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Author Topic: Parasitic Chokes  (Read 53515 times)
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Steve W8TOW
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« on: January 28, 2005, 09:14:40 AM »

OK guys, who can answer this?
Friend of mine called me last night rebuilding a old amp.
The Parasitic chokes are burned up and wanted to know where to find
"Carbon" 47 ohm resistors.
Our local parts store only stocks flame-proof resistors now...
Will the metal flame-proofs work or to "construct"
a parasitic choke do you need to use the carbons ones? & why?
73 steve w8tow
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
w3jn
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« Reply #1 on: January 28, 2005, 09:56:16 AM »

Allied.  www.alliedelec.com

Quick and easy.  You hafta pay a handling charge on orders <$25, but they still have carbon resistors, high voltage capacitors, etc.

BTW I was looknig the other night in their catalog and found some FB high-power noninductive resistors.  They look like a transistor and are intended to be mounted on a heatsink, but  they claim <80 nH inductance.  Just what the doctor ordered for a rhoimic termination, grid swamping network, or a dummy load.

73 John
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W2VW
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« Reply #2 on: January 28, 2005, 11:08:39 AM »

Quote from: w3jn

BTW I was looknig the other night in their catalog and found some FB high-power noninductive resistors.  They look like a transistor and are intended to be mounted on a heatsink, but  they claim <80 nH inductance.  Just what the doctor ordered for a rhoimic termination, grid swamping network, or a dummy load.

73 John


Watch out for situations where you might accidentally over dissipate some of those packages. They can go off like M80's. I had one on a power amplifier screen supply go kaboom. Pictures at 10.
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Steve W8TOW
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« Reply #3 on: January 28, 2005, 11:35:22 AM »

Yes, 2 watters, will do...
RF parts only has 1 watters
tks es 73 steve
8tow
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
Jim, W5JO
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« Reply #4 on: January 28, 2005, 11:47:13 AM »

A little discussion about these parasitic chokes, please.  I have heard three schools of thought about them.  1.  I understand Art Collins had an understanding about them and used nichrome wire for the coil around globar resistors of about 100 ohms each.  2. The current thing Steve has mentioned, 3 to 7 turns of buss wire around a carbon comp. resistor of about 47-51 ohms.  And

3. Are they really necessary at all?  I have seen several shortwave, high power stations that do not use them at all.  

So are they really necessary, and if so, what configuration?

Are the parasitic suppressors necessary to supress parasitics that result from poor construction practices?
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wavebourn
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« Reply #5 on: January 28, 2005, 01:35:21 PM »

Quote from: Jim, W5JO
Are the parasitic suppressors necessary to supress parasitics that result from poor construction practices?


You bet 100%
But sometimes "poor construction practices" includes tubes, switches, etc., that can't be easy redesigned.
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Jim, W5JO
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« Reply #6 on: January 28, 2005, 04:00:18 PM »

Ok if they exist, what kind of construction?
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WA1GFZ
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« Reply #7 on: January 28, 2005, 04:31:52 PM »

I've used flame proof resistors at RF with no problems for years.
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wavebourn
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« Reply #8 on: January 28, 2005, 06:11:06 PM »

Quote from: Jim, W5JO
Ok if they exist, what kind of construction?


Length of tube electrodes is more than zero, distances between them are less than infinity.
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N8ECR
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« Reply #9 on: January 29, 2005, 09:45:45 AM »

I found an old gentleman in Florida that stocks or has new old stock of resistors, I believe he specializes in them only  1/8 watt to 1000 watt.  Most any thing you want.  Unfortune has it, there is a minimun order

http://minresistors.com
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k4kyv
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Don
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« Reply #10 on: January 29, 2005, 02:42:00 PM »

Art Collins put a ham transmitter on the market in the mid 1930's.  I forget the model number, but it used a single triode (211 or 203A) in the final and ran about 100 watts on AM.  It used the "Collins coupler" tank circuit, an early form of pi-network while nearly all other ham rigs, both commecrial and homebrew, were using inductive coupling in the output.  

What was unusual about this transmitter was how smoothly it operated.  Most ham homebrew rigs were squirrelly to say the least.  The secret:  Collins had figured out that the spastic operation of hf class-c finals was due to vhf parasitics (known as uhf back then).  His rig included the now well-known parasitic choke consisting of a noninductive resistor with a coil of wire wound over it.  With parasitics tamed, the tx tuned smooth as velvet.

I have used 5- to 10-watt carbon resistors for my parasitic suppressors.  Found them in junkboxes at hamfests.  I think they were standard items of manufacture in the WW2 era.  The Gates BC1-T uses a bundle of 1-watters in parallel.  Glo-bars are excellent if you can find them.  I would be leery of "flameproof"  resistors.  It has been my experience that they instantly break out in flame when severely overloaded.  Maybe they don't combust, but the arc that results when they self-destruct sometimes produces a spectacular flame.  Flameproof resistors go excellently with solid state electronics, since both self-destruct immediately once a threshold of instantaneous overload is exceeded.

In one of my  rigs I use a set of Ohmmite parasitic chokes manufactured circa 1939.  They look like standard "brown devil" type 10-20w wirewound resistors with a coil of wire around them, typical parasitic suppressor style.  I'm not sure, but I suspect these are "noninductive" wirewound resistors, with two layers of wire connected in parallel, each wound in the opposite direction as the other.  They serve their function perfectly.  I have a set of spares, but am not interested in parting with them.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
wavebourn
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« Reply #11 on: January 29, 2005, 04:49:55 PM »

Quote from: K3MSB
Quote from: Steve w8tow
Yes, 2 watters, will do...
RF parts only has 1 watters
tks es 73 steve
8tow


Stuck out Steve.  I have several packs of 47 1W only.  They test at 52 ohms (not surprising).  If you still want some, let me know.


It's Ok, it's 10% tolerance.
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K6JEK
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« Reply #12 on: January 30, 2005, 07:26:24 AM »

Where else besides in the parasitic supressor in the plate lead is non-inductive important?  I'm bringing a linear back to life which had carbon resistors as the plate current meter shunt in the B- cathode lead -- 3 1W 4.7 ohm carbon comp resistors in parallel but on the power supply side of an RF choke for two 3CX800A7's grounded grid.

I think the carbon comp resistors can be replaced by a wirewound 1.5 ohm because (a) they're on the DC side of the RF choke and (b) the inductance of the wirewound 1.5 ohm resistor is small anyway, about 200 nH.  

Is this wishful thinking on my part?

Jon, K6JEK
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w3jn
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« Reply #13 on: January 30, 2005, 10:21:21 AM »

200 nH combined with parasitic capacitance = FB resonant circuit at VHF.

3CX800s are expensive enough as it is without risking them for lack of a 50 cent resistor.

Why not just order them from Allied?

73 John
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Steve W8TOW
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« Reply #14 on: January 31, 2005, 10:20:44 AM »

So this weekend I found a 47 ohm 2 watter and decided to conduct a bit of an exeperiment.  In one of my HB rigs, (1940 PP Taylor T40's....807 driver and 6V6 osc) I replaced an existing 47 ohm paracitic choke I previously constructed using a flame-proof resistor. This resistor was located on the plate of the 807.

One of the grids of the T40s has a Ohmite Z-1 to it.

This rig has always behaved oddly. Tune-up was tricky, esp on 40m.
When modulated, the waveform never looked as good as I would like.


AFTER replacing the paracitic choke on the 807 plate this weekend with a CARBON 47 ohm resistor, it tunes much better AND the ragged edges on the modulation envelope are now gone!!!
If you can get by using the flame-proof resistors, more power to you,
but with old buzzard circuits, find a carbon resistor for the chokes form...
COnsider this:
If the flame-proof resistor has a foil surounding the composite innards, how will the coil you wrap around it behave that close to the foil?
The "Q" of the R-L circuit will change, as will the resonant freq....
I am off to find carbon resistors, got 4 more to change out!!!
73 steve
8tow
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
WD8BIL
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« Reply #15 on: January 31, 2005, 11:16:33 AM »

Quote
3CX800s are expensive enough as it is without risking them for lack of a 50 cent resistor.


Yes.... and keep it authentic. There's nothing like the nostalgic smell of a burned carbon comp parasitic !!!
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N8ECR
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« Reply #16 on: January 31, 2005, 11:49:49 AM »

What about using Ferrite Beads?Huh  I have an old SBE 33 slopbucket rig and for parasitics it uses ferrite beads on the plates leads going to the 27GB5 output tubes.

Anybody got any input on this idea?Huh?
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Paul, K2ORC
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« Reply #17 on: January 31, 2005, 12:24:49 PM »

Quote from: N8ECR
What about using Ferrite Beads?Huh  I have an old SBE 33 slopbucket rig and for parasitics it uses ferrite beads on the plates leads going to the 27GB5 output tubes.

Anybody got any input on this idea?Huh?


Interesting, Mike.  Are the beads along the whole length of the lead or are there just a few near the cap?
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WA1GFZ
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« Reply #18 on: January 31, 2005, 12:39:36 PM »

Beads and wire wound resistors will mess up the tank Q.
The resistor blowing up means one of two problems.
Bad layout or excessive L in the suppressor.
Sometimes a high VSWR will also take them out.
Flame proof film resistors can't handle over load as well
as carbon comp but the resistance is more stable.

A strapper amoung us puts his suppressor in the grid where it belongs.
This requires a superior layout though.
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N8ECR
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« Reply #19 on: January 31, 2005, 02:20:07 PM »

My SBE 33 has only one bead near to the plate cap on each tube.

I wonder if the SBE34 might be the same way??
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WA1GFZ
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« Reply #20 on: January 31, 2005, 02:45:59 PM »

Usually 1 bead will have a series Z of about 50 ohms. Compare that to the plate Z of 2000 ohms. As you go higher in frequency the bead will have more effect on the tank coil. The bead material will be lossy at VHF
to kill the Q at self oscillation frequency. A resistor across an inductor is doing the same thing. The bead will heat up just like the resistor.
The material can be damaged if it gets too hot. I would avoid a bead
in a high power rig where there is a lot of ambient heat.
layout is still the key to a stable rig. The tube plate to plate tune cap
needs to be as short as possible and the cap to ground is also a big deal.
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wavebourn
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« Reply #21 on: January 31, 2005, 03:48:58 PM »

Quote from: WA1GFZ

layout is still the key to a stable rig. The tube plate to plate tune cap
needs to be as short as possible and the cap to ground is also a big deal.


Also, cathode bypass and plate filter caps should be connected as close as possible to each other and to the cathode of the output lamp, also the input cap of the antenna tuner should be connected to them by the shortest way. It is the output current's path. Inductances of conductors, be it a part of a chassys, or just a wire, must be always considered (it is a common mistake!). Also, RF currents are significant in the PA, so if a "bias filter" cap is grounded between antenna tuning breadslicer and a RF plate filter cap there will be a parazitic feedback.

Just draw a full equivalent diagram that includes all wires with their inductances and resistances, and capacitances between elements, to see better how to optimize the layout.
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K6JEK
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« Reply #22 on: January 31, 2005, 06:28:19 PM »

It's a world gone mad.

I was thrilled that my local surplus store had a supply of 1 W 4.7 ohm carbon composition resistors.   Now I can replace the three paralleled 1/2 W carbon resistors that blew with these 1 W carbon.  Imagine my surprise when I measured the inductance of the carbon resistors and discovered it is higher than the metal film 1 W guys.  Both, of course, are much lower than the wirewound I almost put in.

Do I put in the metal film because lower inductance is better or do I put in the carbon composition because everyone knows you should use carbon composition in situations like this -- the cathode lead of a pair of 3CX800A7's, grounded grid.

I think I better have a drink and think it over.
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w3jn
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« Reply #23 on: January 31, 2005, 07:11:54 PM »

Metal films are always better.  Carbon comps were used in the old buzzard days because that's all they had (within a reasonable cost target).  They're more stable, less inductance, less noise (not that it matters in a parasitic supressor), and if they overheat reasonably they're not permanently damaged as easily as carbon comps.

73 John
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wavebourn
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« Reply #24 on: January 31, 2005, 07:35:56 PM »

I have a different experience, John. Yes, metal films are more stable and less nolsy, but when overloaded they usually produce a smoke and a tiny spark running accross the body... Being a student I used them when did not have a cigarette lighter. I covered one such resistor by a small piece of a cotton from a first aid kit, and using plyers put it into the 220V socket. One Puff! and I'm smocking my cigarette... Wink
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