Parasitic Suppressor

[N4LTA]

I need four parasitic suppressors on the amp build. I usually use the normal 47 ohm 2 watt carbon comp resistor with 5-6 turns of wire. These resistors are getting very hard to find. I have seen metal oxide resistors used but not sure about these. I do have 10 -  22 ohm Allen Bradley 2 watt carbon comp resistors. I would think these should work fine. Any reason that they would not?
I would think they would work fine in de Qing the inductor?

Thanks in advance and Happy Holidays!

Pat
N4LTA

[KB2WIG]

Consider these Ohmite Ceramic Composition, OX/OY Series resistors.



https://www.mouser.com/datasheet/2/303/res_ox_oy-1265382.pdf


klc

[DMOD]

An option is to use two 100 Watt carbon comps in parallel with about 5 turns of #16.


Phil - AC0OB

[DMOD]

Consider these Ohmite Ceramic Composition, OX/OY Series resistors.



https://www.mouser.com/datasheet/2/303/res_ox_oy-1265382.pdf


klc

I second those. These are even better because they are more stable over the long term.

Phil

[N4LTA]

Thanks


I just ordered ten at Mouser -


Pat
N4LTA

[K1JJ]

I noticed the 2W ones are rated at 400V.

When using a 1500 - 2KV plate voltage, how much voltage can we expect across the parasitic R/L suppressor when things get hairy and there IS a real parasitic at VHF frequencies?
IE, if there are no problems, the smaller resistors will probably be fine?

I usually use the big glow bars and have never lost one, but have lost the smaller resistors. I don't know if it was due to heat or voltage arc-over, however.

There is a little bit of both art and science getting the proper parasitic suppressor to work when there really are VHF parasitics to deal with, especially above 20M operation.

Though I know one HB guru I often consult with who says if the rig is built right (short, fat RF connections)  parasitic chokes  are not needed. I have seen one of his amps using 4 tubes in parallel without any suppressors.   I am not usually that lucky, however... :-)

T

[N4LTA]

My understanding is the parasitic suppressor resistor is a short circuited at normal design frequency by  the coil and does not really act as a resistor. At VHF, the coil  is a substantial reactance and the resistor across it lowers the Q so it cannot easily sustain oscillation.

Pat
N4LTA

[K1JJ]

My understanding is the parasitic suppressor resistor is a short circuited at normal design frequency by  the coil and does not really act as a resistor. At VHF, the coil  is a substantial reactance and the resistor across it lowers the Q so it cannot easily sustain oscillation.
Pat
N4LTA

Yes, VHF operation is what I am worried about.  If the suppressor coil is the main reactance in the plate circuit for VHF and there is a real parasitic that is sustained despite the resistor shunt, how high can the voltage soar across the suppressor ... can it hit plate voltage levels?   I dunno - just thinking out loud.

T

 

[WA5VGO]

I’m not sure there’s anything magical about 47 ohms. I’ve arbitrarily built suppressors with whatever I had handy. I’ve used resistances as low as 33 ohms and as high as 120 ohms and they’ve always worked fine. At times I think Johnson used some especially high resistances with apparently good results.

[AG5UM]

Carbon Composition resistors have the lowest Inductance, the chart I found shows:
wirewound : .03-56 micro-Henry
Foil : "less than" 0.08 micro-Henry
metal oxide : 3-200 NH
Film : "less than" 2 NH
(many modern types are "spiral" cut- making them a coil.)
The charts and spec's I've found sofar just say Carbon Comp's are "lowest Induction of all", or "no Inductance".
I don't see a spec for Inductance on your spec sheet.
73 AG5UM

[DMOD]

Here is a calculation I did one time using the Grove coil equations because I wanted to know what the real inductance was at each end of Spectrum: F @ 3.5 MHZ, [F @ 28.5 MHZ]

Parasitic Choke Inductor Wound Around a 2 Watt 47 ohm CC Resistor:
Dia.                                = 0.36"
Leng.  length of the coil    = 0.6"
Number of turns              = 4.5          
Wire d                             =#16, 0.051"
Pitch                               = 0.13"


SRF             = 926 MHz
Cp               = 0.3 pF
Inductance   = 0.084 μH  [0.1 uH]
XL               = 2.1 Ω       [18 Ω]
DC R           = 0.034
Q                = 130 [350]


Phil  AC0OB

[WBear2GCR]

Seems to me that a certain well known (now SK) ham advocated using nichrome wire of
suitable resistance coiled - no resistor.

[kb2vxa]

Ah, I see someone has already hit on why carbon comp is preferable, essentially no inductance since its only purpose is to lower the Q of the coil. Right, 47k isn't a magic number, I've seen them from 22 to 100 ohms, and 2W makes them an ideal diameter to wind a few turns of bus wire on.

Then there was the mother of all parasitic chokes in a 1KW RF welder I dismantled for parts. It was a metal bracket holding a 3/8" diameter 50 ohm carbon stick with gold plated ends in a clip with a 1" diameter 4 turns of bus wire coil. The assembly without the coil made a handy dummy load with one drawback, I got a call from a local who heard me testing a few miles away.

[KD6VXI]

Rich said use nichrome because it exhibits certain qualities that Fowler says are needed for parasitic chokes.

His nemesis makes the stance that parasitic suppressors aren't needed for tubes that operate at higher frequencies. 

I've used Rich's methods with good success, but they tend to glow when used on bands higher than say 15 meters. 

Toms theory seems to hold true as well.  He says that parasitic are more prone to the 'old tech' or style tubes that have a hard time amplifying higher than 50 to 100 mhz. I've found this to pretty much be spot on.  IE, tubes that do well at 2 meters are pretty stable at 10 and lower.

The tubes you have to work to get in 6 meters, the 572, 811s, etc have to have parasitic suppression! 

The 4-1000s on sane plate voltage don't seem hyper critical to the design of the suppressor.  The 3-500z tube at 3kv or lower with directly bonded grid pins is fairly stable too. At 4400 volts, you HAVE to have the suppressor.

The pair of 4-1000s at 8kv I built needed suppressors on both the anode and snubbers at the cathodes to be stable.

Another stability necessity is having some C AT the cathode.  I read somewhere that it helps to quench 3rd harmonic energy. I have gone to making most of the 10 meter capacitance for the input pi at the pins of the tube and then subtracting that out of any C needed at 15 to 80/160.  Efficiency goes up and stability does too.

I've used mox style resistors and they seem to work OK.  However the ohmites above are the kittens meow.

--Shane
KD6VXI

[W1ITT]

Using the conventional "handbook" parasitic suppressors with the carbon resistors and a coil, the first 4-1000A rig that I built almost 5 decades ago had a wonderful parasitic that I could tune through the top end of the FM broadcast band and up into the aviation allocation.  Not wishing to invite B52s into our cornfield, I experimented a bit to stabilize the rig.  I ended up putting some large FB-43 ferrite beads in series with the regular suppressor.  My thinking was that the ferrite beads would be lossy at VHF and not have much effect at HF.  The plan worked out well and I ran various 4-1000A iterations that way for many years, at around 4.5 KV. 
The Rich Measures nichrome suppressors sounded interesting, but the caustic discussions and ad hominem attacks on the Amplifier Reflector and elsewhere were even more entertaining.  I was tempted to chime in with the ferrite method but, uncharacteristically, I kept my trap shut.
73 de Norm W1ITT

[DMOD]

Here is a MatLab run of Frequency verses Impedance of a parallel LRC circuit where the suppressor coil inductance = 0.084 uH, Cp = 0.3 pF, Rp = 47 ohms:


Fc = 1.7 MHz; Z = 0.9
Fc = 3.57 MHz; Z = 3.3
Fc = 5.4 MHz; Z = 7.00
Fc = 7.3 MHz; Z = 11.3
Fc = 13 MHz; Z = 23.4
Fc = 15 MHz; Z = 26.5
Fc = 18.5 MHz; Z = 31.5
Fc = 28 MHz; Z = 38.6
Fc = 30 MHz; Z = 39.5


Phil - AC0OB

[w4bfs]

.....

The 4-1000s on sane plate voltage don't seem hyper critical to the design of the suppressor.  The 3-500z tube at 3kv or lower with directly bonded grid pins is fairly stable too. At 4400 volts, you HAVE to have the suppressor.

The pair of 4-1000s at 8kv I built needed suppressors on both the anode and snubbers at the cathodes to be stable.

Another stability necessity is having some C AT the cathode.  I read somewhere that it helps to quench 3rd harmonic energy. I have gone to making most of the 10 meter capacitance for the input pi at the pins of the tube and then subtracting that out of any C needed at 15 to 80/160.  Efficiency goes up and stability does too.

I've used mox style resistors and they seem to work OK.  However the ohmites above are the kittens meow.

--Shane
KD6VXI

 

a tip learned from Phil K4DPK for pair of 4x1's  .... make a series resonant circuit of 20pF and adjustible L to resonate at 40 mHz from the cathode of the tubes to ground .... the pair of 4x1's physical size tends to promote this resonance and the added series resonant circuit is a low impedance to ground

I am curious as to what you used for a snubber .... a choke sounds murderous enuff  

[KD6VXI]

The snubber I used was fairly similar to your description.

I got it from Senor Vu.

A  .01 uf disc to ground with a resistor in series. Resistor chosen to keep the parasitic at bay.

As to Norm's ferrite in series with the plate lead, I knew a guy build g ceramics with handles in the 80s that really made his living with sweep tube stuff.  He used the same method, type 43 beads at the plate connection.

I believe Pal (Cirestick antennas) and a couple other mass produced Cb amps did as well.

--Shane
KD6VXI

[km6sn]

The whole point of a (VHF) parasitic suppressor is to supply a low Q inductor. Nichrome wire is ideal. In parallel with a resistor. Do not put the resistor inside the inductor. Research Mark Mandelkern "Protecting Tetrode Amplifiers" in QST.

[AG5UM]

Resistors have voltage ratings also.
A metal oxide 2-watt has a volt rating of 350 volts while a 1-watt carbon comp has a working voltage of 500 volts.
the surge, etc. voltage ratings are higher.
73