadio] Pi-Net vs Link Couple|
w5omr at satx.rr.com
Thu Sep 22 17:03:36 EDT 2005
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
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,
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