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Author Topic: Parasitic Suppressor Resistors in Amplifiers.  (Read 29126 times)
KG6YV
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« on: September 26, 2011, 07:00:18 PM »


I need to replace the parasitic suppressors in an 8877 linear.  With carbon composition resistors becoming extinct almost, what other resistors could be used.  The predominate sources today sell metal oxide and metal film.  Can either be used in parasitic suppressors?

Greg
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N4NYY
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« Reply #1 on: September 26, 2011, 07:10:51 PM »

Metal film will not come in high watts. You will need metal oxide or ceramic resistors.

When I have seen bubbly nasty looking suppressors, I usually replaced them with metal oxide, but I went up a watt size (3 watts up from 2). However, that may not be appropriate, as I have read some discussions here. I have always kept the same resistance.

I am not an amp guru, and there are numerous here that can answer the watts sizing. But metal oxide or ceramic is the way to go.
« Last Edit: September 26, 2011, 07:12:48 PM by N4NYY » Logged
KB1LKR
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Posts: 1897




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« Reply #2 on: September 26, 2011, 08:57:58 PM »

From W8JI who would know: http://www.eham.net/ehamforum/smf/index.php/topic,77494.15.html

... either an Ohmmite OX or OY series metal COMPOSITION.

http://www.ohmite.com/catalog/pdf/ox_oy_series.pdf
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W8JI
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« Reply #3 on: September 27, 2011, 02:49:52 AM »


I need to replace the parasitic suppressors in an 8877 linear.  With carbon composition resistors becoming extinct almost, what other resistors could be used.  The predominate sources today sell metal oxide and metal film.  Can either be used in parasitic suppressors?

Greg

I see people advertising general metal oxide films as non-inductive, but that isn't true. Some are quite inductive.

Your saving grace is you have an 8877. If you grounded the grid ring proprly with a low impedance short path to the chassis and have a good layout, it shouldn't even need a suppressor at all. So what you do for a resistor won't matter.

If we assume every tube in every layout needs a suppressor or the world isn't right, then the proper type for general suppressor designs  are composition metal or carbons.

http://www.w8ji.com/vhf_stability.htm

73 Tom

 
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AD4U
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« Reply #4 on: September 27, 2011, 05:32:29 AM »

I recently bought a "strip" of 25 NOS Ohmite 47 ohm 2 watt carbon resistors on ebay.  This is the typical value and wattage used in many parasitec suppressors.  I checked the value of each resistor with my Fluke 87 VOM and each is still within 5% of rated value.  In other words they have not increased in value with age.

These resistors are listed on ebay quite often.  I am very happy with my $6.50 purchase.  Your results may vary.

Dick  AD4U
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W8JI
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« Reply #5 on: September 27, 2011, 05:39:16 AM »

I recently bought a "strip" of 25 NOS Ohmite 47 ohm 2 watt carbon resistors on ebay.  This is the typical value and wattage used in many parasitec suppressors.  I checked the value of each resistor with my Fluke 87 VOM and each is still within 5% of rated value.  In other words they have not increased in value with age.

These resistors are listed on ebay quite often.  I am very happy with my $6.50 purchase.  Your results may vary.

Dick  AD4U

Did you cut one open to be sure they had a solid carbon core?
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N4NYY
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« Reply #6 on: September 27, 2011, 05:50:01 AM »

Quote
I see people advertising general metal oxide films as non-inductive, but that isn't true. Some are quite inductive.

Yikes. I have been using metal oxide until you specified ceramic. Is there any easy way to tell? Would an LCR meter be able to measure it?
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AA4HA
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« Reply #7 on: September 27, 2011, 07:08:45 AM »

Just by the very nature of how a metal oxide resistor is manufactured leaves it with an inductive component. They are either metal that is sputtered in a spiral pattern on a piece of quartz or with an engraved groove on the piece of quartz (or glass) and filled in with a conductive material. Lasers are frequently used to slice away excess material to bring the resistor value closer to spec. This is why you can get these types of resistors in 1% (or better) values, they are precision made and trimmed by a laser.

Carbon comp resistors usually use a clay/graphite mix and run it into presses to make resistor bodies with the leads embedded in each end. I would think that the mixture ratio of clay/graphite and mold pressure would set the value. Since this is rather imperfect you end up with 10 and 20% values.

I would think that you "could" make a non inductive metal oxide resistor. It would look like a series of parallel lines placed around the circumference of the quartz or glass rod and each trimmed to be approximately the right width. In reality it would be more like a bunch of parallel resistors (each being a single line). Trimming would be the tricky part as if you get one too narrow it will act like a fuse and burn open, causing the resistor value to increase by whatever ratio that one strip was for the entire array of resistive lines. (more like Litz wire).
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W1BR
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« Reply #8 on: September 27, 2011, 07:17:03 AM »

Quote
I see people advertising general metal oxide films as non-inductive, but that isn't true. Some are quite inductive.

Yikes. I have been using metal oxide until you specified ceramic. Is there any easy way to tell? Would an LCR meter be able to measure it?

I think the inductance is in the nH range, probably be hard to read on a meter that
most of us would have. A fancy RX meter might show the problem.

Pete
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W8JI
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« Reply #9 on: September 27, 2011, 09:46:27 AM »

I would think that you "could" make a non inductive metal oxide resistor. It would look like a series of parallel lines placed around the circumference of the quartz or glass rod and each trimmed to be approximately the right width. In reality it would be more like a bunch of parallel resistors (each being a single line). Trimming would be the tricky part as if you get one too narrow it will act like a fuse and burn open, causing the resistor value to increase by whatever ratio that one strip was for the entire array of resistive lines. (more like Litz wire).

Ohmite makes metal compositions.

Only Stackpole makes carbon comps, and they are special order.
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W1BR
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« Reply #10 on: September 27, 2011, 10:53:28 AM »

Hey Tom,

Has anyone ever experimented with high C to L resonant
parasitic suppressors, tuned to the parasitic frequency
and heavily swamped withr resistance?

I'd wonder if the losses in the amp's upper operating ranges
could be reduced slightly?

Pete
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AD4U
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Posts: 2541




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« Reply #11 on: September 27, 2011, 12:45:04 PM »

Too much L in the suppressor network may (will) cause the resistors to dissipate more heat which equals fired resistors and perhaps problems on 10 meters.  Over my ham years I have built around 10 different amps running anything from 4-6KD6 sweep tubes, 4-811A's, 3-500's, 4-1000A's (and multiples there-of), and 8877's.  Once I even started (but never finished) an amp running a 3CX15000A.

In all my homebrew amps I used a single 47 ohm 2 watt carbon comp resistor with 3 turns of wire wrapped around it - aka - Heathkit amps.  This may not be the optimum value of L or R, but it always seemed to work for me.

Dick  AD4U
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W8JI
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« Reply #12 on: September 27, 2011, 01:02:13 PM »

Hey Tom,

Has anyone ever experimented with high C to L resonant
parasitic suppressors, tuned to the parasitic frequency
and heavily swamped withr resistance?

I'd wonder if the losses in the amp's upper operating ranges
could be reduced slightly?

Pete

Yes.  I've use both series resonant and parallel resonant suppressors, as well as ferrites on cavity walls.

There are many ways to accomplish the end results.

The AL811 is an interesting system. It uses intentionally inductive resistors (I hope they have not changed it) series tuned by capacitors, in parallel with the normal suppressor coil. Bottom of this link:

http://www.w8ji.com/vhf_stability.htm

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AG6K
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Posts: 1




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« Reply #13 on: September 27, 2011, 06:12:36 PM »


I need to replace the parasitic suppressors in an 8877 linear.  With carbon composition resistors becoming extinct almost, what other resistors could be used.  The predominate sources today sell metal oxide and metal film.  Can either be used in parasitic suppressors?

Greg
 

Greg -- MOF resistors work well is VHF/UHF parasitic oscillation suppressor service provided they have fewer than about two spirals in their conductive film to keep L to a minimum.   The Matsushita MOF 100Ω 3w resistors we use in our suppressor retrofit kits are  able to withstand overloads better than carbon-comp resistors,   Without forced air cooling this unit will dissipate 12w continuous for 1-hour with c. a 9% change in resistance.  At 12w the DUT can be seen glowing in a dark room.    cheers
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AG6K
Member

Posts: 1




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« Reply #14 on: September 28, 2011, 04:14:23 PM »

Hey Tom,

Has anyone ever experimented with high C to L resonant
parasitic suppressors, tuned to the parasitic frequency
and heavily swamped withr resistance?

I'd wonder if the losses in the amp's upper operating ranges
could be reduced slightly?

Pete

Yes.  I've use both series resonant and parallel resonant suppressors, as well as ferrites on cavity walls.

There are many ways to accomplish the end results.

The AL811 is an interesting system. It uses intentionally inductive resistors (I hope they have not changed it) series tuned by capacitors, in parallel with the normal suppressor coil. Bottom of this link:

http://www.w8ji.com/vhf_stability.htm



  Parasitic suppressors are anti-resonant/low-Q  devices, not resonant ones.  Since higher anode RL means more amplification and lower RL means less amplification, VHF parasitic suppressors perform their job by decreasing the VHF RL presented to the anode by the parasitic resonance in the anode circuit.  This resonance is formed by the anode-C, the L of the leads and DC-blocker C between the anode and the L and the C of the Tune-C plus the L of the return path through the chassis and the tube.  Example:  A TL-922:  The total anode C is c. 10pF, the L of the conductors is c. 190nH, and the Tune C is typically 30 to 120pF.  Thus the resonant freq. is c. 120MHz.
  The factory stock parasitic suppressors in a 922 have a Q of 5.5 at 100MHz.  The presents a higher than desirable VHF-RL to the anodes of the 3-500Zs - which produces more VHF amplification - so much so that occasionally there will be a 120MHz parasitic oscillation.  The fix is to lower the VHF O of the suppressors in order to lower VHF gain.  There are two ways of doing this:  1.  Add more L to the suppressor inductance and increase  the resistance of suppressor R.  Unfortunately this increase the heat generated in the R exponentially** at 28MHz so that a much higher dissipation R is required for 28MHz operation.  or 2.  Increase the resistance of R,  use low-L resistors capable of dissipating more heat, and use resistance wire to lower the Q the inductor.  When this is done with double-suppressors per tube the VHF-Q can be reduced to 1.5 at 100MHz, thereby lowering VHF amplification  -- which in turn lowers the ability to sustain oscillation.  end
**  "Calculating Power Dissipation in Parasitic-Suppressor Resistors" March, 1989 QST, page 7,
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