[AMRadio] FW: [Central-Electronics] CE 20A Phase shift question


James M. Walker chejmw at acsu.buffalo.edu
Tue Jan 31 08:31:49 EST 2006


Humm,
Snippage here >

> With that VFO, I wonder how much you had to fiddle with it to make the
> output tuned circuit track the input tuned circuit? I gave up on mine, and
> added a second variable capacitor to tune separately. When operating on 80
> meters, turning that 1625 plate tuning capacitor sure pulls the
oscillator,
> upwards of 15 Kc. The 1625 is neutralized, or was when there was two of
> them...

My VFO458 units are stock, as in just like the manual, with changes
says, and I have not experienced this phenomenon at all!

>
> Your carrier null at -72 db seems rather optimistic, and I'd wager hard to
> hold.
Snippage agn.

Carrier null on several of my units is in the -80 db range, and stays that
way, while in use.

I did one 20a once where I used discreet resistors (3) along with a 5K
> pot to in effect use the pot as a "fine null", while maintaining the
circuit
> impedances as when stock. The 20a I have on the air now has a stock
balanced
> modulator pot setup, and I'd swear that my null can degrade 10db by just
> looking cross at the pots! Later 20a's have a temp compensation circuit in
> there so you can hold the null better while the rig warms up.

Snippage insert -->

Again, I found that my VFOs are stable, even though the units sit
on an AV cart and I tended to move the cart around, so I could
work on other things while on the air. Also turning the rig off for
a few days, and returning to operate later the vfo was right where I
left it during prior use.

>
> My 20a's are modified for QRO, and therefore I lost 15 and 10 meter
> operation because of additional capacitance from the tube plate(s) to
> ground. Still, with a stock 20a to get on 10 meters the VFO needs to put
out
> say 29 +/- 9, which is 20 or 38 Mhz. There is a CE module kit for 10
meters
> that goes into the BC-458 that has 3 tubes, and a crystal in it. I recall
it
> putting out 38 Mhz for 29 Mhz transmit frequency.

My CE gear is all stock with refurbishing done to them to bring to
manual specs and ranges. The 10 meter output on them are at 37.0
to 38.7  Mhz.
I have run them on 10 driving a Johnson Thunderbolt, without any
drifting or squirelly effects, and great power out right around 300
watts carrier to the antenna.

Without the optional VFO
> kit, your limited to 160 to 15 meters unless you use a crystal. Keep in
mind
> that when using a crystal, say 3740 for 80 meter CW, the balanced
modulator
> doesn't work...making this a old time Novice rig. A crystal that beats
> against the 9 Mc should work OK for phone work.

Snippage insert -->

The manual calls for 115 VAC to the power transformer, at
that input voltage, and with good tubes, 5U4 and 6AL5 the
voltage levels are pretty close to the manual, 310VDC output
in my case. The 6AG7s normally run hot in "AB(*) operation.
CE put out change notices to lower the screen voltage to the
proper level and also to move the paracitic suppressors to the
output of the tubes and not the input.

My 20As run right around 7.5 watts output on AM, and 12.5
on CW. If you are going to drive a linear amp like the Thunderbolt
you really need to monitor the power output from the 20A to the
Tbolt, and keep it around 2.5 watts.


>
> I wonder what power you get from yours? On 160 and 80 meters you should
see
> the maximum. The 6AG7's run pretty darn hot, and a little 12 volt DC fan
on
> top of the 20A case can cool things a whole bunch. You could 1/2 wave
> rectify the filament voltage and with a capacitor filter get 8-9 volts for
> the fan. Running down there makes it almost silent. The 6AG7's run class
> AB1, and with enough drive, they go Ab2 and draw grid current. If you can
> hold the bias stable, several more watts RF output are possible. What
> happens stock is when the peak RF grid swing exceeds the -10 bias, the
bias
> just gets more negative. This is self limiting. Still with today's line
> voltages (like 125) you might get upwards of 20 watts from that 20a, and
> maybe 25 watts with stabilized bias. On 40 meters where the VFO has to
> triple, drive is a problem, so output is usually much lower. 20 meters
> should provide good output as well.
>
The rigs were never designed to provide 20 - 25 watts, their main
intent was to be a low cost low power exciter with clean output, to
drive a higher powered linear amplifier. Such as the Tbolt or the
600L pair of 813s.

Jim
WB2FCN
Hope this wasn't too confusing.

> Keep up the good work! Maybe we can work each other on 20 meters when your
> ready?
>
> Regards,
> Jim
> WD5JKO
>
>
>
> -----Original Message-----
> From: central-electronics-bounces at mailman.qth.net
> [mailto:central-electronics-bounces at mailman.qth.net]On Behalf Of K3PID
> Sent: Monday, January 30, 2006 8:04 PM
> To: central-electronics at mailman.qth.net
> Subject: Re: FW: [Central-Electronics] CE 20A Phase shift question
>
>
> Jim, Thanks for the tips!!! I had completely overlooked the 9 MHz getting
> into the audio. I added both of your mods!
>
> I could only hold the circle from about 250 Hz to just under 2500 Hz,
after
> the mod, the circle remains pretty true ( your eyes start to play tricks
> after you stare at it for awhile) through just over 4000 Hz ( maybe higher
> :>) ).  I can suppress the carrier to about -72db and I can get the
unwanted
> sideband a little lower.  I just finished converting a BC-458 to the
> multiband VFO and while I haven't tried it on 15 or 10 yet it seems to be
> working fine on 160 ( my favorite band) and 80, 40 & 20.
>
> I haven't had this much fun since I first got my ticket in 1960......
>
> Next... feed the 20A into the T-368 for some real punch!!
>
>
> 73 & Tnx
> K3PID
> Ron H.
>
>
>
>
> ----- Original Message -----
> From: "Jim Candela" <jcandela at prodigy.net>
> To: <central-electronics at mailman.qth.net>
> Sent: Sunday, January 29, 2006 12:44 PM
> Subject: Fwd: FW: [Central-Electronics] CE 20A Phase shift question
>
>
> >
> >
> > Ron,
> >
> >   Hats off to you for remembering how to use Lissajous
> > Figures! I forgot about that stuff, and you are
> > correct that two signals 90 degrees apart fed into
> > scope in X:Y mode should make a nice circle. I did set
> > up my 20a a few times with the scope, and with
> > perfectly compensated 10X probes (OK, best I could do)
> > I was able to get pretty close to correct setup by
> > varying the two audio pots within the 20a. Figuring
> > out 90 degrees phase shift was pretty tough, but 90
> > +/-5 was doable. Heck next time I'll try Lissajous
> > figures! Setting the RF phase shifter with a scope
> > (looking for 90 degrees shift) is more challenging,
> > and a 100 Mhz high quality scope with two matching
> > probes helps. Still, doing it the CE way is best as
> > described in the original alignment instructions.
> >
> >    The fuzz in your circle was probably the 9000 Khz
> > from the crystal oscillator. Those bypass capacitors
> > C11, and C12 don't filter out the 9 MC very well, and
> > they do it differently which is I think due to
> > different circuit layout between the two audio
> > channels. I bet the circle intensity modulation maxima
> > and minima you saw was due to power supply ripple
> > beating with the tone injected from your audio
> > generator.
> >
> > I do use the RCA jack in the back for a scope input
> > (Trapezoid pattern on AM), and the 9 Mc signal is
> > pretty high. I filter it out with a simple R-C of 1.2K
> > series, and 2700 pf shunt at the jack.
> >
> >    So over what range of frequencies can you maintain
> > a circle? My experience is that the 20a phase shifter
> > does a decent job from 300-3000 hz, and beyond that
> > the sideband rejection falters. There is a low pass
> > filter in the audio (RC at gain control input), but
> > the drop off is not all that much up to 5 Kc. I chose
> > to fiddle with the phase shifter so it behaves better
> > up to 5 Kc. What I did was to bridge R15 (400 ohms)
> > with 1.8K, and then make a series R-C consisting of
> > 390K and 180pf. The RC goes between PS-1 pins 2 and 9.
> > After the mod, go back and set up the pots again, and
> > see if things are close to 90 degrees phase shifted up
> > to 5 kc.
> >
> > I just got off 20 meters USB with my 20A.....
> >
> > Regards,
> > Jim
> > WD5JKO
> >
> > >
> > > -----Original Message-----
> > > From: central-electronics-bounces at mailman.qth.net
> > >
> > [mailto:central-electronics-bounces at mailman.qth.net]On
> > > Behalf Of K3PID
> > > Sent: Sunday, January 29, 2006 11:12 AM
> > > To: central-electronics at mailman.qth.net
> > > Subject: [Central-Electronics] CE 20A Phase shift
> > > question
> > >
> > >
> > > Setup:
> > >
> > > CE 20A to dummy load
> > > 1 KHz audio input tone
> > > scope accross the output of the phase shift network
> > > ( 90 deg shift)
> > >
> > > Observation:
> > >
> > > The X-Y scope display shows a very nice circle which
> > > varies in diameter with
> > > the amplitude of the input and also varies in
> > > diameter (although less
> > > variation) with the input frequency. The inside of
> > > the circle always looks
> > > to be a well defined circle but the thickness of the
> > > line has some variation
> > > around the circumference.  Thin and sharp at 6:00
> > > thru 10:00 and a little
> > > thicker from 12:00 to 2:00.
> > >
> > > The two signals look great in the time domain just
> > > like two sine waves out
> > > of phase.
> > >
> > > Question:
> > >
> > > Is there something going on that needs correction?
> > >
> > > K3PID
> > > Ron H.
> > >
> > >
> > >
>
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