[JMS] Millen 90800/90831 power supply and 90881 amp

[james_millen_society@mailman.qth.net]

All: 
I am gathering the parts for the 90800/90831 combo to use with the 90881 amp. First off, I was concerned about power supply design concerns. I used to think that to make a "good filter", one need only slap on two appropriately voltaged caps and a choke. Taint so! I found out that it is important that in order to have good regulation involved, the choke must be appropriately loaded, especially when operating CW (since the load is not always constant at that point). Well, with the 600 VCT 250 mA xfmr I had on hand, I was going to full wave rectify the unit. That meant that the total voltage out should be 300 VAC times 1.414. Turns out I would have 420 VDC after the two diodes. That meant that with a 12 H choke I had around, the minimal amount of current for the choke would be 35 mA (or 420V/12H). With the choke, however, I may be in for a slight voltage drop but regulation should be fine with the p.s. slightly loaded. Now with the xfmr rated at 250 mA and the choke rated 240 mA, that means now I have a headroom of 205 mA. In any event, the transmitter and modulator combo needs at least 400 VDC and 200 VDC for the plates and the screens. I think the supply will handle both units quite adequately, including the filament output of the xfmr (which is 6.3 VAC at 6A but I won't be using all of that). 

I'm also thinking about running another filament transformer "bass-ackwards" from the 5.0 VCT tap so I can have a bias supply for both the transmitter and the amp. They need -45 VDC and -48 VDC respectively (with the amp at class B p-p operation). However, I'll probably switch the bias over to something higher than -48 VDC for the amp during STANDBY periods. I know the grids of the 812A's will handle about -100 VDC so I'll switch between the -100 and -48 via a relay. 

The bridge rectifiers are on the way. They are rated at 6KV at about .5-.6 amp. I'll team them up with three 560 uFd/400 VDC caps in series along with the appropriate voltage dividers. I'll have to figure some surge current protection but the ARRL Handbook as a few of these configurations handy. 

O.k. That's about it. I'll keep you apprised of the situation. Cal, N6KYR. 



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