[AMRadio] Re: need help


Edward B Richards zuu6k at juno.com
Thu Oct 20 23:29:57 EDT 2005


Thanks, Larry.

Ed K6UUZ

On Thu, 20 Oct 2005 13:29:53 -0400 "ne1s" <ne1s at neandertech.com> writes:
> 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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