Hello all!

This is my first post here so please forgive me if this is posted in the wrong spot. If wrong please let me know where this should be!

I need to rebuild the plate parasitic suppressors, both sides of course.

I can't find any 47 ohm, 2 watt carbon composition resistors. I need two. I did find a listing on eBay for 10 for $30, but I only need two. I suppose I could re-sell the other eight, but that's a pain in the neck.

RF Parts is also out of them as well. However, they do have others that are close to that value and I'm not sure if it really matters all that much what the value is as these mainly provide a form for the parasitic coil in the first place. Does anyone have a recommendation about choosing other values?

Thanks in advance and 73,
David NE7G, ex-K6DMA

Contact Just Radios (click on the ad on the right -->)

I'll try to whip up an order, but there is a $25.00 minimum order
I'll have to figure out what else to order.

Thanks for pointing that out!

73 NE7G

PM me. I may have a small stash of that vaiue but it'll be Tuesday before I can rummage for them.
--Wayne

Thank you. I might have a local source, but I'm not sure at this point. I need two.

OK. Unless I hear that you found some, I'll plan to look for them Tues.

VHF parasitic suppressor design is tube dependent in that the L and C values relate roughly to where the tube oscillates in frequency. But, the range is somewhat broad and therefore the L and C values don't have to be precise. You don't have to have exactly 47 ohms for example. Root around; maybe you have a pair of 100 ohm 2 watt carbons you can parallel. Those will work fine. For coils I like either copper strap cut to a width of 1/4 inch or AWG 14 solid copper wire with the insulation stripped off. In this case I'd go with the wire. Use something for a coil form to get the diameter; wind the coil, solder everything together and away you go. Any resistance 50 to 100 ohms is okay but make sure you use non-inductive resistors at least two watts.

Rob
K5UJ

I think carbon film would be OK.

Mouser has some: https://www.mouser.com/ProductDetail/TE-Connectivity-Holsworthy/CFR200J47R?qs=DDevMFOh4svFPIRYYOSbXg%3D%3D

I believe you can get away without a minimum order if you send a check/mail order?

Rich

Modern resistors have increased inductances over the vintage carbon film resistors that were used in those kits. Even a so called modern carbon resistor is spiral trimmed using lasers and has different characteristics than the originals that came with the kit. There are some options that will work, but I do not recall the exact specs. Some searching for W8JI comments on suppressor resistors, or a deep search on EHam.net might yield more definative answers.

Surplus Sales of Nebraska has them $8.00 each

Eight dollars for a resistor?!?!? I think I'd scrap this before I'd pay that. Worth it's weight in GOLD. Fair Radio Sales has never been high on any list if mine.

Yes, I know that modern resistors have many differences, so that would be my last choice.

The antique radio parts place in the right margin is my first choice, but a local says he has some. I'll wait on that as my first choice.

We have a huge number of older Hams (like me!) here and several collect older radios as a way to preserve them. It's very dry here and thongs don't corrode very quickly. He has HUNDREDS! (really)

Thanks for the help es 73
David NE7G

Greetings to Dave and the Forum:

Someone (I am embarrassed to say that I don't recall who) gifted me a batch of NOS Allen-Bradley 47 ohm 5% 2 watt carbon resistors some time ago... when I was working on an SB-200 in fact.

Anyway, I have a couple; they are drifting a bit high but still within 5%. One is 51.2 ohms and the other is 50.3 ohms. If you are interested and you would be willing to defray my mailing costs, I will send them to you. Please PM me with your address.

73,

I am amazed. 2 watt carbon comp resistors seem to be a thing of the past!

I checked both Mouser and Digi-key.

Rich

Rich: yes, you're right. Just old stock in someone's parts bin

Anyway a local ham came up with FOUR. So, I'm more than covered.

Thank you all for your suggestions. Maybe I should fabricate two others and make them available. Or I could just hoard them for a rainy day. I do have two of these.

73, David NE7G

Why do you think you need new parasitic suppressors anyway" . Discoloration means nothing. Do you have a parasitic problem? Don't fix it if it isn't busted. If you do need new, $16 is nothing in the world on amplifier restoration. A resistor is every bit a complex part as any other an an engineer can spend a career on resistor desighn and application. I would worry much moire about your SB-220 power switch but it is just a switch, right?.

Because the wire broke of at the resistor leaving me no lead to complete the connection the the plate cap.

as a temporarily expedient if the coil/resistor connection is still good, as it usually is , Take some solder wick, work it through the end of he coil with possibly a little prying with a very small screwdriver or awl, solder it to the resistor/coil end and use it. I have a SB-220 running like that for years now because I am lazy. Small solder wick will easily carry the l few hundred mils of current. Good luck with the SB-200, I have had several and they work well. PS- if the coil/resistor assembly has actually broken from the resister body than I haven't been helpful and I apologize and you have a very unusual situation.

2 watt, 47 ohm, ceramic, $1.34 a head. Better than the 'ol carbon.

https://www.mouser.com/ProductDetail/KO ... 94Wg%3D%3D


KLC

These have a metal cap on each end which will probably affect the coil measurement.

DM

With these old amplifiers, the good 'ol carbon composition resistors have to be used. Someone with a whole lot more smarts than me would have to reengineer the function to arrive at something suitable for a parasitic suppressor. Changing the basic design of the resistor would have consequences.

Some (including me) might tend to brain-storm possible scenarios. Mine would be that the end-caps on the resistor would act as capacitors and force RF at around 300 watts or so to arc across causing the NED effect (Noise Emitting Diode - makes a loud bang....once). Then I'd have to start all over again.

Anyone want to try this? It would be an interesting experiment. Electronics is easy...unless it's not.

Anyway, on to the next challenge.

David NE7G