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on air tuning because of stability issues




 
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Patrick J. / KD5OEI
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« on: January 02, 2017, 12:34:30 PM »

While I am able to tune nearly everything into a 50 Ohm load, at some point I almost always have to do a short tune up on the air to finalize a couple things like grid drive (of all things). This is a 4-1000 plate modulated.

At carrier the scope is zoomed in to show the RF wave and a couple things touched up to make sure it's smooth and nice. there can otherwise be very slight notches in the waveform's sides. These are eliminated by tuning the grid off resonance very slightly one way but not enough to affect the reading on the  dual needle meter between the exciter and class C stage.

Then with a sine wave 100% modulation making sure that at max. modulation the peaks of the RF envelope are clean of noise. Usually boosting the grid drive is all that's needed, if that issue shows up.

I can use the sine wave for this tuning because the audio is well-processed and the peaks are all the same height, voice or sine wave.

It's an annoyance but seems to be a result of operating the unit at a lower power than it was designed for. Meaning the 4-1000 does not like less than 3500V and is very fussy at 2700V where the power level is more reasonable but the grid current and screen current tend to escalate.
It's only happy with a 600-700W carrier. Maybe a 4-400 which likes lower voltages would be netter in there but it won't fit the socket.. The tuner in use won't take any more than 'legal limit' AM and I don't even want to go down that road. I have not tried a 3-1000 but its drive is an entirely different mess.

I hate to have to fiddle with this on the air. How embarrassing.  It must partly be due to some instability and for whatever reason may not be well enough neutralized.

Neutralization is by a copper rod #8 gauge placed 1" from the chimney. How about adding a 1" square metal plate to that top of that rod. like the plate of a cap and the other plate is the plate of the tube? Can I over-neutralize this?
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« Reply #1 on: January 02, 2017, 03:25:38 PM »

hi Patrick ... how much screen V are you running ?    It would be interesting to see if the tx is more stable at lower Vsc

a neutralizer needs a variable cap to exactly balance the amp stage .... either the feedback stub cap or the cap to gnd in the usual single ended circuit .... various ways to adjust and check
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« Reply #2 on: January 02, 2017, 07:08:43 PM »

Hi Patrick -

There are several ways to tame down a final amplifier that is RF unstable.  I've had mucho experience with the 4-1000A in both class B and plate modulated class C service.

1) The first way is neutralization. You've already done this, but be sure that it is working properly. I usually "cold" neutralize it. With just fils on, inject a signal into the grid as usual and hook the scope probe to the 50 ohm C2 plate loading cap. You should see a definite null in pass-thru signal as the optimum neutralization point is passed. You have a variable way to tune the feedback, right?  

2) Another VERY effective method is to add a loading cap/resistor from grid to ground right at the tube pin. Usually a .001 uF (heavy duty brown 1"X1" type) in series with a non-inductive resistor like  50 watt Glowbar works well. The resistor value can be anywhere from 600 ohms to 6K. The lower the value, the harder to drive, but the more stable the amplifier. All of my big rigs have a grid loading cap/resistor just as good practice, starting with about 3K or so.  A very stable amp may not need it below 40M, but some will.

3) Be sure to sprinkle a lot of .001 1KV BYPASS disc caps onto ALL incoming leads. This means keying lines, AC, grid voltage input, screen voltage in, etc.  Use a C-L-C input for the grid D.C. and screen D.C. close to the tube pins. (500V > .001, 2.5mH, .001 > tube pin)

4) Be absolutely sure the RF INPUT leads and parts cannot see any RF OUTPUT leads and parts. Watch the paths and routing of other DC leads too. Keep RF leads away from ALL other stuff. Use shielding/barriers. Study the underside and top of chassis and look for possible coupling. A common problem can be the antenna relay that might have the input and output leads close together on a common relay. Two separate relays with a shield between them is a better idea.    I always use a barrier strip on the back of the rig with .001 caps bypassing every incoming lead before entering the critical inside.. Feed-thru caps work well too .

There's more, but try these first and report back. I am quite confident that the .001/ 1K-2K non-inductive resistor(s) from grid to ground will do the trick. Be sure to figure how much power this power resistor will dissipate and rob from input drive watts. Good thing that  it is under the chassis so that the blower air will help it cool a lot.

Let us know.

T
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« Reply #3 on: January 04, 2017, 09:37:44 PM »

hi Patrick ... how much screen V are you running ?    It would be interesting to see if the tx is more stable at lower Vsc

a neutralizer needs a variable cap to exactly balance the amp stage .... either the feedback stub cap or the cap to gnd in the usual single ended circuit .... various ways to adjust and check

The screen is running at 200V or so which is close to the low range on its variac control. The screen voltage is run low because the plate voltage is run low (2700V) because the carrier is already 400W.
There is a 2K resistor and 8H choke in series with the supply and screen that drops the voltage to 200 from whatever it otherwise would be, about 400V. They can be shorted out in CW mode and the screen voltage is much higher. The screen is very well modulated without question, has been checked with a scope.
The supply is a typical FWCT, two section choke input type, 800-0-800 @225mA, 5R4, 8.5H, 4uF, 8.5H, 2uF setup. It's very standard old school type.

There never has been an adjustment for balance or the neutralizing capacitor, but a kind of design where the grid to plate capacitance of 25pf is balanced by a 25pf cap in the grid circuit.


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« Reply #4 on: January 04, 2017, 10:03:03 PM »

Hi Patrick -

There are several ways to tame down a final amplifier that is RF unstable.  I've had mucho experience with the 4-1000A in both class B and plate modulated class C service.

1) The first way is neutralization. You've already done this, but be sure that it is working properly. I usually "cold" neutralize it. With just fils on, inject a signal into the grid as usual and hook the scope probe to the 50 ohm C2 plate loading cap. You should see a definite null in pass-thru signal as the optimum neutralization point is passed. You have a variable way to tune the feedback, right?  

No adjustment but it may be time to consider it.
Quote

2) Another VERY effective method is to add a loading cap/resistor from grid to ground right at the tube pin. Usually a .001 uF (heavy duty brown 1"X1" type) in series with a non-inductive resistor like  50 watt Glowbar works well. The resistor value can be anywhere from 600 ohms to 6K. The lower the value, the harder to drive, but the more stable the amplifier. All of my big rigs have a grid loading cap/resistor just as good practice, starting with about 3K or so.  A very stable amp may not need it below 40M, but some will.

I did this before with a 3K globar NI resistor of 50-60 and IIRC the stage was stable. I removed it because I needed the existing front panel switch to change input coils on the MB-40 tank. I don't recall if there was some tuning issue or not with the resistor.
Quote

3) Be sure to sprinkle a lot of .001 1KV BYPASS disc caps onto ALL incoming leads. This means keying lines, AC, grid voltage input, screen voltage in, etc.  Use a C-L-C input for the grid D.C. and screen D.C. close to the tube pins. (500V > .001, 2.5mH, .001 > tube pin)

This has to be revisited. LC filters are presently at the rear panel but not on all connections. The pins of the tube except grid drive are all very heavily bypassed right at the socket.
Quote

4) Be absolutely sure the RF INPUT leads and parts cannot see any RF OUTPUT leads and parts. Watch the paths and routing of other DC leads too. Keep RF leads away from ALL other stuff. Use shielding/barriers. Study the underside and top of chassis and look for possible coupling. A common problem can be the antenna relay that might have the input and output leads close together on a common relay. Two separate relays with a shield between them is a better idea.    I always use a barrier strip on the back of the rig with .001 caps bypassing every incoming lead before entering the critical inside.. Feed-thru caps work well too .


The input is well separated from the output and the RF jacks are some 25" apart, although the input and output coaxial cables are through the same hole in the overall cabinet. There is no TR switch on it and a separater antenna is used for receive. The grid tank circuit compartment is under the chassis in a box with just the grid lead, neutralizing cap lead, and the coax cables for 50 Ohm drive and grid bias poking through. Bias and 50 Ohm input RF drive go from the back panel to that box through shielded cables. No output, HV, or other thing comes through the chassis from the plate area except the neutralizing 'capacitor' rod.

I should find pictures because there is probably room for improvement.
Quote

There's more, but try these first and report back. I am quite confident that the .001/ 1K-2K non-inductive resistor(s) from grid to ground will do the trick. Be sure to figure how much power this power resistor will dissipate and rob from input drive watts. Good thing that  it is under the chassis so that the blower air will help it cool a lot.

Let us know.

T


Ok so I should try the globar resistor and capacitor right at the socket? That can be done. This picture is old, the globar resistow was removed.


* 102_0019.JPG (769.05 KB, 1500x1125 - viewed 116 times.)
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« Reply #5 on: January 06, 2017, 11:50:24 PM »

Hi Patrick -

Yes, I would add the 3K globar and series .001 right at the socket pin. You can even go lower, like 1.5 K if the unit is still unstable. 3K will have a slight effect whereas 1.5K will have a big stability effect and the drive increase is not that bad.

The neutralization circuit: By all means you MUST make it a variable adjustment. Because while looking at the feedthrough signal you will see it get smaller as the coupling is increased and then hits a NULL - but once you continue with more coupling adjustment, the feedthrough signal will get larger again. IE, it's like tuning a tank to resonance - there is just one optimum point or else you can easily make the instability worse. This may be a large part of your problem. It might even be better to have NO neutralization rather than a setting that is random like that... :-)  As you will see, the absolute null is fairly critical and it will be deeper if your layout is good so that the input circuitry cannot see the output circuitry.  The only appreciable coupling should be the feedthrough capacitance through the tube elements, which you are attempting to cancel out thru neutralization.

This 813 schematic shows an easy way to do adjustable neutralization using a small plate facing the tube. http://www.amwindow.org/tech/htm/813/813.htm    
Once you get the sampling metal plate size and spacing in the ballpark, you will find a good null using the 300pF variable capacitor.

Other than those two items, the picture looks good from what I can see so far.

Oh, just be aware that the 4-1000A metal socket (made for audio service) doesn't have the best air flow and has longer connections.. The black baked-mud-plastic type are best. Consider drilling a circle of 3/8" holes (or 1/4", whatever fits) around the inside edge on the socket - bordering the glass chimney to increase the air flow. I have done this in the past and it made a big difference.   (When using the mud sockets, the holes are drilled thru the chassis inside the chimney border diameter.)

T
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https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #6 on: January 08, 2017, 07:50:28 PM »

Next time I can get it out I'll see about adding the globar resistor back in. Maybe next weekend if my helper comes.

The MB-40 is designed to have its rotors/chassis at RF ground but the only reason I can see for that is the faraday shield on the input link. If I recall, hanging a big cap to ground off one side of the MB-40 caused it to tune to two frequencies, that is, there were two peaks for any given frequency, one for one side and one for the other. It would also not go to 10M. If I can get the MB-40 chassis isolated from RF ground it might work better or avoid this trouble. The MB-40 appnote says 'conventional neutralization can be used' but gives no single ended example. The push pull examples are the typical setup.

I'm fairly confident about air flow because the tube never runs anywhere near its ratings. If I ever have the socket out I should work on it.
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