CE 20A QRO Update,
I am nearing completion of my QRO'd second 20A. Many changes
from below, and lessons learned:
The first rig was extremely sensitive to power line voltage changes,
and I had to put a bias pot in the back to adjust the PA idle current.
Still the maximum power available was changing day to day as the
exciter, and PA B+ changed. I also did not like the external power
supply requirement.
I was able to get the higher B+, and other voltages using the
original power transformer. I now have +550 (at 50 watts output), +375
(3 tube series regulator, good for 80 ma), +290 (280-310 for line V
117-123), -100 V regulated (5 watt zener), and -17 volts regulated
(adjustable -10 to -30 with TL431 adjustable shunt regulator). The
unregulated voltages +550, +290 are both from choke input circuits, so
the regulation with these is decent. The Key up high B+ only rises to
about 600 volts. Funny thing here is that with choke input filtering,
the power transformer runs cooler than a stock 20A which uses a
capacitor input filter circuit. Lowering the B+ on the low level stages
also cuts way back on the heat produced. Recall that a stock 20A will
run the low level stuff at ~ 400 volts B+ with today's 120V + line
voltages. The major problem I encountered is finding room for an added
choke, and discovering that stray magnetic fields from the choke
migrated into one of the audio transformers. This caused 120 hertz hum
in my carrier null, and the null was not as deep, i.e. more carrier
output on SSB. I had to find a suitable choke in a metal (not aluminum)
enclosure. The problem was solved. Seems that the CE audio transformers
use a non ferrous metal enclosure. Sigh!
As for tubes, I first chose the EL34/6CA7 because they are cheap
($9-11), and have the beefy ratings I needed. Oops! The plate to grid1
capacitance is 4-5 times that of the 7591. The EL34's worked like a
champ on 160 meters, and on 80 they fell apart. Using one tube on 80m
also worked good (about 30 watts output), but that feel apart on 40
meters. It might be possible to beef up the 12BH7 cathode follower...
Instead I went back to the GREAT but Pricey 7591's. It was like getting
out of a cold swimming pool, and then going into a nice heated Jacuzzi.
Got 50 watts out on 160, 80, 40, nice and linear. Boy I like those
7591's. The Cgp of the EL34 is about 1.1 pf, and .25 pf for the 7591.
Interestingly the 9 pin version of the 7591 is the 6GM5, and it has .13
pf Cgp, and the Novar version 7868 has .15 pf Cgp. The original 6AG7's
are 0.06 pf Cgp. I also tried a 6L6 with similar results as the EL34,
except it took more grid swing (Lower Gm).
If anyone is interested, contact me for additional details.
Regards,
Jim Candela
WD5JKO
Here is my Revision 'A' article:
>
>
>
> Hi all,
>
> I have been working on a 20a for some time now, and I'd like to
> share
> something here with the group.
>
> The 20a has low RF output from the 2 X 6ag7 RF linear amplifier
output.
> Mine
> was about 3 watts AM carrier, or maybe 12 watts PEP on 80 meter SSB.
> Getting
> a 600L linear amplifier is next to impossible, so I took another
approach.
>
> I gave a 4 times 572 G-G linear that needs about 80 watts in for 800
watts
> out (10 db gain). If only my 20A could put out more...
> Using my linear on AM with the 20a gives me 30 watts, and heats the
ham
> shack. I did this for a while, and people laughed at me for such a
silly
> lash up.
>
> Here is what I have done:
>
> 1. replace 6ag7's with 7591's, and rewire tube sockets (could use
7868
> with
> socket type change)
>
> 2. Add extra B+ supply (200v from small 150 vac transformer, bridge,
and
> cap) in series with existing 300v for ~ 500v 7591 Plate voltage
>
> 3. Remove 10k resistor across 6ba7 tank circuit. This provides more
drive
> to
> 7591 grids
>
> 4. Adjust 7591 grid bias for -22 volts, and screen grid volts for 380
> volts
>
> The results have been stunning:
>
> 50 watts CW output (B+ at 475V), or ~ 60 watts PEP SSB (B+ idles at
550v)
>
> 10 watts AM with plenty of headroom for > 100% + modulation. At 10
watts
> out, each 7591 dissipates 15 watts (19 = max), and the % efficiency is
25%
> (40 watts DC plate input for 10 watts rms output). The efficiency
doubles
> at
> 100% modulation (sine wave).
>
> 15 watts AM at 100% +/- modulation, and 150 watts from linear
amplifier.
> Efficiency a little higher here.
>
> The 7591 is about the size of a 6V6, yet has higher Gm (gain), and
plate
> dissipation. It fits nicely in the confined space of the 20a.
> Unfortunately
> the 7591 can be pricey, as the audio nuts have bid the price up. Look
on
> EBAY, they are there. The 7868 is the same, and cheaper, but requires
a
> socket change.
>
> If any of you are interested in this, and want more details,
just
> contact
> me, and I'll share it with the group.
>
> Regards,
> Jim Candela
> WD5JKO
>
> _______________________________________________
> AMRadio mailing list
> AMRadio@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/amradio
>
> _______________________________________________
> Central-Electronics mailing list
> [EMAIL PROTECTED]
> http://mailman.qth.net/mailman/listinfo/central-electronics
