[AMRadio] Re: need help

[ne1s]

With a 50 ohm grid load on 4-400s, I think you'll find you'll get very 
little amplification from the stage - the 40400 grid(s) want(s) to see more 
voltage. I went throught this exercise one (on paper), so went with a 1:16 
balun into a 800 ohm non-inductive resistor network in the actual design. 
Problem was, I couldn't make it broadband enough to cover more than 3 
consecutive bands at a time, and finally resorted to a T network on the 
input, loaded with about 2000 ohms worth of resistors. 

-Larry/NE1S 

Gary Schafer writes: 

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: [EMAIL PROTECTED] [mailto:amradio-
[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED]
Sent: Wednesday, October 19, 2005 10:46 AM
To: amradio@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 <[EMAIL PROTECTED]>: 

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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