The AM Forum

John FXE has a new HB 813 rig on the air.

It has a big arse commercial mod tranny in the hole.

John reported that when he unkeyed, there was a huge pulse and it tripped the overload relays, pegged the plate meter. He had installed a manual (iirc) relay that he keyed across the mod iron before unkeying the rig to kill the pulse. There's been some discussion on the air about the problem...

The cathode of the 813s has the usual relay that shunts a resistor to when it is time to operate, and opens it, making bias and turning the tube off when it is time to standby. FXE does not key the B+.

Earlier today in a QSO on 75 I suggested that he might try a cap across the resistor (50kohm) so that when the relay pops open, the tube doesn't instantly go offline, creating a nice pulse. Gave John a call after figuring out a time constant that seems about right - between 0.5ufd and 0.1ufd - and suggested he try it and see if it helps.

Got a call back a little while ago, and he found two 0.33ufds that he put in parallel, and it seem that there is now no pulse on the plate meter, the overload does not kick out, and he says there is a small blue spark across the relay contacts that he had in there.

He's got a spark gap on the primary, and we discussed putting one on the secondary now, and getting the relay out of the secondary path entirely.

There was also some discussion about relay timing and checking to see if the Ranger that drives the big rig is still dumping RF into the grids when it's being unkeyed... might be adding to the problem?

Anyhow, this seemed to fix the problem by making the slope of the pulse on unkey somewhat slower. John's going to try some slightly larger values, and see if the small blue spark he is now getting can be reduced even further without compromising the time it takes for the rig to fully unkey...  ;D

Since I haven't heard of anyone trying this, (although it probably has been done many times before?) I thought I'd mention it in case someone runs into this problem.

In the meantime FXE's rig is putting out a FB signal and sounding great!
I'll try to get over there and post some pix of the beast... if it didn't get foggy here this afternoon, I'd be there now. :'(

        73s

           _-_-WBear2GCR

Yup it kicks butt. Also I think his antenna realy opens up before all the energy is out of the choke. The vlotage then goes very high without a load. Slope control is a good idea as long as the antenna stays connected during the slope.