John,
That sounds like something I should look into. My 20V3 transmitter was never meant to be cycled on and off as it is here. The plate transformer and others in this rig would probably live much longer with inrush currant limiting. I'll have to look at the manual and schematic to figure out how many and of what current I'll need. I think the parts list gives the transformer ratings.
Since the transformers are all 240 volts I'll need one limiter on each ac leg to each transformer? Thanks for bringing this up.
Mike
I have been using these thermistors in receivers (R-390A) etc, and they do a great job of taming that initial inrush surge. Stops that thumping, "cheewang" sound. They do get hot..VERY hot, but they are designed to work at high temps. I agree with John in that they should be installed in ALL OB receivers.
Now, I recently have been refurbing and modifying a DX-100 - SS supplies, etc. I put a 16 ohm cold at 4 amp Thermatronics thermistor in the HV plate xfrmr primary and it works FB. When I key up the rig, the plate current rises fairly quickly to full value without a huge inrush surge, the thermistor quickly becomes very hot and all is well. The problem I forsee with this method of inrush taming is when the QSO is not an old buzzard type of roundtable. If I allow 30 seconds to a minute or so, the thermistor cools off and is ready for another round of suppression. If I key up for a few seconds, unkey for a few seconds, then key up again, (break-in ops), the thermistor is still hot and presenting very low resistance so it is useless for it's intended purpose.
When I did up my Gates BC-1G a few years back. I used SS time delay relays that short out 10 ohm 25 watt resistors (one in each leg) after about 1/2 to 1 second. They work FB for break-in with the big Gates that was certainly not intended for this type of operation. I even added one to limit the initial surge of the filament inrush. I have since added this system to my T-368. These SS TD relays can be had fairly cheaply on Ebay. I try and get the ones that have a fairly low time range (.1 to 5 seconds). You can get the relay, the socket and 2 resistors for $10-15.
Here is a pic from my webpage of the one for the plate step-start in the Gates:
http://webpages.atlanticbb.net/~w3np/images/bc1g/Gatescontrol906.jpgI wished there was a way to quickly cool the little thermistors for more effective use in smaller transmitters as they are cheap, easy to install and don't take up much room.
Any thoughts???