|[AMRadio] N3RHT Shack Update|
recycler at swbell.net
Sun Mar 6 09:09:36 EST 2005
I have the same phenomenon with the 872's or 575's (whichever
happen to be installed) in a homebrew rig. 3 light but one is
dark, and the phenomenon stays at that socket. No obvious other
issues.. how curious.
> From: "Merz Donald S" <merz.ds at mellon.com>
> Subject: [AMRadio] N3RHT Shack Update
> Just thought I'd post an update on recent activity here....
> I have this homebrew transmitter (I didn't build it) that has a Globe King 500B RF deck (running a 4-400 in place of the 4-250) and the rest home made. It's in a 6 foot rack with PP 811A's for modulation. The rack was professionally re-painted in black powdercoat and it is the nicest looking rack I have ever seen. It has brackets to support each chassis, locking rear door and tapped bolt holes in the rack rails.
> Unfortunately, after doing such a nice job on the cosmetics, the builder seems to have gotten lost in the electronics. The AC wiring was a lamp cord disaster and had to be all ripped out. High voltage connectors were old-fashioned ceramic posts with standard hookup wire used to make HV connections. The speech amp had every manner of abominable construction practice on display. There was no push-to-talk. He did not bias the 811A's but instead ran the (separate) modulator supply on a Variac to keep the plate voltage in the 1200V region. The enamel on the variac's windings had turned purple--so it was obviously under some strain. The final plate volt meter was junk.
> All of that has now been undone, though there is still plenty to do. The rig is putting out 275 watts of glorious AM as of last night. The 4-400 is being run at 1800 volts--so it is loafing. The 2 supplies run two 3B28 rectifiers each. 3 of them run with no visible indication at all. You can't even see the filament. But one gets a dramatic glow under load. I have not seen this before--is that gas in the tube?
> I'm dumping the Variac and biasing the 811A's. But I haven't added the bias supply yet. I was running the 811A's on 1500 volts with zero bias last night with no apparent ill effects. But there's no reason to push them like that.
> My buddy Rich Sperling rebuilt the speech amp and did a great job. He added push to talk and some control wiring that will make the rig easy to use.
> So with any luck I should have the rig on the air sometime in April. This will be the highest power AM TX in my shack when it's done.
> 73, Don Merz, N3RHT
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