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eHam Forums => Amplifiers => Topic started by: KG6YV on September 26, 2011, 07:00:18 PM



Title: Parasitic Suppressor Resistors in Amplifiers.
Post by: KG6YV 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


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N4NYY 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.


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: KB1LKR 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


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W8JI 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

 


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AD4U 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


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W8JI 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?


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N4NYY 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?


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AA4HA 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).


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W1BR 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


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W8JI 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.


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W1BR 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


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AD4U 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


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W8JI 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



Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K 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


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K 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,


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W8JI on September 28, 2011, 04:49:00 PM
  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. 


One way to test a theory is to take it to the limits the theory sets.

If low "RL" is the goal, the lowest possible RL comes from no suppressor at all. A dead short.

If high "RL" means highest gain, an infinite impedance load to the anode would offer highest gain.

Obviously you are missing something. Your statement is a gross oversimplification, like all 3-500Z filaments should be set for 4.8 volts.

The optimal suppressor impedance depends on the impedance of the anode path in which it is inserted at the problematic frequency. While one answer to all problems sounds great to some people, it is not often correct.

Quote
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.
 

That is the system resonance, and not the suppressor resonance.

The suppressor is an independent system in series with the anode lead to the tank. The suppressor independently has an equivalent impedance from terminal-to-terminal. Much like the trap in a trap antenna, it can be resonated to peak at a problem frequency and shift more current into the resistor at the problem frequency. This is commonly done in applications where grid resonance that primarily determines frequency of instability approaches operating frequency.   

Quote
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.

Occasionally? One a week, one a month? What makes it stop and start?

Quote
The fix is to lower the VHF O of the suppressors in order to lower VHF gain.


What is VHF "O"?

Quote
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. 
 
 
......and of course the best or only cure for this "oscillation", the one that occasionally or very randomly pops up and damages a part and then goes away,  is the one you sell.     :-)


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W1BR on September 28, 2011, 05:25:59 PM
My question to Tom was  if you know the self resonant frequency of the
tube in question, would using a parallel resonant supressor with a high C to L ratio be something
that is worth investigating.
  
The goal would be to minimize reactive losses at 30 MHz and below by using a LC tuned circuit that
would provide a low impedance path at HF, and a very high impedance path at the parasitic frequency.
this would force the VHF energy through the resistor, killing the Q at the self resonant, possible
parasitic requency.

Using nichrome wires in the path seems to be counter intuitive to minimizing HF loss.



Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W8JI on September 28, 2011, 08:45:59 PM
My question to Tom was  if you know the self resonant frequency of the
tube in question, would using a parallel resonant supressor with a high C to L ratio be something
that is worth investigating.

Absolutely. It is a good method for minimizing loss on desired frequencies while maintaining effective suppression at a prolematic frequency.
  
Quote
The goal would be to minimize reactive losses at 30 MHz and below by using a LC tuned circuit that
would provide a low impedance path at HF, and a very high impedance path at the parasitic frequency.
this would force the VHF energy through the resistor, killing the Q at the self resonant, possible
parasitic requency.

There will be a certain optimum dampening resistance. My parasitic suppressor page goes into all of this.




Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: G3RZP on September 29, 2011, 11:10:01 AM
If you look at older ARRL Handbooks, you will see that one of the suggested parasitic suppressors was a resistively loaded tuned circuit coupled to the plate through a small link.

A straight RLC network was used in a 80kW tx for this purpose in the 1960s.


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K on September 30, 2011, 10:15:17 AM
  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. 


One way to test a theory is to take it to the limits the theory sets.

If low "RL" is the goal, the lowest possible RL comes from no suppressor at all. A dead short.

  SO IT MIGHT SEEM, BUT THE PARASITIC SUPPRESSOR IS NOT CONNECTED FROM THE ANODE TO GND.  Murphy was right about things being more complicated than they first look. 

If high "RL" means highest gain, an infinite impedance load to the anode would offer highest gain.

  Correct Tom but since the VHF parasitic suppressor is not in parallel with the anode the engineer must do a series to parallel transformation to find out what the VHF RL is.  This procedure is covered in "Calculating Power Dissipation in Parasitic-Suppressor Resistors", March, 1989 QST, page 7, 'Finding Impedance by Solving for Admittance'. .® The American Radio Relay League, Inc. - note - Series/parallel transformations require considerable thinking and minimal drinking.   

Obviously you are missing something. Your statement is a gross oversimplification, like all 3-500Z filaments should be set for 4.8 volts.

  In "Care and Feeding ..." Eimac recommends operating  Th-W filament tubes at the lowest potential (+1%) that provides no decrease of peak emission - but Not at a potential below the Eimac's recommended minimum filament V.  The minimum recommended  for the 3-400Z and the 3-500Z filament is 4.75V.   What do you find wrong with 4.80V Tom?

The optimal suppressor impedance depends on the impedance of the anode path in which it is inserted at the problematic frequency.

 say what ?

While one answer to all problems sounds great to some people, it is not often correct.

  A Low VHF Q  parasitic suppressor produces a relatively low VHF-RL at the anode.  Lowering VHF-RL produces lower VHF amplification.   Lower VHF amplification =s less chance of VHF oscillation.   - note - osc. is due to the unavoidable feedback path between the input and the output of all tubes. . 

Quote
  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.
 

That is the system resonance, and not the suppressor resonance.

  suppressors aren't, they are low-Q anti-resonant devices that produce low parallel-equivalent R at the anode. 

The suppressor is an independent system in series with the anode lead to the tank. The suppressor independently has an equivalent impedance from terminal-to-terminal. Much like the trap in a trap antenna, it can be resonated to peak at a problem frequency and shift more current into the resistor at the problem frequency. This is commonly done in applications where grid resonance that primarily determines frequency of instability approaches operating frequency.   

Quote
  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.

Occasionally? One a week, one a month? What makes it stop and start?

  Who knows what made the fuel pump crap out on my 1973 Dodge Maxivan?  How come the original brakes are still good?  Forrest Gump said "Shit happens".  Maybe the brakes have the good kind and the fuel pump had the bad kind?  IME intermittent VHF parasites are somewhat mysterious but I feel like I know how to discourage them.

Recommended reading:  Fyler, G. W. ''Parasites in Transmitters'', Institute of Radio Engineers journal. Sept. 1935

Quote
 The fix is to lower the VHF O of the suppressors in order to lower VHF gain.


What is VHF "O"?

  X/R at some frequency between 30MHz and 300MHz.  At 100MHz a SB-220's VHF parasitic suppressors have a Q of c. 5. 

Quote
  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. 
 
 
......and of course the best or only cure for this "oscillation", the one that occasionally or very randomly pops up and damages a part and then goes away,  is the one you sell.     :-)

  chortle.  If an owner of a SB-220 is uncomfortable knowing that his amp has suppressors with a Q of 5, he can buy one of our retrofit kits, do some 221ºC Ag-Sn soldering, and decrease the suppressor Q to 1.5.  If he/she thinks that the amplifier is not more stable with low-Q parasitic suppressor he/she gets a refund cheque from yours truly.    cheers


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N2EY on September 30, 2011, 12:50:02 PM
In "Care and Feeding ..." Eimac recommends operating  Th-W filament tubes at the lowest potential (+1%) that provides no decrease of peak emission - but Not at a potential below the Eimac's recommended minimum filament V.  The minimum recommended  for the 3-400Z and the 3-500Z filament is 4.75V.   What do you find wrong with 4.80V Tom?

I don't know about Tom, but here's my take on it:

If I had an amplifier with a genuine Eimac 3-500Z or 3-400Z, and it could be guaranteed that the filament voltage would never, ever drop below 4.75 volts if set at 4.8 volts, I might consider running the tube that way. Maybe.

But such conditions are simply not realistic in 99.99% of amateur radio situations. Here's why.

To meet the above conditions, you need:

1) A true-RMS AC voltmeter with accuracy better than 1%. Otherwise, when you set for 4.8 volts you could actually be at 4.75 volts or lower.

2) A setup that can measure the filament voltage at the tube pins while under full-power operating conditions.

3) A line supply and filament transformer that are regulated to better than 1% under all conditions so that the filament voltage doesn't ever drop below 4.75 volts

4) Test equipment to measure the IMD, power out, etc. as recommended by Eimac, to be sure that 4.8 volts really is OK.

IOW, to run at 4.80 volts when the lower limit of the specification is 4.75 you need a filament supply and voltage measuring system where the total combined errors from all factors is less than 1% under all conditions. Otherwise there will be times when the minimum voltage specification isn't met, the tube runs too cold and useful life is actually reduced.

The plain and simple fact is that 99.99% of hams, including me, just don't have the resources to run that close to the edge. Not even close.

So what 99.99% of hams should do is to aim for the design-center voltage - 5.0 volts under load. That way the chances of being out of specification are minimized and tube life is maximized.  

Which is the whole point of the filament-voltage discussion.

Of course it is counter-intuitive that running too cold can be as bad as running too hot. But that's how it is. Transmitting tubes with thoriated-tungsten filaments are not the same things as light bulbs.

73 de Jim, N2EY


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W8JI on October 01, 2011, 05:04:44 AM
AG6K wrote:
Quote
SO IT MIGHT SEEM, BUT THE PARASITIC SUPPRESSOR IS NOT CONNECTED FROM THE ANODE TO GND.  Murphy was right about things being more complicated than they first look. 


The common suppressor is indeed connected from anode to ground in a grounded grid amplifier. The path is anode through the anode leads through the blocking capacitor and through the tuning capacitor to ground, which is also where the grid is.

The suppressor is normally, although it does not have to be, in series with the tank connection.

I said:
If high "RL" means highest gain, an infinite impedance load to the anode would offer highest gain.

AG6K said:
Quote
  Correct Tom but since the VHF parasitic suppressor is not in parallel with the anode the engineer must do a series to parallel transformation to find out what the VHF RL is.


Sorry Rich, but the suppressor is in SERIES with the anode path. The easiest way to find the suppressors effect at VHF is to look at series impedance of the path for a normal series connected suppressor.

AG6K: 
Quote
This procedure is covered in "Calculating Power Dissipation in Parasitic-Suppressor Resistors", March, 1989 QST, page 7, 'Finding Impedance by Solving for Admittance'. .® The American Radio Relay League, Inc.

Since the suppressor is in series with the anode path, one only has to look at the series equivalent impedance and calculate heat with current. There is no magic to nichrome. It de-Q's the system at HF much more than a regular suppressor, while not offering anything that cannot be done some other way at VHF or UHF. It's 1920's breadboard suppression technology.

AG6K wrote:
Quote
In "Care and Feeding ..." Eimac recommends operating  Th-W filament tubes at the lowest potential (+1%) that provides no decrease of peak emission - but Not at a potential below the Eimac's recommended minimum filament V.  The minimum recommended  for the 3-400Z and the 3-500Z filament is 4.75V.   What do you find wrong with 4.80V Tom?


If you can't understand what is wrong with your suggestion of something so simple as setting a voltage, how can you ever hope to understand more complex things like suppressors and how they work?? 

If the minimum operating voltage is 4.75 volts and you are the single source telling Hams to set voltage at 4.8 volts, you are causing virtually 100% of people following your advice to set voltage below published voltage levels. This is because you give no instructions to test emission, find the lowest (and/or  highest) mains voltage, what meter to use, or even where or how to measure.

Look at the mess it caused everyone because QST and an author used your 4.8 volts as a target reference! The nichrome is not much different than the filament voltage, and neither is moving a balun to the input of a floating tuner network.

73 Tom


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N4NYY on October 01, 2011, 08:02:04 AM
OK, I am trying to follow this thread, but the one dude is putting everything in quotes, including his replies. That is not helping.


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W3LK on October 01, 2011, 10:22:32 AM
OK, I am trying to follow this thread, but the one dude is putting everything in quotes, including his replies. That is not helping.

Because the poster doesn't know how to use the quote function correctly. :(


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N4NYY on October 01, 2011, 11:39:18 AM
Quote
Because the poster doesn't know how to use the quote function correctly. Sad

Yep. Maybe he can figure it out.


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: KA5N on October 01, 2011, 12:24:12 PM
Any thread longer than two yards long will take you to places you don't need to be.
Time to tune out.
Allen


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N2EY on October 01, 2011, 12:51:25 PM
OK, I am trying to follow this thread

It's really pretty simple.

Tube type HF amplifiers used by hams often have oscillations at frequencies other than the desired ones because there are unavoidable resonances in the circuit. Usually these are in the VHF region. A typical HF amplifier circuit looks different to VHF frequencies in such a way that the wiring itself becomes a tuned circuit.

These "parasitic oscillations" or "parasitics" can't usually be completely cured by neutralization, bypassing or shielding. (Those things help, and are good practice, but most amplifiers need more).

The usual cure is to insert one or more "parasitic suppressors" in the VHF oscillator circuit, in such a way that VHF oscillations can't start or be sustained.

The typical parasitic suppressor is a small inductance in parallel with a noninductive resistor. The idea is that the inductor value is small enough that it is essentially a short-circuit at HF, yet large enough at VHF that the resistor is effectively in the circuit for VHF. The resistor introduces enough loss at VHF that parasitic oscillations can't start or continue.

That's the theory. In practice, suppressing parasitics can take a bit of trading-off, particularly in amps covering the higher HF bands like 10 meters. This is because an inductor-resistor combination with values of L and R large enough to stop a parasitic may have some loss at HF too.

One possible option to reduce losses at HF is to add a capacitor in parallel with the inductor so that the combination becomes a tuned circuit at the VHF parasitic frequency. This works, and in fact the Ancient Ones sometimes did it as a TVI reduction measure. But it has drawbacks:

1) More parts

2) Only suppresses one parasitic frequency

3) Requires adjustment intially.

The usual resistor-inductor combination has been used for decades with great success when properly applied. W8JI's website description of cleaning up a TL-922 is IMHO a good example (complete with pictures).

73 de jim, N2EY

   


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N4NYY on October 01, 2011, 01:45:19 PM
Thanks, Jim. I only have minimal experience with suppressors as I typically replace the original, as in Heathkits case. I do not use non-original setups, unless there was a service bulletin to correct a flaw. I try to use these threads to try and learn something, but it is difficult to follow, especially when some answers were put in a quotes highlight.


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W8JI on October 01, 2011, 01:57:02 PM
Thanks, Jim. I only have minimal experience with suppressors as I typically replace the original, as in Heathkits case. I do not use non-original setups, unless there was a service bulletin to correct a flaw. I try to use these threads to try and learn something, but it is difficult to follow, especially when some answers were put in a quotes highlight.

I'll try to expand what I have on this including photos, so if something is unclear let me know.

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

Tubes with the greatest stability problems are older designs with long thin grid leads. Many modern tube designs in good layouts require no suppression at all. There are about a dozen different ways to skin the same cat, but generally the best way is the way that affects the desired frequencies the least and the indesired frequencies the most.

73 Tom


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K on October 01, 2011, 04:09:43 PM
In "Care and Feeding ..." Eimac recommends operating  Th-W filament tubes at the lowest potential (+1%) that provides no decrease of peak emission - but Not at a potential below the Eimac's recommended minimum filament V.  The minimum recommended  for the 3-400Z and the 3-500Z filament is 4.75V.   What do you find wrong with 4.80V Tom?

I don't know about Tom, but here's my take on it:

If I had an amplifier with a genuine Eimac 3-500Z or 3-400Z, and it could be guaranteed that the filament voltage would never, ever drop below 4.75 volts if set at 4.8 volts, I might consider running the tube that way. Maybe.

 Heath SB-220s run their 3-500Z filaments close to this potential when operated from 234vac.  One of the tubes in my SB-220 is an Amperex that was mfg in 1967 and it still does its job. 

But such conditions are simply not realistic in 99.99% of amateur radio situations. Here's why.

To meet the above conditions, you need:

1) A true-RMS AC voltmeter with accuracy better than 1%. Otherwise, when you set for 4.8 volts you could actually be at 4.75 volts or lower.

2) A setup that can measure the filament voltage at the tube pins while under full-power operating conditions.

3) A line supply and filament transformer that are regulated to better than 1% under all conditions so that the filament voltage doesn't ever drop below 4.75 volts

  And what happens when a 3-500Z's filament potential drops slightly below 4.75v ?  In tubes with good vacuums, nothing.  However if the tube is slightly gassy. the filament's 1.5% Thorium (Th) content  becomes contaminated and emission decreases.  The fix is to raise the filament V temporarily to expel the O2 and N2 atoms that have contaminated the Th.

4) Test equipment to measure the IMD, power out, etc. as recommended by Eimac, to be sure that 4.8 volts really is OK.

IOW, to run at 4.80 volts when the lower limit of the specification is 4.75 you need a filament supply and voltage measuring system where the total combined errors from all factors is less than 1% under all conditions. Otherwise there will be times when the minimum voltage specification isn't met, the tube runs too cold and useful life is actually reduced.

  but the fix is fairly easy. When I had an 8170 rebuilt by Econco I was surprised to find out that the recarburized 7.5V, 75A filament had so much emission that  I could turn down the potential to 6.7V with no decrease in peak output.  Since the Eimac recommended minimum filament potential was 7.15V, I phoned Dave at Econco, explained what Iwas seeing, and he explained that going below the factory recommended V was only a problem if peak output decreased or if the tube had many air atoms inside. 

The plain and simple fact is that 99.99% of hams, including me, just don't have the resources to run that close to the edge. Not even close.

  the edge is not serious. 

So what 99.99% of hams should do is to aim for the design-center voltage - 5.0 volts under load. That way the chances of being out of specification are minimized and tube life is maximized.  

  only in the minds of those who don't know. Operating a Th-W (thorium-tungsten) filament  more than 2% above the V that produces max PEP reduces tube life by the ratio of the filament potentials raised to the 23.4 power.  IOW, each 3% increase in filament potential reduces emissive life by 50%.  Example 10v/10.3v ^23.4 = 0.5, 10.3v /10v = 2. 

Which is the whole point of the filament-voltage discussion.

Of course it is counter-intuitive that running too cold can be as bad as running too hot. But that's how it is.

   "It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."
 — Mark Twain

>Transmitting tubes with thoriated-tungsten filaments are not the same things as light bulbs.

  Correct.  Light bulbs have 100% W filaments while electrom tubes have 98.5% W filaments.

CHEERS JIM

73 de Jim, N2EY


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K on October 01, 2011, 04:15:21 PM
My question to Tom was  if you know the self resonant frequency of the
tube in question, would using a parallel resonant supressor with a high C to L ratio be something
that is worth investigating.

Absolutely. It is a good method for minimizing loss on desired frequencies while maintaining effective suppression at a prolematic frequency.
  
Quote
The goal would be to minimize reactive losses at 30 MHz

  reactances don't incur losses Tom. they change the phase relationship between E and I. 

and below by using a LC tuned circuit that
would provide a low impedance path at HF, and a very high impedance path at the parasitic frequency.
this would force the VHF energy through the resistor, killing the Q at the self resonant, possible
parasitic requency.

There will be a certain optimum dampening resistance. My parasitic suppressor page goes into all of this.





Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K on October 01, 2011, 05:24:46 PM
OK, I am trying to follow this thread

It's really pretty simple.

Tube type HF amplifiers used by hams often have oscillations at frequencies other than the desired ones because there are unavoidable resonances in the circuit. Usually these are in the VHF region. A typical HF amplifier circuit looks different to VHF frequencies in such a way that the wiring itself becomes a tuned circuit.

  Amen Jim, well put. 

These "parasitic oscillations" or "parasitics" can't usually be completely cured by neutralization,

  Indeed, in fact they are unaffected by neutralization because neutralization is only at the operating frequency. 

bypassing or shielding. (Those things help, and are good practice, but most amplifiers need more).

  The capacitive feedback path for VHF parasitic oscillation exists inside the tube so dealing with it not a piece of angelfood cake with cream cheese icing. 

The usual cure is to insert one or more "parasitic suppressors" in the VHF oscillator circuit, in such a way that VHF oscillations can't start or be sustained.

  Correct, and a way to do this is to reduce VHF amplification by reducing VHF RL on the anode.  This can be done by using a low VHF-Q parasitic suppressor.

The typical parasitic suppressor is a small inductance in parallel with a noninductive resistor.

  There is no such thing as a non-inductive R, but there are low inductance resistors.

 The idea is that the inductor value is small enough that it is essentially a short-circuit at HF, yet large enough at VHF that the resistor is effectively in the circuit for VHF.

  good

The resistor introduces enough loss at VHF that parasitic oscillations can't start or continue.

  Well sort of, but a VHF suppressor's specific job assignment is to reduce amplification at the VHF resonance so that there will be not enough VHF amplification for VHF  parasitic oscillation.   

That's the theory. In practice, suppressing parasitics can take a bit of trading-off, particularly in amps covering the higher HF bands like 10 meters. This is because an inductor-resistor combination with values of L and R large enough to stop a parasitic may have some loss at HF too.

  good point.  There's a tradeoff to almost everything. 

... ... ...  cheers Jim. 
   


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K on October 01, 2011, 05:40:30 PM
OK, I am trying to follow this thread, but the one dude is putting everything in quotes, including his replies. That is not helping.

So how does one do it the right way?   tnx


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N2EY on October 02, 2011, 04:45:22 AM
OK, I am trying to follow this thread, but the one dude is putting everything in quotes, including his replies. That is not helping.

So how does one do it the right way?   tnx

Simple:

All quotes begin with a quote statement in brackets and end with a /quote statement in brackets.

Your own added verbiage has no such statement.

Look at your own post which I quoted and hit "modify" to see how it is done.

73 de Jim, N2EY


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K on October 02, 2011, 05:53:36 AM
OK, I am trying to follow this thread, but the one dude is putting everything in quotes, including his replies. That is not helping.

So how does one do it the right way?   tnx

Simple:

All quotes begin with a quote statement in brackets and end with a /quote statement in brackets.
like this Jim?

Your own added verbiage has no such statement.

Look at your own post which I quoted and hit "modify" to see how it is done.

73 de Jim, N2EY

[/quote]


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N2EY on October 02, 2011, 06:18:48 AM
OK, I am trying to follow this thread, but the one dude is putting everything in quotes, including his replies. That is not helping.

So how does one do it the right way?   tnx

Simple:

All quotes begin with a quote statement in brackets and end with a /quote statement in brackets.
like this Jim?

Your own added verbiage has no such statement.

Look at your own post which I quoted and hit "modify" to see how it is done.

73 de Jim, N2EY

[/quote]

Almost there. Note the extra /quote in brackets at the end.





Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: K9FV on October 02, 2011, 06:57:44 AM
Quote from: AG6K
So how does one do it the right way?   tnx

Note the
Quote from: ag6k....
that appears at top of reply window.
then verbiage of their quote you wish to use. then the

Be sure to put your words after the last [/quote] and this will allow your text to appear separate from quoted text.

 Mr Measures, from your last post I think you have the idea, but maybe this will help.

73 de Ken H>



Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K on October 02, 2011, 01:32:07 PM
OK, I am trying to follow this thread, but the one dude is putting everything in quotes, including his replies. That is not helping.

So how does one do it the right way?   tnx

Simple:

All quotes begin with a quote statement in brackets and end with a /quote statement in brackets.
like this Jim?

Your own added verbiage has no such statement.

Look at your own post which I quoted and hit "modify" to see how it is done.

  I CLICKED MODIFY AND THERE WERE NO CLUES.  MAYBE IT DOESN'T WORK WITH APPLE COMPUTERS?


73 de Jim, N2EY

[/quote]


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K on October 02, 2011, 01:55:15 PM
Quote from: AG6K
So how does one do it the right way?   tnx

Note the
Quote from: ag6k....
that appears at top of reply window.
then verbiage of their quote you wish to use. then the

Be sure to put your words after the last
and this will allow your text to appear separate from quoted text.

 Mr Measures, from your last post I think you have the idea, but maybe this will help.

73 de Ken H>


[/quote]

  tnx Ken.  So when I want to place comments within certain parts of a quote I have to type [/quote] before and after each comment I write ?


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: N2EY on October 02, 2011, 02:01:04 PM
In the interest of legibility I will try to explain the quoting process. If you already know how this works, please go on to the next posting.

----

When someone writes a posting and someone else replies by hitting "quote", an editing box appears with the original verbiage plus what I call "quote control phrases".

The quote control phrases are encased in brackets []. The beginning of a quote has the verbiage "quote author = ........." with lots of stuff about the author, topic number, message number, date, etc. with the whole thing encased in brackets. The end of a quote has just the word "/quote" encased in brackets.

eham sees those quote control phrases and puts the quotes in shaded boxes to differentiate the quote from the reply. However, you have to tell it when to start and stop by cutting and pasting the correct phrases in the correct places.  

For example, here's what a quote looks like when eham before processes it, with the brackets []replaced by braces {} so you can see the quote control phrases:

{quote author=AG6K link=topic=77773.msg542975#msg542975 date=1317516030}
So how does one do it the right way?   tnx
{/quote}

See the beginning and ending quote control phrases? If you replace the braces by brackets, they disappear because eham recognizes them.

Here's what it looks like after eham processes it (brackets used):
So how does one do it the right way?   tnx

When you just stick your own verbiage into a quote without an end-of-quote control phrase, eham doesn't know the quote is over and so makes your words part of the quote.

eham permits multiple/nested quote levels but tfirst get the basic working.

It has nothing to do with Apple or PC.

73 de Jim, N2EY


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: W8JI on October 02, 2011, 02:01:58 PM
Proper protocol would be to go to the site talk forum for help, and not totally ruin a running useful thread with unreadable posts and noise.   :-)


Title: RE: Parasitic Suppressor Resistors in Amplifiers.
Post by: AG6K on October 02, 2011, 02:50:58 PM
In the interest of legibility I will try to explain the quoting process. If you already know how this works, please go on to the next posting.

----

When someone writes a posting and someone else replies by hitting "quote", an editing box appears with the original verbiage plus what I call "quote control phrases".

The quote control phrases are encased in brackets []. The beginning of a quote has the verbiage "quote author = ........." with lots of stuff about the author, topic number, message number, date, etc. with the whole thing encased in brackets. The end of a quote has just the word "/quote" encased in brackets.

eham sees those quote control phrases and puts the quotes in shaded boxes to differentiate the quote from the reply. However, you have to tell it when to start and stop by cutting and pasting the correct phrases in the correct places.  

For example, here's what a quote looks like when eham before processes it, with the brackets []replaced by braces {} so you can see the quote control phrases:

{quote author=AG6K link=topic=77773.msg542975#msg542975 date=1317516030}
So how does one do it the right way?   tnx
{/quote}

See the beginning and ending quote control phrases? If you replace the braces by brackets, they disappear because eham recognizes them.

Here's what it looks like after eham processes it (brackets used):
So how does one do it the right way?   tnx

When you just stick your own verbiage into a quote without an end-of-quote control phrase, eham doesn't know the quote is over and so makes your words part of the quote.

eham permits multiple/nested quote levels but tfirst get the basic working.

It has nothing to do with Apple or PC.

73 de Jim, N2EY
tnx Jim.  cheers