[AMRadio] Re: need help


ne1s ne1s at neandertech.com
Thu Oct 20 13:29:53 EDT 2005


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: 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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