[AMRadio] Re: need help


Gary Schafer garyschafer at comcast.net
Fri Oct 21 22:48:29 EDT 2005


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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