[AMRadio] Re: need help


Edward B Richards zuu6k at juno.com
Fri Oct 21 23:56:33 EDT 2005


Thanks, Gary.

Ed K6UUZ

On Fri, 21 Oct 2005 21:48:29 -0500 "Gary Schafer"
<garyschafer at comcast.net> writes:
> I just looked up the 4-400 and I see it requires more bias than I 
> had
> remembered for AB1 operation. It requires -130 volts with 2500 on 
> the plate
> and 750 on the screen.
> So a 50 ohm resistor with 100 watts would only provide around 100 
> volts peak
> drive voltage. Not quit enough.
> 
> A 100 ohm resistor should provide around 140 volts peak with 100 
> watts. That
> should work with little drive to spare.
> Probably the easiest would be a 200 ohm resistor (close to the 170 
> ohm
> resistor). A 4:1 balun should match it close to 50 ohms.
> A 200 ohm resistor and 100 watts should provide close to 200 volts 
> peak
> drive voltage.
> 
> I did this setup (balun and resistor) with a pair of grid driven 
> 1625,s
> driven by my 20A. It worked well. Voltages were less of course!
> Stray capacitance will change things a bit.
> 
> 73
> Gary  K4FMX
> 
> 
> > -----Original Message-----
> > From: amradio-bounces at mailman.qth.net [mailto:amradio-
> > bounces at mailman.qth.net] On Behalf Of ne1s
> > Sent: Thursday, October 20, 2005 12:30 PM
> > To: Discussion of AM Radio
> > Subject: [AMRadio] Re: need help
> > 
> > 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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