[Central-Electronics] My QRO 20A update

[Jim Candela]

     Last week my 20A with the 7591 finals crapped out pretty good. First the frequency started shifting, and then the audio went to heck on SSB, and on AM no audio. Well I went in circles for a while. Turns out I had two problems.


1.) One of the modulation transformers opened up. I had 3 spares in a box, all used; two looked good, and the third had an open primary also. I put one in and also did the para-feed trick, and used a 22K 1W plate load resistor. To get the thing to bias at 1/2 plate voltage I tried a 12AU7 instead of a 12AT7, and that got the plate voltage to 150V. 

The change was rather simple. After adding the two 22K 1W resistors from each modulator plate to the B+, I took the two RED modulation transformer wires off the B+ terminals and keeping them together, I then bypassed that junction to ground with a 1 uf 275V AC rated polypropylene capacitor. 

The lower gain of the 12AU7 was offset with a higher audio pot setting. One thing I've seen now on multiple tube rigs is a form of waveform distortion when the audio gain pot is at the center. At the center the wiper is at the highest impedance, and depending on the load impedance, this can cause some distortion that goes away at higher or lower pot settings like 1/4 turn or 3/4 turn.  

Years ago on this reflector I sent R-C additions to the audio phase shifter where I did get good response from about 200HZ TO 4 khz. After the change to para-feed it improved to about 150 hz to 6 Khz before the sideband suppression dropped to about 30 db. Over the range of 200 to 4500 Hz the sideband rejection is very good.  Apparently there was phase shift on those transformers that was reduced when the DC magnetization current was eliminated. For sure, on AM, I can pass a triangle waveform with less ramp non-linearity than before. A sine wave is near perfect all the way to 100% modulation with a 12.5 watt carrier level.

2.) The frequency shift seems like a heater cathode short on the 9 Mhz oscillator tube. I use a 6MU8, and after replacing it the problem seemed to go away. I was quite upset over this since I had the Huff & Puff (Cumbria Design) addition to the VFO, and a crystal oven for the 9 Mhz crystal. Prior to this issue I was enjoying a very stable 20A that was ready to talk on SSB after a mere 5 minutes warm-up. Time will tell if I am back to that again. 

I got everything back on the air two hours late for the Wednesday evening 7:30 PM CST 3870 Boat Anchor net... 

back in the saddle again with the 20A..:-)

Jim
WD5JKO