[JMS] 90800/p.s. progress

Sat, 21 Feb 2004 22:18:28 GMT

Today, I built up the "first half" of the ongoing 90881 transmitter/amp story. I finished building the filtering circuits for the modulator power supply. Two 33 uFd caps with a 12 H .240A choke and the 300 VCT/250 mA xfmr comprise this setup. Also, I went to the local electronics store. They were out of my required 6k/15 watt units so I settled for a 6.8k and 5.6k 25 watt set. The extra watts should give me plenty of cushion if things heated up too much. Well, I did jumper up everything. Here are the results:

450 VDC out. In the bleeder string, I placed the 6.8k ohm resistor as the bottom. That had an output of exactly 240 VDC, perfect for the modulator! 

Well, having done all that be satisfied with the test, I decided to take out James Millen transmitter 90800 (s/n 4068) and test it out with this supply. I had ten 9 volt batteries in a string for an old regen radio project and I used five of these for the bias supply (the 90800 required -45 VDC at the grid of the 807). I found the two 42082 coils needed for 75/80meter operation. I also slipped in a new 807 and a working 6L6G tube. With no VFO on hand, I got an FT-243 80m CW xtal (op freq at 3655 kcs) and used some solder (not melted) to act as shims to fit the xtal into the 5 pin VFO socket. Switches were set to cathode out and I fired up the transmitter. The best I could get out of this transmitter is 60 mils at 430-440 VDC. That's about 25 watts input or about 16 watts output (at 66% efficiency). Well, to me, that was great news! That means I could use this power supply with both the modulator and the transmitter. Not only that, I had just about the ideal drive for the power input to the 90881 812A pair. So, things seem to be going along just fine. I brought #4068 out of its dormant state (which was probably not transmitting for so many decades) and made the power supply for both units! I guess this project will be doing just fine. 

73 de Cal, N6KYR. 

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