Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
RF Choke question




 
The AM Forum
January 16, 2019, 01:48:31 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: RF Choke question  (Read 560 times)
0 Members and 1 Guest are viewing this topic.
W4RFM
Member

Offline Offline

Posts: 308



WWW
« 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
Logged

BOB / W4RFM
KA2DZT
Member

Offline Offline

Posts: 2157


« 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
Logged
WBear2GCR
Member

Offline Offline

Posts: 3617


Brrrr- it's cold in the shack! Fire up the BIG RIG


WWW
« 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
Logged

_-_- bear WB2GCR                   http://www.bearlabs.com
KB8TWH
Member

Offline Offline

Posts: 13


« 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
Logged

Ed, KB8TWH
w4bfs
W4 Beans For Supper
Contributing
Member
*
Offline Offline

Posts: 1239


more inpoot often yields more outpoot


« 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
Logged

Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
KK4YY
Member

Offline Offline

Posts: 117


Gol’ na vydumku khitra


« 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
Logged

Without having a purpose in life, you're just a meatball wandering through space.
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1234


« 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
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
W4RFM
Member

Offline Offline

Posts: 308



WWW
« 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.
Logged

BOB / W4RFM
W4DNR
Member

Offline Offline

Posts: 48


« 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
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7697


"Let's go sailing, Tommy!" - Yaz


« 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
Logged

“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
KD6VXI
Contributing
Member
*
Offline Offline

Posts: 1938


Making AM GREAT Again!


« 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
KD6VXI
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 7018



WWW
« 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.
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
KK4YY
Member

Offline Offline

Posts: 117


Gol’ na vydumku khitra


« Reply #12 on: January 06, 2019, 09:23:46 AM »

That's the "Crazy Bias" scheme.
Oh I get it. Grin
Logged

Without having a purpose in life, you're just a meatball wandering through space.
KB2WIG
Contributing
Member
*
Offline Offline

Posts: 4253



« Reply #13 on: January 06, 2019, 12:28:04 PM »



" Franklin Bias " ?


KLC
Logged

What? Me worry?
W4RFM
Member

Offline Offline

Posts: 308



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

I certainly appreciate the "un Biased" input.  Grin
Logged

BOB / W4RFM
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.059 seconds with 18 queries.