[AMRadio] need help


crawfish crawfish at surfmore.net
Wed Oct 19 23:50:40 EDT 2005


I believe this is called a "passive grid" amplifier configuration.
                                 Joe W4AAB
----- Original Message -----
From: "Gary Schafer" <garyschafer at comcast.net>
To: "'Discussion of AM Radio'" <amradio at mailman.qth.net>
Sent: Wednesday, October 19, 2005 8:02 PM
Subject: RE: [AMRadio] need help


> The input impedance should be very near what the value of the resistor is.
> In this case 170 ohms. The 4-400's will most likely be run in AB1 so no
grid
> current.
> I would put in a 50 ohm resistor instead. You should get enough drive with
> it. Figure what the bias voltage will be on the tubes. Then figure what
the
> peak voltage will be from the exciter at 50 ohms. If the peak voltage will
> be greater than the bias voltage on the tubes then you have enough drive.
>
> 73
> Gary  K4FMX
>
> > -----Original Message-----
> > From: amradio-bounces at mailman.qth.net [mailto:amradio-
> > bounces at mailman.qth.net] On Behalf Of esieb at sympatico.ca
> > Sent: Wednesday, October 19, 2005 10:46 AM
> > To: amradio at mailman.qth.net
> > Subject: RE: [AMRadio] need help
> >
> > I'd have used an MFJ 259B to actually measure the input Z.
Alternatively,
> > you  can always use a  small tuner to "tune" the input.  I do that
anyway
> > with my Drake L4B, (use a small MFJ mobile tuner with meter).
> >
> > 4-400's, eh?  Nice amp!  If your plate voltage is high enough, you ought
> > to
> > get serious power out of that baby!
> >
> > 73, Ed, VA3ES
> >
> >
> > From: Edward B Richards <zuu6k at juno.com>:
> >
> > I want to drive a linear amplifier with a rice box that requires a 50
ohm
> > load.
> > The linear amp uses an input to a 170 ohm, 80 watt swamping resistor to
> > ground, then
> > through a .001 mfd capacitor, then through a VHF parasitic suppressor
> > consisting of  4 turns of wire around a 47 ohm resistor, to the grids of
> > a pair of parallel connected 4-400A tubes.
> >
> > What I need to know is the impedance of the input. Is it close to 50
ohms
> > or do I need to use a matching network between the rice box and the
> > linear amplifier.
> >
> >
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>
>
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