[AMRadio] N3RHT Shack Update

Merz Donald S merz.ds at mellon.com
Fri Mar 4 12:18:56 EST 2005

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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