[AMRadio] Pi-Net vs Link Couple

W5OMR/Geoff 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 
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,

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