Non Inductive Resistors

[K1ETP]

Anyone have a good source for non inductive resistors of the tubular type?  (Please...not Rip-off Sales of Nebraska).  Looking for something about 2K at 50w.  Also, has anyone had any luck using the heat sinked TO-220 style NI's for grid swamping, especially above 40 meters?

Rich

[WA1GFZ]

Rich the TO220 resistors are fine as long as you keep them cool.
They will be less inductive than a 5 inch long resistor.   
I think you can also get them in a TO247 for higher dissipation.
try Caddak or Caddok Not sure on the spelling. 

[K1ETP]

Frank-- I know some have a ceramic/metal back, but what about the C between the metal back and the heat sink.  Would that cause problems on the higher bands?

Rich

[KM1H]

The big Caddocks are only 100W rated at 25C which means a big heat sink and air cooling or oil if your swamping out a lot of power for a big tetrode.

I blew a few up in testing.  http://www.caddock.com/

Big Globars show on Ebay and the oddball values are dirt cheap plus they love abuse.

Carl
KM1H

[WA1GFZ]

Rich,
I have them in a solid state final as feedback resistors and they work to above 6 meters. The ones in my amp bolt directly to the heatsink without an isolator. 50 watt dissipation heatsink will need a good 200 square inches of surface area to keep it cool. As Carl says you need to keep them cool just like a final transistor.

[K1JJ]

Lich,

The good thing about swamping resistors is that they are usually mounted under the chassis.  If you use forced air with a chimney, (a good idea even for 813's) it's a natural cooling system... :-)  You can get away with much smaller resistors.

I usually use Globars myself, but they are getting harder to find these days.


I take it you are adding some grid swamping for the higher bands in your new 813 rig?  Have you experienced instability problems or just planning ahead for 10M?

T

[WA1GFZ]

Tom Vu,
Look at the feedback resistors on the irb amp. Nice and small but they need a good heatsink. Without a heat sink maybe a couple watts.

[K1ETP]

Tom--

I'm using 4 X 1200 10w in series parallel right now and they work fine but take up room.  I figure if I increase the R value a tad, I can reduce the drive requirements  and free up some room under the RF deck.  No pressurized forced air, just blowing air across the tube which is fine for 2KV.

[K1JJ]

Sounds good, Lich. 

Yes, if the final is reasonably stable via neutralization, you might be able to get away with a much higher value, like 4K -7K.  Try it and see by putting those four 1200 ohmers in series = 4.8K. 


Frank: Those Irb resistors are pretty small -  would need some air and heatsink too.

T

Below:  Yaz modeling his new hand-sewn coat. It gets cold out there sailing... :-)    I used an old down filled coat of mine to make it for him.

[K1ETP]

Tom--

We don't use no stinkin' neutralization ... just 1.2K of swamping. 

[Jim, W5JO]

If you are interested in where to find Globar resistors you can go here:
http://www.globar.com/

Contact them and they will tell you where you can get their resistors.  I am not sure, but I would bet you better wear your wallet when you go shopping.