[AMRadio] 833 AM rig

John Coleman jc at pctechref.com
Fri Feb 13 10:57:19 EST 2009

        As I recall, the swinging Formica bar was about 3/8 inch across and had feed through solder eyelets.  I'm sure it was roommy enough to allow the center lead to pass around with out much trouble.  I believe that his reasoning was to allow for more room and also to reduce the capacitance change at the center of the tank coil.  His belief was that this would cause an unbalance in the tank, little though it may be.

        Otis has never run push pull 833 RF finals, as for as I know, but for sure never in the rig since I have known him.  He said "I never want to be accused of running more that the 1KW input legal limit"  Of course the 833 could have run more than the 1KW input it would not have been significant enough to make a large difference.  And most of the tubes he had were pulls that would hardly hold grid drive up for high plate current.

        The double RF choke for shunt coupling is a pain.  I had so much trouble with hot spots on chokes that I wound a special RF choke for the purpose.  It was wound on two 1.5 inch by 5 inch ceramic forms.  I wound with #20 enamel wire 4 sections each, about 15 or 20 turns on each section, on each form.  The two forms were mounted on a plexiglass plate supported in the middle by a cermic insulator.  The tops of the coils went to the plate of the 304TLs and the common connection at the bottom went to another RF choke of high inductance but not a low capacitance choke.  The other choke went to the bypass and then to the modulated high voltage.  I found a real buy on some 500 pf 20KV coupling caps rates at about 7 amps of RF.  These monsters were about 3.5 inch in diameter and 2.5 inches thick with a metal plate on each end and a screw hole for a 1/4-20 screw.  The coupling capacitors then went to the ends of the tank circuit.  The center of the tank coil had a RF choke to ground.  With the coupling capacitors and the plate RF choke bypass the total capcitance as seen by the moulator was about 2000pf which is pretty high for most rigs but I ran about 1600 volts at 600 ma so the modualator load was a low Z and a little extra capacitace didn't hurt much.

        The 304TLs showed little color and the output was just under 800 watts.  The low impedance load that the final represented to the modulator was the reason for four 813s class AB2 in the modulator instead of just two.  I soon found that with good audio design in the speech amplifier and coupling that more voltage was needed to get the positive peaks up to where they would not flat top when the negative peaks wher just about the cut the carrier at 100%.  After raising the modulator supply voltage to 2700 volts I was able to keep the peaks from flat topping on the scope at 3 time the carrier level.

John, WA5BXO

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