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Author Topic: RF Choke question  (Read 5031 times)
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W4RFM
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« on: January 03, 2019, 09:27:29 PM »

In building the circuit shown, RFC 3 is called for a National R-60 4 uHy choke.  Well I have a 2.4 uHy, and a 5.5uHy. Is this a critical value or just "close enough" will do?  Thanks. Bob
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KA2DZT
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« Reply #1 on: January 04, 2019, 09:48:00 AM »

Probably not critical just use the 5.5uhy.  The builder of the original circuit probably used the 4uhy cause that's what they had on hand.

Fred
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WBear2GCR
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« Reply #2 on: January 04, 2019, 10:12:56 AM »

It's a low pass filter, designed to get the RF off the B+ line.

Cap to ground, choke, cap to ground.

You can figure out the curve of the filter it forms.
Which will be a 3rd order, or 18db/octave. There are free online calculators that graph these
things... (you can guess at the input and output Z being the Plate Z for the tube and the B+
side being related to the voltage and current available - ohm's law)

Adjusting the cap value closest to the B+ input can be done to optimize
the filter curve. And to lower the -3dB point of the filter.

Also, although not necessary, one could add another choke and cap to ground in series,
making it a 30db/octave filter.

The larger the choke, the lower (better here) the rolloff frequency goes.

                          _-_-

But as Fred said, use the larger one! Cheesy
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K8DI
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« Reply #3 on: January 04, 2019, 11:04:45 AM »

In building the circuit shown, RFC 3 is called for a National R-60 4 uHy choke.  Well I have a 2.4 uHy, and a 5.5uHy. Is this a critical value or just "close enough" will do?  Thanks. Bob

Notwithstanding RFC3, that schematic looks like it has the bias supply diodes backwards!

Ed/KB8TWH
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w4bfs
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« Reply #4 on: January 04, 2019, 11:56:57 AM »

not so fast ... the first order (ideal) approximation is satisfied by what has been said .... the problem is the second and third order approximations (real world) cannot be ignored .... it is necessary to explore behavior of the circuit with a grid dipper or impedance bridge looking backwards to see if there are any parasitic resonances that may excite .... a typical example is a nice big plate choke (rfc2) having a series resonance near the 10 to 15 meter bands .... the problem gets worse as things get bigger, etc
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KK4YY
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« Reply #5 on: January 04, 2019, 05:33:08 PM »

Notwithstanding RFC3, that schematic looks like it has the bias supply diodes backwards!

Ed/KB8TWH

I believe the "SR" stands for "Selenium Rectifier". It does appear to be drawn backward, although the "K", indicating cathode, looks correct to me.

Don
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« Reply #6 on: January 04, 2019, 06:31:34 PM »

And let's not forget that the small value RFC3 was also used as a "last chance" fuse with a nominal current rating of 2 X IP max.



Phil
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W4RFM
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« Reply #7 on: January 04, 2019, 08:16:56 PM »

I thought the bias rectifiers were backwards also. should be negative voltage appearing in that circuit.  I would have wired them correctly, this is not the first mistake I have encountered in the Radio handbook designs. ARRL book stuff always seemed to work "as-is". West Coast handbook, I dont know...

Also this TX is only going to be on 80 meters, so higher band problems are not a concern.
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W4DNR
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« Reply #8 on: January 04, 2019, 08:32:15 PM »

The Collins 30S-1 Amp shows the bias diodes "backwards" also.   

Selenium vs silicon   

The Collins purists will dig out the selenium from the diode case and replace  with a 1N4007 and cover the evil deed with a yellow epoxy ( leads reversed, of course ).

I just let the guy on the other end guess if I did a proper ( undetectable ) upgrade.


Don W4DNR
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« Reply #9 on: January 04, 2019, 11:12:00 PM »

The selenium polarity confusion (and fireworks) has gone on for many years.  I've never used them - always replaced them with regular silicon. I think I made the same polarity mistake years ago and was baffled....

Check out this thread where they discuss the how and whys of what really happened:   Grin

http://audiokarma.org/forums/index.php?threads/st-70-selenium-rectifier-vs-diode-polarity-enigma.741937/


BTW, RFC3 and cap are not really needed and most rigs do not have them. But any additional filtering is always a good thing, why not?  The RFC3 choke does not take as much abuse as the main plate choke since the plate RF is about blocked/attenuated/bypassed by that point.  Just about anything will work there.



Here's a sample of the selenium thread:

"This problem dates back to the old (erroneous) way of teaching electron current flow, and as such had the positive output side of a selenium rectifier marked with a + symbol and red dot. After all, selenium rectifiers were relatively new at the time, and that was the point at which you could first measure a positive DC output in a conventional rectifier circuit of an actual unit, so it made sense -- at the time. Notice also that the manual makes no reference to the terms "anode" or "cathode" as you are doing. Therefore, it (only) becomes confusing today when you start applying the terms and indicators of the 60 year old text, with today's correct understanding of true current flow. Today, with all symbols based on absolute current flow, the old selenium rectifier's + terminal equates to today's cathode/negative/banded terminal on a silicon diode. Therefore, both you and the text are correct for the period of reference: The manual text AND schematic are correct -- the manual being correct for the installation of the component used produced at the time the kit was produced, and the schematic being absolutely correct regardless of time period. Therefore, the bottom line answer to your question is: Follow the schematic -- and now you know why."



T
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« Reply #10 on: January 05, 2019, 10:22:34 AM »

One of the tricks I've used is to replace the RFC3 with a well bypassed 5 to 10 ohm wire wound.

It's sacrificial.

It has inherent inductance.

It will limit current if you have a meltdown.

5 ohms is sometimes lower R than some of the chokes I've seen used! It's thr first rf choke, closest to the anode, that takes the brunt of the rf.  If you have residual rf getting to RFC3, you probably have a problem with something else.

--Shane
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« Reply #11 on: January 06, 2019, 12:56:25 AM »

In building the circuit shown, RFC 3 is called for a National R-60 4 uHy choke.  Well I have a 2.4 uHy, and a 5.5uHy. Is this a critical value or just "close enough" will do?  Thanks. Bob

Notwithstanding RFC3, that schematic looks like it has the bias supply diodes backwards!

Ed/KB8TWH

That's the "Crazy Bias" scheme.
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KK4YY
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« Reply #12 on: January 06, 2019, 09:23:46 AM »

That's the "Crazy Bias" scheme.
Oh I get it. Grin
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« Reply #13 on: January 06, 2019, 12:28:04 PM »



" Franklin Bias " ?


KLC
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W4RFM
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« Reply #14 on: January 06, 2019, 01:54:58 PM »

I certainly appreciate the "un Biased" input.  Grin
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