Parasitic Chokes

[W2VW]

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!!!

8tow

I wrapped some elestric toaster wire around a 100 Ohm carbon resistor and stuck it across my mic input lead. Now, no one will answer my CQ.

[Steve W8TOW]

I doubt very much that metal film resistors "L" measured at 30 mhz and above is lower than a carbon resistor.
I swept several this weekend,
At 1 khz, they measure about the same "L", but at 1 mhz, the metal film (same R values) goes up x 10, and at 50 mhz, the L of a metal film is outta sight,
so.... I think if ya wanna get rid of VHF/UHF paracitics, you would not use a
metal film resistor as a form, but, that is just me....
73 to all

[w3jn]

I'll measure some this weekend - I have an old buzzard tube-type Rohde&Schwartz network analyzer.  What values did you measure, Steve?

73 John

[WA1GFZ]

I have checked the failure modes of most resistor types. Only wire wound
resistors burn open. All others I have tested go low in resistance before
they burn open. This can cause cascading failure modes. A wire wound resistor always increases in resistance until it opens providing it doesn't flash over with high voltage.

[N8ECR]

So what if it ain't a carbon resistor???  I have some so called noninductive resistors around here that appear to be  two resistive wire windings that counter each other, they are called noninductive.

 Could these be used as part of a parasitic choke?

I have some of these used as dummie loads, and they do have SWR reflected of about 1.2 to 1.

[Steve W8TOW]

JOhn, I checked the following values (both flamers and carbons)
39 ohm
47 ohm
56 ohm
63 ohm
I made a typical P choke for an 807, 6 turns of #18 on
the resistor.
oh, these were measured on a HP LCR analyzer
73 steve

[WA1GFZ]

6 turns is a lot of L and the 807 is a real VHF generator.

My 4CX3000A has 1 turn of 1/4 inch strap 5/8 inch I.D. with 5 220 ohms 2 watt carbons in parallel. 6 turns in a linear may have problems on 10 meters. I think the SB220 had 2 turns on a 47 ohm 2 watt.   fc

[w3jn]

Steve, I'm not sure that flameproofs are metal films, or at least the same type of metal films I'm talking about.  I'm talking about RN55 1 percenters as well as 1/4w film resistors as purchased from DigiKey.

Flameproofs are generally used as current limiting resistors in the primary circuit of siwtch mode power supplies, TV sets, etc., and are not intended for any sort of RF service.  The reason they're "flameproof" is so that they don't present a fire hazard if they overcurrent (although someone posted the exact opposite experience!).

I have a HP digital L-R-C bridge as well.  I'll run a bunch thru and see what I find.

Y'know, I'd think a wirewound resistor would make a swell plate RF choke - it's a great lossy inductor, which would go a long way towards quelling unwanted resonances.  Anyone tried this?

73 John

[WA1GFZ]

John,
I had an old MARS article of a 4CX1000A linear. The builder blew the
B&W 800 plate choke and rewound it with wire removed from a power
resistor. He claimed it worked great. The B&W 800 has a resonance
around 25 MHz. The wire resistance should kill the q of the choke.
The trick is finding resistor wire low enough resistance that is long enough.
I know a number of guys use resistance wire for parasitic chokes.

Flame proof resistors are only supposed to contain the flame. My testing did not include these I was doing RLR, RNC, RWR, and RN60 style mil parts.
I found film resistors dropped a lot in value as they got red. We were sensing EED squibs so needed to limit current under all failure contitions.
fc

[Steve W8TOW]

John, these resistors were "NTE" off the shelf "flameproofs"
2 watters..
I took a hammer to one and inside is a foil around the carbon...
perhaps if a person wanted to, they could crape away the foil,
but I think its easier to buy a old carbon resisitor or perhaps use some
pencil lead!
steve

[w3jn]

I have some of those, too, Steve.  I'll do some experimenting with various resistors this weekend - 's the way to find out fer sure!

73 John

[KL7OF]

I used 3 in parallel 50 ohm wirewound inside 4 turns of #12 copper 3/4 in dia on my 4x1... works FB on 80, 40 and 20.......does not get hot.....Before I put this on top the tube, I had some jumpy tuning and some arcing of the plate tune cap air variable...I tried the wirewounds cause I didn't have any carbons....

[WA1GFZ]

I think a wire wound may have a problem with the end termination where the lug is located acting like a shorted turn at each end. RWR resistors have a cap at the end that may not act like a shorted turn. Take a 10 watt or 25 watt wire wound with a shorted turn at each end I suspect this could be a problem. Say you had a 175 watter at a fairly low resistance there
may be enough inductance to act like a plate choke.
When I first built my 4-1 rig I had 4 turns also. It didn't work well on 15 and 10 meters. I eventually got it down to two turns 1/2 inch I.D. and worked well on ten meters. It took a number of layout changes to get it right. It is not easy with a tall tube.   fc

[KL7OF]

Yes ...I'm still experimenting with the 4x1...When I come across some carbons, I'll try them...I am using a B and W tank coil and a breadslicer @ 4KV and so far it won't go to 10 meters.......

[wavebourn]

Gentlemen, why don't calculate how a wire is acting, how end short turns are acting, what's inductance, what's resistance we need, and so on, having in mind what do we need to supress parasitics oscillations but don't affect too much output on the high end of the band, instead of just reading tea leaves? :lol:

[w3jn]

There's the engineer in you showing, Tolly!

The problem is that we don't know a lot about the actual construction of these resistors.  Molded wirewounds are almost impossible to see how many turns are inside, and whether there is a shorted turn or not.

However, there are certainly computer programs that could model a parasitic supressor - however, they're based on a more or less ideal resistor = and how the resistor affects the performance of the suppressor is the whole thread here.

BTW did you find a manual for your R-392?  If not, there's one here:

http://bama.edebris.com/manuals/military-kg7bz/r392/

73 John

[WA1GFZ]

The real problem is the mechanical layout not so much the tube.
I agree with Tolly but we all live out of our junk boxes. VERY low resistance RWR style resistors may work very well.
I actually used them on the modulator plates of my PDM Viking 2. I used
6.8 ohm 3 watt RWR resistors at the plates of the 6DQ5s
They have been there for 20 years now. I never measured the inductance
since the PWM is running at 75 khz. Small pulses still make it throught the circuit.  fc

[wavebourn]

Thanks John, downloading now!

I hope after disassembling I'll find a way to resolder that filaments inside of the crystal osc chassis...

[WA1GFZ]

Tolly,
You might be able to get a new module from Fair Radio Cheap.
Good luck!

[wavebourn]

Thank Frank,
I have a nice audio chip from TV, it is Hi-Fi amp and was powered exactly from 28V source! However, I'll need to protect it simehow, because a squelch relay will defenitely damage it shorting output.

[WA1GFZ]

Tolly that is great. Just mount it to the side wall for plenty of heat sink
then use the base of a dead tube as a plug to connect it to the audio output tube socket. Mil receivers like the Racals and Cubics use a series resistor at the output of the audio IC to protect the output. I think Cubic uses 33 ohms if I remember. This way the IC can handle a dead short
on the output. This would be in the line level path. I have a RA6830 ISB
board with resistors at the aduio amplifier output for each channel that
drive little board mounted 600 ohm line level transformers. I suspect
the R392 does the same thing.Racal and Cubic both
have the connection to the speaker between the IC and resistor. This is not protected. The R392 audio output is 600 ohms. I suspect a 28 volt
audio IC will make a lot of power to drive the output transformer with a
series resistor. You should have no problem driving a speaker if it is matched to 600 ohms. The R392 audio output was under 1 watt if I remember. You might want to AC couple the output of the Amplifier
with around 470 to 2000 UF to get good low end response. fc

[wavebourn]

Thank you Frank again! Corrections: it was my mistake, the squelch relay don't short output trans, it shorts one more trans that is input trans, I forgot about it.
I am going to mount a heatsink on top of a tube base. And one more redesign: instead of plugging input of the amp into the 6BJ5'th socket I'll crocodile it to the volume regulator through the shielded wire.

Also, I figured out how to convert filament feeding without resoldering wires on tube sockets. What bothered me, were couple tubes buried inside of the crystal oscillator subassembly. Fortunately, a resistor that drops 12V for that tubes is conveniently located underneath the RF chassys and may be easily shotred.

So, 12BA6 and 12AT6 may be easily installed!

[WA1GFZ]

AH very good converting to 12 volt tubes. You could use 3 terminal regulators to regulate the 28 volts down to 12V or put pairs in series
to operate off 28 VDC. I thought the 2 tubes in the crystal oscillator
were in series. My 392 was so clean inside I hated to tear into it.
I sold it when I got an R390A.  I have friends who installed relay controlled mechanical filters to help bandwidth skirts.  fc

[wavebourn]

AH very good converting to 12 volt tubes. You could use 3 terminal regulators to regulate the 28 volts down to 12V or put pairs in series
to operate off 28 VDC. I thought the 2 tubes in the crystal oscillator
were in series. My 392 was so clean inside I hated to tear into it.
I sold it when I got an R390A.  I have friends who installed relay controlled mechanical filters to help bandwidth skirts.  fc

Yes, filaments of 2 tubes in crystal oscillator are in series, and in series with 80 Ohm resistor. If to short the resistor the whole set may be easily converted to be powered from 12 V source so 12V version of tubes may be used. However, tubes from AF desk must be in such case removed and replaced by a hi-fi IC.  One more store is a crapstal calibrator, where transistors may be inserted in the tube sockets instead of 12AU7.

[WA1GFZ]

Tolly, 12AU7s can be wired for 6.3 volts. There are 3 heater pins someone
correct me it has been a lot of years.  This allows you to put both heaters in parallel. good luck!