[AMRadio] Need some help here

[Bill Coleman]

Dan, not sure what freq you are aiming at, but have included the results of
a PI and PI-L calc spreadsheet I built from W5FD's Q based formulas.   If
you have Lotus 123 I'b be glad to send it to ya.   Or hollar & I can plug in
other values.  The Efficiency% stuff is only used to show plate dissipation
& power out, does not alter the tank values.  I used the 2KV & 400mA values
you noted.

73, Bill   N2BC

     INPUT DATA       PI Calculations   PI - L Calculations
        PA Ep 2000 Plate Load 2500   Q1 10.83   Ql 2.24
        PA Ip (Amps) 0.400 Input Watts 800   Q2 1.17   Qoutx 9.76
        Efficiency % 60 Output Watts 480   XC1 230.8   Q1L 7.38
        K 2.00 Plate Dis 320   XC2 42.78   Q2L 2.38
        Rout 50       XL 253.6   XL2 112
        ImageR 300             XP2 134
        Qout 12   K Values Class A 1.3     XC1L 338.6
                Class AB 1.5 to 1.7     XP1 126.1
                Class B 1.57 to 1.8     XL1 439.6
                Class C 2     XC2L 65.0

      Freq C1 (pf) L1 (uH) C2 (pf)   Freq C1 (pf) L1 (uH) C2 (pf) L2 (uH)

      1.800 383 22.42 2067   1.800 261 38.87 1360 9.89
      1.900 363 21.24 1958   1.900 247 36.82 1289 9.37
      2.000 345 20.18 1860   2.000 235 34.98 1224 8.90

      3.500 197 11.53 1063   3.500 134 19.99 700 5.08
      3.750 184 10.76 992   3.750 125 18.66 653 4.75
      4.000 172 10.09 930   4.000 118 17.49 612 4.45

      7.000 99 5.77 532   7.000 67 10.00 350 2.54
      7.300 94 5.53 510   7.300 64 9.58 335 2.44


~~~~~~~~~~~~~~~~
Bill Coleman    N2BC
http://home.stny.rr.com/n2bc
----- Original Message -----
From: "Dan Hall" <kj7fx at tdn.com>
To: <amradio at mailman.qth.net>
Sent: Monday, March 04, 2002 7:08 PM
Subject: Re: [AMRadio] Need some help here


> Thanks Bill:
> I will comment on your comments with some snippage.
>
> > Well, Dan, I am of the firm belief that these transmitters are
> > single-frequency beasts only.
>
> I have gathered that, and yes I will go back to a single frequency scheme.
>
>
>   I have
> > tried to stay with the existing circuitry as much as possible, even to
the
> > extent where in most cases to the transmitter can be retuned back to the
> > broadcast band.
>
> Me too.
>
> (exciter coil)
> > I have found that some coils can't be tapped down without excessive
loss.
> > The unused turns should be shorted out with a strap.
>
> I'll take another look at shorting things before I take out the original
> coil and wind a new one. The bias comes to the PA tubes from a resistor
> network to ground via the center of this coil.
>
>
> Can you look at this
> > signal with a 'scope?  If it isn't a clean sine-wave, then you may have
to
> > use other sets of taps on the coil or replace the coil.  If you see a
> > sine-wave with a big dip where the peak belongs, you indeed have
problems
> > with the unused coil turns.
>
> Bingo! Very ugly sine at the grids of the 833As and it changes drastically
> when tuning the exciter through resonance. The purer the waveform the
lower
> the output it seems.
>
> >> 2000 Volts?  That sounds low, but perhaps that is correct for 833's
At
> any rate, they should put out well over
> > 1000 watts at full output.
>
> The main power transformer has two HV taps one at 2230VAC and the other at
> 3300VAC. I'm using the lower voltage tap to reduce the power output.
>
>
>   I favor a PI-L arrangement if you have a choice,
> > and can help you configure one if you need.
>
> It's a PI-L, I'm using 145pf fixed C1,  around 9mmh L1, 750pf C2 and
around
> 4.5mmh L2. The coils are variable with shorting of the unused portion and
C2
> is a 80-1300pf  variable in parallel with a 300PF fixed. I can make this
> fixed with a 3000pf and a 1000pf in series.
>
>
> .  You may
> > have to also short intermediate turns so the coil can't adsorb energy in
> the
> > unused portion.  Use a scope to verify the output waveshape is a good
> > sine-wave.
>
> After I get the drive in shape I'll look at this
>
>
>
> > Another problem may be the RF choke in the B+ line.  Make sure it isn't
> > resonant.  I have had to replace the RF choke in several transmitters.
>
> I'll check
> >
>
> > Not sure why the PA would ever act as a doubler, but sounds like tank
>
> This when waaaay out of tune, experimenting
>
>
> >Regarding the
> > tank circuit, how are you tuning it?  I calculate the tube impedance
from
> > specified voltage and current, multiplied by a factor, think it is 0.8.
I
> > then take a film resistor of somewhere around that value (surprisingly
it
> > has turned out to be 3300 ohms in many cases) and, with power
disconnected
> > to the transmitter, attach it between one of the PA plates and ground.
I
> > then connect an Autek RF-Analyst to the antenna terminals, and adjust
the
> > plate tank circuitry for an SWR of 1:1.  It must be 1:1, anything higher
> is
> > not acceptable.   MFJ units don't seem to work in this application.  I
> then
> > remove the resistor (important!), turn on the power, and the trasmitter
is
> > tuned and ready to go!
>
> Sounds like a plan, do you use a guessed at current based on .7 efficiency
> and desired output?
> I am using an AADE L/C meter to set values based on the charts in the 1990
> handbook. At that, C1 is way out of wack but I attributed it to stray
> capacitances.
>
> Lets say for sake of arguement that I want to shoot for 2000VDC at 400mils
>
> Thanks Bill, and I think this stuff is of great use for most on the list.
>
> Dan
>
> .qth.net/mailman/listinfo/amradio
>
>
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