[AMRadio] need help


Gary Schafer garyschafer at comcast.net
Wed Oct 19 21:02:19 EDT 2005


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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