RCD resistors between HVPS and Plate Choke.

[Naysayer]

I’m nearing the end of my 4cx rebuild -for this year anyway- and the resistor I’ve had between HVPS and plate choke has seen better days. It’s a single long body ww from almost 2 decades ago. After checking the W8JI website I see there is a better part for this function: RCD Part Number: 175P-10R0-FBW 10R 10W with the recommendation of one for every kv.

I've got 2.3kv yet these resistors serve a fuse-like purpose. Would I use 2 or 3? Then again, the RCD datasheet provides no less than 6 options. Does CTR stock these so I can move-on to the next issue?

neil, n2eye

 

[nomadradio]

A "glitch" resistor in line between the HV + and the anode choke does serve to place a current limit on a short pulse, like an arc inside a tube. If it's short enough, the resistor will survive. And if the fault current lasts long enough to pop the resistor, it still served to protect other stuff that's more expensive.

That part number appears to be a 1%-tolerance part. Not sure why the added expense of a high-accuracy resistor pays for itself in this application. The spec sheet isn't telling my why a generic 10W wirewound isn't just as good.

My Heathkit SB1000 has a series string of eight 25-ohm 30-Watt wirewounds for the glitch resistor. I use it to test iffy 3-500Z tubes. It does serve to make the max fault current from a dead short no more than 15 Amps. That's what Eimac's data sheet recommends. Found out one day it works okay. Just pulled it off the shelf and plugged it in. Didn't look inside. Thought a tube was in it. As it turned out, no tube, and the plate heat dissipator was touching the underside of the top cover. When I flipped the switch there was a very brief hum, and the click of the circuit breaker. Took of the cover and my idiot light came on really bright. Turns out it passed this unintended test. The breaker reset, everything still worked. I suppose a single 200 ohm 240-Watt monster wirewound resistor would work the same way, but I had the 30-Watt part on hand from a local closeout.

73

 

[Naysayer]

OK, I think I get your meaning. IOW, a standard 10w 25R ww part is fine for my purpose.

 

[Naysayer]

Sooner or later the roller inductor must go. One problem w/ roller I suspect is that it’s too small for two 4cx300’s. Before it gets replaced (looking at Variometers,UR4LL’s website has a few Russian pieces that may work HF 10-40), I want to experiment with connecting the unused ends of the roller and compare results with the ends unconnected.

As it was (unconnected), I saw ½" arcs up and around the 1/4" ceramic shaft coupler and to chassis from roller shaft Thinking maybe try an RF termination resistor, something like 50 ohms. For fun I ordered a 250w, 50ohm tab style RF Termination Resistor. Can’t be much worse than unchecked arcing. Maybe a Vacuum switch to put it in and out when needed. I’m just spit-balling a bit but for some reason I view a fixed coil & band switch as my last resort. Hard learned lessons are least repeated, perhaps.

Looking at variometers: I see the inductance range more than doubles when the 2 coils are wired in Series and these items appear to be made for doing just that. Most pieces I see have really high inductance values but some are HF workable (.9-25uH, rated for a kw for $50 plus shipping from Russia, pictured).

I want my roller to work but I’m starting to realize it won’t ever hold more than 5-600w.

 

[nomadradio]

Not fond of roller inductors. They seemed sexy when I was young, but they are nothing but a maintenance headache. Hmm. Sexy = high maintenance. Been there.

Variometers are used in the tuning unit at the base of AM broadcast towers to compensate for seasonal influences on antenna tuning, especially in a multi-tower directional site. Definitely want to hear how it works as a pi-network coil.

73

 

[Naysayer]

I have lots of length and height but only a bit over 3-1/2" of width available. This is going to take some time to find a larger unit that fits, width-wise. Meanwhile, I will try shorting the unused turns of the roller with RF termination resistors. Waiting on 50R/250w part now. Perhaps a vacuum switch to only use it when the unused turns are arc-prone. soon. Thanks for your input.