[JMS] Power supply design concerns

[james_millen_society@mailman.qth.net]

All:
Just wanted to keep you abrest of the situation with regards to my Millen transmitter project. Yesterday, a 10.5" by 19" steel rack panel came in (which will be the basis for my power supply chassis). I ran into some design problems along the way. Thinking that I can supply both the modulator and transmitter, I started on paper with my 600 VCT 250 mA Hammond transformer and a 12H/240 mA choke. 

The minimal bleeder value to use the 12H choke is 35 mA (if you use the formula B+/L to give the current in mA). So 420 VDC (that's 300 * 1.414) div by 12 H. I think this is to ensure that the choke will already be in its operational curve to ensure regulation. On CW, this will be important. I did build into the circuit a voltage divider so to maintain a minimum of 35 mA, a 12kohm resistor at 30 watts should be used. That meant that I could use a string of two 6kohm resistors to split the 420 VDC down to 210 VDC (ideal for the modulator screen voltage). 

Unfortunately, I started to add up the current draw needed to power the transmitter AND the modulator: 35 mA for the bleeder, about 40 mA for the mod screen, probably about 140 mA for the modulator plate (if a minimum of 55 Watts is needed for cathode modulating the 90801, and another 90-100 mA for the transmitter. That is well beyond the 240 mA limit for the choke! Not only that, if the supply gives the transmitter around 400 VDC for the 807 plate, were talking about a 40 Watt power input. The 90800 transmitter seems to exhibit a 66% efficiency so that translates to 26 watts out to the 812A's. You need only 17.5 watts to drive the 812A's according to the Millen data sheets. Already, it looked bad. Luckily, I had another transformer lying around that would take away some of the "pressure" on the supply. I have a 375 VA/205 VAC (no c.t.) transformer lying around. Instead of loading the power supply with both the modulator and the transmitter, I decided to power the plate supply of the 90800 with this power supply setup. Besides, power input to the 812A's comes out to be around 19 watts, much closer to the spec of the Millen data sheet! Well, now the modulator supply seems to be fine with 35 mA at the bleeder, 40 mA at the screens of the 6146's, and 140 at the plate. That's 215 mA total current. I have a little 25 mA headroom but to be quite honest, I don't think I'll be using the full 215 mA on this supply. So, that's the situation with the power supply. Looking forward to your thoughts. Cal, N6KYR.

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