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Using an SB220 on AM




 
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Jim, W5JO
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« Reply #100 on: April 28, 2010, 03:08:46 PM »

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Not to belabor a point, but where did this originate?  I have been fooling with amplifiers for years and never have read it anywhere. 

You can see it first hand at your station with a good scope.
Tune up with a tone (I use 1kHz) at 100% in the "low power" position. Then go to High power and you'll find the load needs tweeked. WHY?

Most hi/lo switched on amps cause an increase in plate voltage. You've just changed the plate impedence!

When you modulate you change the voltage and current. Peak values present a different impedence than nominal.

So yes, it's all about impedence.

That is what I was speaking about Buddly.  Changing the tune and load controls simply changes the impedance between the final and load.  If you tune for max power, you tune for max power plus matching the plate impedance to the load impedance and should you want to "Load lightly" then you can set the load control either side of peak power and accomplish the same thing.  Change the impedance and less power will be transferred resulting in less current in the plate and grid.  You simply have one impeadance at the load undercoupled and less power then the same thing but with a different impedance overcoupled.

Changing the plate voltage does affect the plate impedance but if you use lower plate voltage to tune then you should retune after changing the plate voltage (or for that matter the current) in any case.  Who was it, Pete that said the last adjustment should be the plate control for final dip, which is correct.  Most high power amplifiers have recommended settings for tuning or at least should.  Nothing I have ever built or used has a high/low siwtch.  I always just start with low drive and work up, but one thing for sure is if you aremismatched at the load, you will have heating in the final tank circuit, so I am not going to do that.

If there are any mistakes or mispellings in my answer it is because the dam reply panel keeps flopping up and down making proof reading impossible.  What is wrong with this thing?
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K5DBX
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« Reply #101 on: April 28, 2010, 06:16:03 PM »

Carl,

At least your logic is moving closer to exactly what I said in my first post. Like I said, I prefer to keep my triode in grounded-grid configuration, and I really don't need NiCr wire, 12 guage solid copper works just fine. When you begin to float the grids with a modest 50pf, a GDO and a SA will show you that the grid resonance at low VHF range increases from about -50db~-40db with grounded grid to close to 0db to +10db floated, and the farther up the band you move, the worse it gets. Add more capacitance, exacerbates it further (you are now creating a triode in a oscillator configuration) For the life of me, I cannot understand this die hard need to rationalize poor engineering practices, unless of course you DO want your triode to be an oscillator.

I would question however, the "double duty" assumption that using NiCr wire somehow provides protection to a catastrophic failure of a valve. I have seen numerous catastrophic failures of tube amplifiers for decades, and I can attest to one thing, using NiCr wire on the anode won't save anything if the anode goes to grid or cathode. At Altec, we saw hundreds of triode based amps come in for service after running 24/7 for decades. When a valve fails, NiCr wire will not respond fast enough to go open circuit, or provide enough "surge current limiting" to do a thing. All you have done is reduce available current to the anode for regular service, and provided a false sense of insurance.
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K5UJ
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« Reply #102 on: April 28, 2010, 06:28:58 PM »

I think it is your browser.  I had that happen to me on some PC I was using with Internet Explorer I think (but might have been something else) and there is something about the form for posting that made it go crazy.

I wonder if "tune for maximum power output" is what manufacturers tell folks now mainly because it is a no-brainer for consumer product hams.  The amp I have from Ten Tec even has a LED output voltage driven watt bar to make tuneup even easier.  All they have to tell people is tune for the maximum reading on the LED watt bar.

To be fair, it is also the case that with a class AB linear amp, the plate current dip is not as pronounced as it is with a class C PA in a plate modulated AM rig.  The Ip meter measures average Ip.   At resonance of the tank, in class C with the tube is conducting for a small part of each cycle the average Ip dips noticeably.  With class AB the conduction is much more of the cycle so there is not as much noticeable dip when the tank is matched the anode to the load.   So the dip and load instructions are harder for people to follow, and making things worse are newer amps that include 160 m. which seems to force some tank circuit compromises to get from 1.8 to 30 MHz.  

Manufacturers could get into what I just got into, but probably find it easier to tell people to just tune for max suds and design the RF deck so if you don't overdrive the amp you can do that and be linear.  

But, if you want to be kind to ur amp   Cheesy you can also drop the load cap value a tad.

Rob
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"Not taking crap or giving it is a pretty good lifestyle."--Frank
K5DBX
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« Reply #103 on: April 28, 2010, 06:36:04 PM »

I think it is your browser.  I had that happen to me on some PC I was using with Internet Explorer I think (but might have been something else) and there is something about the form for posting that made it go crazy.

I wonder if "tune for maximum power output" is what manufacturers tell folks now mainly because it is a no-brainer for consumer product hams.  The amp I have from Ten Tec even has a LED output voltage driven watt bar to make tuneup even easier.  All they have to tell people is tune for the maximum reading on the LED watt bar.

To be fair, it is also the case that with a class AB linear amp, the plate current dip is not as pronounced as it is with a class C PA in a plate modulated AM rig.  The Ip meter measures average Ip.   At resonance of the tank, in class C with the tube is conducting for a small part of each cycle the average Ip dips noticeably.  With class AB the conduction is much more of the cycle so there is not as much noticeable dip when the tank is matched the anode to the load.   So the dip and load instructions are harder for people to follow, and making things worse are newer amps that include 160 m. which seems to force some tank circuit compromises to get from 1.8 to 30 MHz.  

Manufacturers could get into what I just got into, but probably find it easier to tell people to just tune for max suds and design the RF deck so if you don't overdrive the amp you can do that and be linear.  

But, if you want to be kind to ur amp   Cheesy you can also drop the load cap value a tad.

Rob

Hey Rob!

You know, over on YouTube Ameritron did a couple tune-up videos....

http://www.youtube.com/watch?v=OT1UaCrVvd4
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Jim, W5JO
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« Reply #104 on: April 28, 2010, 08:28:56 PM »

Rob it is probably my browser, but I am not sure so I can't say.  All I know is the board design parameters should be for compatibility with anything.  But that aside.

Tune for max is matching the tube plate impedance to the load impedance.  If you tune for a mismatch on the load side you have higher circulating current in the tank circuit.  I have seem many B&W 850 tank coils that are melted because of this kind of thing.  A pair of 4-1000s in parallel with 6 kv on the plate with as much current as the pole pig would develop into something that either was mismatched or broken.  That caused the coil supports to melt, I don't care who designed it or what frequency of operation.  Push the knot to the end of the rope and it disappears like parts in high power amplifiers.
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KM1H
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« Reply #105 on: April 28, 2010, 10:01:42 PM »

Carl,

At least your logic is moving closer to exactly what I said in my first post. Like I said, I prefer to keep my triode in grounded-grid configuration, and I really don't need NiCr wire, 12 guage solid copper works just fine. When you begin to float the grids with a modest 50pf, a GDO and a SA will show you that the grid resonance at low VHF range increases from about -50db~-40db with grounded grid to close to 0db to +10db floated, and the farther up the band you move, the worse it gets. Add more capacitance, exacerbates it further (you are now creating a triode in a oscillator configuration) For the life of me, I cannot understand this die hard need to rationalize poor engineering practices, unless of course you DO want your triode to be an oscillator.

I would question however, the "double duty" assumption that using NiCr wire somehow provides protection to a catastrophic failure of a valve. I have seen numerous catastrophic failures of tube amplifiers for decades, and I can attest to one thing, using NiCr wire on the anode won't save anything if the anode goes to grid or cathode. At Altec, we saw hundreds of triode based amps come in for service after running 24/7 for decades. When a valve fails, NiCr wire will not respond fast enough to go open circuit, or provide enough "surge current limiting" to do a thing. All you have done is reduce available current to the anode for regular service, and provided a false sense of insurance.

You seriously insist on missing the point of the reasoning behind the floating grids. I really havent noticed your logic as it doesnt seem to follow any pattern. They do work when the proper values are used and the classic 2 tone static IMD test shows about a 2-3dB IMD improvement over direct grounding.

You also totally missed my point of insisting on using a real wire wound resistor. I said nothing about using nichrome as part of the anode circuit wiring. The proper use of a HV surge resistor is to limit fault current to a safe value until a fuse or other safety device trips. It is also supposed to survive. Since internal gas arcs are a lot more common than a grid falling into a plate it works rather well, Ive yet to see one blown open and many tubes have survived. Ive worked around, designed as a job, and repaired amps for all applications (not just ham) for close to 50 years and can say Ive about seen all possible failure modes.
A classic enameled wire wound fits the requirement, a cement resistor does not.

Complaining about the small voltage drop in normal operation is ludicrous at ham power levels.

Carl
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K5DBX
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« Reply #106 on: April 29, 2010, 06:51:13 AM »

Carl,

Believe me, I know how to use triodes as an oscillator. To try to squeeze 2db~3db improvement in IMD at the 3rd or 5th order region, when it is already -40db~-50db below the primary two-tone, in my opinion, doesn't justify elevating the grid resonance in the VHF range from -40db~-50db to 0db~+10db or more, and creating for yourself the issue of oscillation. +/-2db variance can be seen in assembly or wire routing or shielding. You more than likely, (with most common transceivers today) will be introducing IMD at the -30db 3rd order region from the exciter. For the medium Mu 3-500z, I'll leave the grids grounded.

I did however, misunderstand you when I thought you were advocating the use of NiCr wire as the anode lead material. I do understand you in using a current limiting resistor in the anode path, but if you want a protection circuit, I would prefer using an active HV crowbar circuit. Back in the 90's when AG6K wrote his article about the SB-220 in QST, there were engineers who spoke directly to this issue about using current limiting resistors in the anode path in their rebuttal to the article. From the Sept. 1994 issue of QST; "The addition of a 10 ohm resistor may only offer a negligible improvement in most circuits. (which I agree with) It is alarming to note (the article which proposes this method) recommends a resistor rated at 10 watts 500v. Any device rated at 500v is subject to several thousand volts. The manufacturer of such resistors absolutely do not endorse the use of this type of resistor in this application. The correct component would be an energy absorbing type of resistor such as the Carborundum SP type, or RDC PCN type. The PCN series resistor for this type of application should be rated at 80 watts or more dissipation to safely handle the stored energy of a 25uF capacitor circuit charged to 3kv."

I guess at the core, I have to ask myself, how many folks do you hear complaining about how horrible the IMD is on 3-500z based linears in a grounded grid configuration, and then ask, why do so many folks seem to have to address the issue of VHF stability in those amps who utilize a floating grid. I would prefer the most stable platform, especially when it's IMD is equal to or well below the exciter I'm driving it with.
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KM1H
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« Reply #107 on: April 29, 2010, 10:52:58 AM »

I dont know where you are coming up with those resonance # as there isnt any reference mentioned.

The actual (high Mu as defined by Eimac) 3-500Z IMD 3rd in a SB-220 as biased is in the mid 30's at best and that is PEP. Adding another 2-3dB on a quiet band is certainly noticable. While this is better than many rice box rigs it falls far short of the Class A exciter options as well as the -40dB and better 6146 rigs of the past. If you really want excellent IMD, run zero bias in the CW position and stuff 560-680uF caps in the PS. Then you can use it as an IPA for a real tube.

With my primary SB-220 interest being 6M conversions, of which Ive done over 250 since the early 70's, Ive gone for stability and SSB signal quality. Customer feedback indicates long term stability with any brand of tube and the grid chokes doing their part during a gas arc. The FB also refutes any claim that the stability falls as the frequency goes up. In fact the amp works quite well on the European 4M band.

Since directly grounding the grids raises the gain it then holds that instability potential increases.  There is no benefit to be gained by trying for another 100W out other than stressing an already marginal PS and tank circuit in a stock amp. Many hams have reported going back to the chokes and later value caps after experiencing switch arcing. Adding a resistor to the input also works.

IMO it is a waste of time to modify a SB-220 grid circuit except for changing to 115pf caps (standard value 110 and 120pf also work). Instead concentrate on new suppressors and adding the surge resistor. Adding one little 10pf 1kv SM cap across the 40-20M switch lugs will act as a snubber to prevent contact arcing.

I could go on about tweaking the suppressor design to reduce heating but thats a whole different subject.

HV surge resistors have been around for many decades before the QST wars. RCA even included them in spec sheets of the 50's. The earliest ham amp use that Im aware of is in the 1963 National NCL-2000 of which I was on the design team as a fresh out of the Navy ET.

Carl

 
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K5DBX
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« Reply #108 on: April 29, 2010, 12:15:41 PM »

Carl,

There are a couple of sites out there that have mirrored what I have seen with my own testing of grid resonance...however my results are from the SB-221 w/Johnson socket.

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

The SB-220 is a fine candidate for use in 6 meter operation. I've seen many at full legal limit. As far as a current limiting resistor in the HV path for catastrophic failure, my point was not unlike Tom Rauch's point, it's negligable, and won't help you in the least if the valve fails. And recommending or using a device not intended for the voltages incurred in the HV path is not good engineering. I prefer other methods to protect the HV area.
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KM1H
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« Reply #109 on: April 29, 2010, 08:53:31 PM »

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The SB-220 is a fine candidate for use in 6 meter operation. I've seen many at full legal limit. As far as a current limiting resistor in the HV path for catastrophic failure, my point was not unlike Tom Rauch's point, it's negligable, and won't help you in the least if the valve fails. And recommending or using a device not intended for the voltages incurred in the HV path is not good engineering. I prefer other methods to protect the HV area.

Rauch is one of the last people Id use as a reference. Your statements above confirm that you really arent that knowledgable about the subjects.

Im thru wasting my time.
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K5DBX
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« Reply #110 on: April 29, 2010, 09:29:11 PM »

Carl,

I wish you well then...have a great day. If you have a beef with Rauch that's your own issue. However, that doesn't address the issue to which I was speaking, nor does it justify poor engineering practices. I can only assume if you can't argue with results of the sweep, you need to discredit the source and dismiss it since it doesn't serve your agenda. Oh well. And BTW, Tom Sowden, (W6KAN) stated, "Warren Amfahr (W0WL), a top Collins engineer when the HK SB-220 was being designed, told me in a phone conversation that the "feedback" idea was originally driven by Mr. Collins. He felt the IMD products of the Eimac tubes were not low enough to meet Collin's standards and therefore he had his engineers draw up a feedback circuit that was later borrowed by Heathkit."

On the SB-221 I own, it's 3rd order IMD in a grounded-grid configuration is at worst -38db, and 5th order -46db on 2 tone in the 75 meter band. Move the bias current greater than 200mA and those numbers begin to vary. It's spectral 2nd harmonic is better than -50db and 3rd harmonic better than -68db. +/-2db of IMD at those levels won't be perceptable to the ear.

I said it before, and will again...I will stick with my grids firmly attached to ground and you can roll your own any way you wish...good luck...  
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