[AMRadio] RE: CE 20A Phase shift question

Jim candela jcandela at prodigy.net
Thu Feb 2 20:01:52 EST 2006

Jim WB2FCN stated in an earlier post:

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

WD5JKO Replies: Yea, my VFO's drift about 800 hz cold to hot, and there are
two thermal time constants. The first is heating around the tubes under the
chassis, and the second is the heating of the oscillator tank that is
enclosed in the metal cage. Mine would improve if I moved that dropping
resistor in the back away from the VFO, or added a little 12 volt DC
computer fan to mix up the air inside that VFO. I went through several 1626
tubes to get mine as good as it is. Some will move 1 kc or more with a 5
volt drop in line voltage, and other 1626's don't care at all. I am sure
glad your VFO is rock stable.

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

WD5JKO Replies: With those carbon 1 turn null pots, a deep null is possible,
but hard to keep there due to temperature sensitive components in the
balanced modulator, and the physical issue with the coarseness of the carbon
pots themselves. The null is limited by hum modulation both 60 and 120 hz.
The 120 hz is easy to fix with an additional RC filter feeding B+ to the two
modulator tubes. The 60 hz component is from stray magnetic fields in the
chassis coming from the power transformer. I did reduce it in mine by
injecting 60 hz in the correct phase and amplitude into the chassis to in
effect "neutralize" that hum as seen in the nulled carrier. The point of
injection is critical, and was best placed at a specific spot on the
balanced modulator diode mounting bracket. My null is at best -60 dbc, and
yours as claimed is as good or better than state of the art equipment
manufactured today.

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 squirrelly effects, and great power out right around 300
watts carrier to the antenna.

WD5JKO Replies: What power does your 20a provide on 10 meters? I assume that
your VFO has the 10m option in it.

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.

WD5JKO Replies: Yes running these on 115 volts is easier on this rig, and
most boat anchor gear as well. Still, and stock 20a plugged into 125 volts
will put out about 20 watts (160, 80m), maybe 15 on 20m, and about 8 watts
on 40 meters (this is where the VFO triples). They run hot that way, and a
fan is required. I took the approach of undergoing a QRO project for the 20a
using different tube(s), and higher plate voltage, but that is another story
that I've covered here before.

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.

WD5JKO Replies: Yes in a stock 20a on 80, 160m, there is plenty of drive
available, and the transition from class Ab1 to Ab2 is at about 12 watts. In
Ab2, the bias source is high impedance, so self limiting occurs, and this is
seen by gain compression between 12 watts and maximum power. Therefore for
AM, 115 vac operation, 3 watts AM (12 watts PEP) works out real nice, and
perfectly matches amplifiers like the T-bolt, and CE 600L amplifiers. At 5
watts AM, a 20a looks like hell on the scope due to gain compression on the
positive modulation. I don't understand how you can achieve 7.5 watts AM on
your 20a when the max cw is 12.5 watts unless you are limiting your positive
modulation to something under 50%

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.

WD5JKO Replies: Amen

Hope this wasn't too confusing.

WD5JKO Replies: No not confusing, but some of what you said sure seems to
indicate that your 50 year old CE 20a is as good as more modern gear, or
maybe your recollection is painting things in a better light.

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