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New 4X1 X 4X1's Rig - Problem with audio spikes - Questions - Pictures




 
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Author Topic: New 4X1 X 4X1's Rig - Problem with audio spikes - Questions - Pictures  (Read 88971 times)
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WD5JKO
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« Reply #25 on: November 29, 2009, 11:36:54 AM »

I could never understand why some many people are worried about keying a transmitter by switching the AC to the primaries on the plate transformers on and off all at once. That's the way most all the old transmitters worked for years and years. So what if the screen supply comes up a few milliseconds before of after the HV plate supply, etc.?

The only main thing necessary to have are the proper chokes on the HV supplies. Big swinging chokes work the best.

   Brian,

   You bring up a good point. The issue here though is that Tom keeps the HV on all the time, and keys the HV B+ with a relay. This unquestionably creates a different monster to contend with. I'm not saying this is bad, but in doing so there might be some unforeseen consequences, along with some different ways to tame an unruly shrew.

Jim
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« Reply #26 on: November 29, 2009, 11:55:50 AM »

I wildly speculate that the RF is biasing some device in the SS amp enough to cause a DC offset in some stage or another but not enough to be noticed in the audio during operation. When the Rf disappears, so does the bias, and so comes the pulse from the SS amplifier. When the rig is keyed, this event inside the SS audio amp is over with before the modulator is powered up enough to be affected by it.
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« Reply #27 on: November 29, 2009, 12:12:22 PM »

Tom,
I'm thinking it is a rate of change transient. Try a series RC across the modulator key contacts to slow down the turn off of the tubes. Make sure the antenna is delayed a bit longer. You want the tube to bias itself off slowly so the iron doesn't see a transient. The driver closed loop may be generating another transient when it sees a load change. IF you have extra drive power put a resistor across the grids of the modulators to hold the tube input Z more stable. A sequencer will work as long as the transient has a place to go. Remember the old point ignition system and how bad the car ran when the condenser across the points went bad. This condenser was a shaping network to extend the turn off a bit to generate a longer spark.  You need to extend it a lot longer so there is no transient. Killing primary power slows down the turn off but you have to live with all the extra hassle of step start noise and shocks to the power supply turning on and off.
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« Reply #28 on: November 29, 2009, 12:21:01 PM »

Great suggestions! That's just what I needed - things to try until I find a solution. Lot's to work on now.

Yes, the HV primary AC is on ALL the time and the HV DC is keyed by a HV relay and routed to the particular rig. Itís one of those long arm swing types on insulators. I do this simply for safety reasons since the HV is on the rig ONLY when I key up and not sitting there all the time. However, for this un-key spike problem, by keeping the HV on the 4X1 rig all the time did NOT have any effect on the inductive spike. I tried that as one of my first tests... sigh.

Bruce - that's a beautifully written and detailed explanation for sequencing and shud be accessible for future use. Well, right now I am partially there with the RF drive being unkeyed first and the TR ant relay last. But the mod tube relay and RF tube final relay come on and off at the same time. I plan today to put in separate relays and unkey the modulators before the RF final as suggested.

BTW, Frank, I already tried putting a cap across the cathode keying relay contact's resistor - no help. I also left the modulators on as an experiment and other relays trials - no help.


Gito - I notice you usually come up with great solutions in other threads.  I will most certainly try one of those spike protected AC strips to see if it helps the SS amp - thanks.

One of the first things I want to try is to put a relay on the output of the SS amp that shuts off early, before all the other relays, as Bruce suggested. Since the spike is obviously coming out of the SS amp, that cud be an easy fix. (The spike problem goes away when I shut off the SS power or disconnect its output from the driver xfmr)


Derb, if I had a tube driver amp I wud use it, but donít.   Iím starting to think that maybe it, too, wud have the same problem if itís a sequencing problem. Weíll see. Iíll post the solution, of course. If I run outa things, Iíll build up a triode direct-connected 813 audio driver. No driver transformers.

Iím running the rig at 250w out so hope to minimize stress. (Yes, by loading it lightly, a 4X1 WILL limbo low)  But still, that inductive kick is pretty severe.   It caused me to find a few weak spots amongst the mounting of the mod parts already that were sparking. Thatís a good thing, actually, but now the mod xfmr arc gaps are taking all the abuse.

I plan to take a pic of the outside mounted blower today. It is encased in a Plexiglas housing that makes it look like a tiny air conditioner. Iíll paint it white to match the house later.


Bear sent me an email about trying a slope capacitor technique to slowly unenergize the mod iron on unkey.  He said it worked for W1RKWís rig in  some past thread here. Anyone know where that is?

Plexiglas:  I donít worry about shielding. As others have seen here, Iíve run many rigs under Plexi over the years, including a full blown pair of 4X1ís PDMed by a pair with no RF in the audio problems I couldnít cure. Radiation calculations work out to be minuscule at 75 and 40M, the bands of operation.  I just love watching 4X1ís operate thru the Plexi.  BTW, Fabioís nickname is gonna be, ďThe Invisible MawlĒ  - named after the Invisible  Man and Invisible Motor toys..  

Thanks again for all the help, OMís.

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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #29 on: November 29, 2009, 12:24:49 PM »

Hey Tom it's a beaut.  You sure do good work.  Can you lift that thing yourself?  Must be pretty heavy.

This probably has nothing to do with nothing but I'm curious about the braid to the anode caps--I've never seen that done before--I've always seen copper or silver plate flat strap.  Maybe I have not been around long enough.  I wonder if the braid has any additional inductance.

73

Rob
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« Reply #30 on: November 29, 2009, 12:40:03 PM »

I wildly speculate that the RF is biasing some device in the SS amp enough to cause a DC offset in some stage or another but not enough to be noticed in the audio during operation. When the Rf disappears, so does the bias, and so comes the pulse from the SS amplifier. When the rig is keyed, this event inside the SS audio amp is over with before the modulator is powered up enough to be affected by it.


Interesting. I wonder how I would test this. I could move the SS amp closer to the oprating position, away from the rig, but then the NFB audio lead coming back wud be much longer.  Right now I just use a longer 1v audio feed to the SS amp.  

Which, BTW, I have a VERY slight 60hz hum in the audio. It disappears completely when I disconnect the low level audio from the SS amp.  The RF carrier is perfect at that point.  I wonder if a 20' run of regular shielded  RS mic cable is good enuff to handle the task from the 1V low level compressor output to the SS amp?  (Next problem to solve)

Right now I'm testing into the Bird dummy load located on the floor under the audio gear. If there wud be RF problems, it wud be now... Wink


Rob - On the silver plated braided shield used for RF connections... I've used it for ground connections for years.  I use it for the plate cap connections to take the mechanical stress off the fragile plate cap. I think copper strap is a little harsh, but it wud probably be OK anyway. It shud be OK for RF despite the mesh.
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #31 on: November 29, 2009, 12:40:52 PM »

Tom,

It sounds like you have a lot of good suggestions to try out for the rig and you should have a good handle on sequencing.  I don't really like the idea of keying the rectified DC since that is a recipe for arcs and transient events.  Although it is less of a problem with the fairly rugged screen structure in the 4-1000A you do want to make sure that the screen doesn't come up before HV and is dropped simultaneously or before the HV is dumped to avoid frying the screen structure.  It is also a good idea to drop the modulator drive prior to dumping HV.  

A scope is your best bet for looking for stray RF in the modulator and also capturing transient events.  Although a storage scope is handy for glitch capture you can do a pretty good job with a regular scope set for single sweep and a decent digital camera (darker room with proper mix of ISO, aperture, and long exposure shutter setting so that the shutter is open during the event).

I may have missed it in your description but do you have RF chokes in the output of your SS amp (bypassed on each end) where it feeds the modulator tubes?  SS stuff does some interesting things when RF shows up where it doesn't belong.  My first attempt at using SS relays to key the plate transformer in my Desk KW produced interesting results until I took care of RF getting into the relays via the control line; the primary voltage was being "modulated" by RF on the control lines which produced a really interesting sounding carrier  Smiley

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« Reply #32 on: November 29, 2009, 12:54:24 PM »

Hey Tom it's a beaut.  You sure do good work.  Can you lift that thing yourself?  Must be pretty heavy.

This probably has nothing to do with nothing but I'm curious about the braid to the anode caps--I've never seen that done before--I've always seen copper or silver plate flat strap.  Maybe I have not been around long enough.  I wonder if the braid has any additional inductance.

73

Rob
K5UJ

Rob,

My TMC GPT-750 also uses regular tin-plated copper braid as the interconnect between the heat radiating plate caps on the 4-250As and the RF plate choke, so it has been done in the past in commercially built rigs.

In my homebrew 2x 4-400A rig, I use fairly flexible soft copper strap for this connection. Works FB.

73,

Bruce
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« Reply #33 on: November 29, 2009, 01:00:14 PM »

I wildly speculate that the RF is biasing some device in the SS amp enough to cause a DC offset in some stage or another but not enough to be noticed in the audio during operation. When the Rf disappears, so does the bias, and so comes the pulse from the SS amplifier. When the rig is keyed, this event inside the SS audio amp is over with before the modulator is powered up enough to be affected by it.


Interesting. I wonder how I would test this. I could move the SS amp closer to the oprating position, away from the rig, but then the NFB audio lead coming back wud be much longer.  Right now I just use a longer 1v audio feed to the SS amp. 

Which, BTW, I have a VERY slight 60hz hum in the audio. It disappears completely when I disconnect the low level audio from the SS amp.  The RF carrier is perfect at that point.  I wonder if a 20' run of regular shielded  RS mic cable is good enuff to handle the task from the 1V low level compressor output to the SS amp?  (Next problem to solve)

Right now I'm testing into the Bird dummy load located on the floor under the audio gear. If there wud be RF problems, it wud be now... Wink


Rob - On the silver plated braided shield used for RF connections... I've used it for ground connections for years.  I use it for the plate cap connections to take the mechanical stress off the fragile plate cap. I think copper strap is a little harsh, but it wud probably be OK anyway. It shud be OK for RF dispite the mesh.

Tom,

Are the leads between the output of the negative feedback ladder/DC blocking cap circuit and the SS audio driver amp shielded?

I'm not suggesting that this is the cause of the transient event that is occuring, but if those leads are unshielded, they are a potential source of noise and RF to find their way into the low level stages of the driver where the NFB is introduced.

73,

Bruce
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« Reply #34 on: November 29, 2009, 01:02:00 PM »

Tom,

It sounds like you have a lot of good suggestions to try out for the rig and you should have a good handle on sequencing.  I don't really like the idea of keying the rectified DC since that is a recipe for arcs and transient events.  Although it is less of a problem with the fairly rugged screen structure in the 4-1000A you do want to make sure that the screen doesn't come up before HV and is dropped simultaneously or before the HV is dumped to avoid frying the screen structure.  It is also a good idea to drop the modulator drive prior to dumping HV.  

A scope is your best bet for looking for stray RF in the modulator and also capturing transient events.  Although a storage scope is handy for glitch capture you can do a pretty good job with a regular scope set for single sweep and a decent digital camera (darker room with proper mix of ISO, aperture, and long exposure shutter setting so that the shutter is open during the event).

I may have missed it in your description but do you have RF chokes in the output of your SS amp (bypassed on each end) where it feeds the modulator tubes?  SS stuff does some interesting things when RF shows up where it doesn't belong.  My first attempt at using SS relays to key the plate transformer in my Desk KW produced interesting results until I took care of RF getting into the relays via the control line; the primary voltage was being "modulated" by RF on the control lines which produced a really interesting sounding carrier  Smiley

Rodger WQ9E

Hi Rodger,

The switched DC is a carry-over from using it with linears here. Seemed to work OK there, but maybe not for plate mod. The RF carrier keying looks very clean with no spikes on the scope Ė without the modulator involved.  Bear in mind keeping the HV on the rig all the time had no effect on this problem, but you bring up a good point about the screen being naked for a few millisecondsÖ.Iíll address that later on for sure.

Interesting on using chokes in series wid the audio leads to the modulator grids. What values shud I use there, like 2.5mh types or less?  Bypass caps on each end of .001?  I have nothing there except for a 5K load power resistor across the driver transformer for protection.

I am going to add a relay to the output of the SS amp that will disconnect it off first, after unkey.  I wonder where I shud place this relay Ė at the 8 ohm output of the amp, or at the driver xfmr output secondary on its way to the mod grids?      The relay idea may only be a band aid since it will probably still be there. I will try the power strip test first, then look at sequencing.

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #35 on: November 29, 2009, 01:08:37 PM »

quote author=WQ9E link=topic=22073.msg158863#msg158863 date=1259516452]
Tom,

It sounds like you have a lot of good suggestions to try out for the rig and you should have a good handle on sequencing.  I don't really like the idea of keying the rectified DC since that is a recipe for arcs and transient events.  Although it is less of a problem with the fairly rugged screen structure in the 4-1000A you do want to make sure that the screen doesn't come up before HV and is dropped simultaneously or before the HV is dumped to avoid frying the screen structure.  It is also a good idea to drop the modulator drive prior to dumping HV.  

A scope is your best bet for looking for stray RF in the modulator and also capturing transient events.  Although a storage scope is handy for glitch capture you can do a pretty good job with a regular scope set for single sweep and a decent digital camera (darker room with proper mix of ISO, aperture, and long exposure shutter setting so that the shutter is open during the event).

I may have missed it in your description but do you have RF chokes in the output of your SS amp (bypassed on each end) where it feeds the modulator tubes?  SS stuff does some interesting things when RF shows up where it doesn't belong.  My first attempt at using SS relays to key the plate transformer in my Desk KW produced interesting results until I took care of RF getting into the relays via the control line; the primary voltage was being "modulated" by RF on the control lines which produced a really interesting sounding carrier  Smiley

Rodger WQ9E

Hi Rodger,

The switched DC is a carry-over from using it with linears here. Seemed to work OK there, but maybe not for plate mod. The RF carrier keying looks very clean with no spikes on the scope Ė without the modulator involved.  Bear in mind keeping the HV on the rig all the time had no effect on this problem, but you bring up a good point about the screen being naked for a few millisecondsÖ.Iíll address that later on for sure.

Interesting on using chokes in series wid the audio leads to the modulator grids. What values shud I use there, like 2.5mh types or less?  Bypass caps on each end of .001?  I have nothing there except for a 5K load power resistor across the driver transformer for protection.

I am going to add a relay to the output of the SS amp that will disconnect it off first, after unkey.  I wonder where I shud place this relay Ė at the 8 ohm output of the amp, or at the driver xfmr output secondary on its way to the mod grids?      The relay idea may only be a band aid since it will probably still be there. I will try the power strip test first, then look at sequencing.

T

[/quote]

Tom,

I would locate this "audio driver muting relay", at the 8 ohm output of the SS driver amplifier.

Being that it is a SS audio amp, and most probably does not utilize an audio output transformer for impedance matching to a loudspeaker load, you will do no damage to the amplifier if the load is lifted from the amplifier output when any kind of audio is driving the amplifier.

And you are correct; this is more of a band-aid over an open wound solution, than finding and solving the cause of the transient.

73,

Bruce
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« Reply #36 on: November 29, 2009, 01:09:46 PM »

Bruce,

Yes, both the input and output of the NFB ladder uses shielded cable.  The NFB works very well and lets me put in MORE NFB than I can use before taking off. It cleans up the deep low end tremendously. I ran a sweep last night and found it clean down to about 25-30hz. Without the NFB, it was good to about 70hz at best. Love that NFB!  In fact, when using the NFB I can run the audio phase in the "shark-fin class E mode"....  which can only be done with a very clean low end.  Normally I need to run the phase the other way. The balanced modulator rigs here are the only ones I can normally do that with.

Will-do on the 8 ohm position for the relay.  - tnx.

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #37 on: November 29, 2009, 01:16:20 PM »

Tom,

Just out of curiousity, how much NFB are you running in that rig?

I use 6 dB from the plates of the 833As to the low-level voltage amplifier stage in my push-pull class A 845 driver circuit. As you and I had discussed some time ago, 6 dB of NFB is fine with all-triodes in the audio line-up, as I am using.

Obviously, as you have verified, the NFB loop is not contributing to any kind of instability.

73,

Bruce
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« Reply #38 on: November 29, 2009, 02:30:00 PM »


Interesting on using chokes in series wid the audio leads to the modulator grids. What values shud I use there, like 2.5mh types or less?  Bypass caps on each end of .001?  I have nothing there except for a 5K load power resistor across the driver transformer for protection.


Tom,

Standard 2.5 mh RF chokes would be my choice.  I am not sure what your audio impedance is at the output of SS audio unit but that will determine the highest value of capacitance you can use without impacting the audio response.  For example a .001 cap will have a bit of 15K of capacitive reactance at 10 Khz. so you would want to compare that to the audio impedance of your circuit so as not to excessive roll off your high frequencies.  There are a lot of handy reactance calculators on the web such as:  http://www.sengpielaudio.com/calculator-RC.htm in case you don't have one handy.

Rodger WQ9E
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« Reply #39 on: November 29, 2009, 02:33:46 PM »

Here's the pics of the outside mounted blower for the 4X1 rig.  I first had it mounted on the back of the sub-chassis in the shack. The noise was horrible!  Blower noise gets into the audio as intermod, just like hum in the audio produces artifacts.  I could not run my compression at all - and still had blower noise with just an EQ and mike.  To cool three 4X1's effectively, you need a lot of air. The squirrel cage itself is what makes most of the motor and air noise via turbulance.

Anyway, once I mounted it outside, the noise dropped to a whisper. With the covered 4X1's in the rack, there is just the sound of exiting air - white noise and almost soothing. I can run full compression now with the blower Variaced up all the way. With the cooler temps outside, I can variac the blower down to 120VAC - it's a 240VAC blower.  

As you can see from the pics, it looks almost like a small air cornditioner - hardly noticable on the house. I mounted the blower itself on 1" of styrofoam to keep vibration off the wall.   I'll tell ya, now I can cool the tubes fully and still have a quiet shack. I'm real happy about that!

T


* 4X1 Rig 303.jpg (344.3 KB, 1280x960 - viewed 536 times.)

* 4X1 Rig 304.jpg (324.82 KB, 1280x960 - viewed 539 times.)

* 4X1 Rig 305.jpg (331.86 KB, 1280x960 - viewed 553 times.)
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #40 on: November 29, 2009, 02:35:37 PM »

AFTER painting - hi hi FB OM/YL/XYL/OW approved!


* 4X1 Rig 311.jpg (342.51 KB, 1280x960 - viewed 559 times.)

* 4X1 Rig 310.jpg (319.61 KB, 1280x960 - viewed 534 times.)

* 4X1 Rig 315.jpg (343.46 KB, 1280x960 - viewed 535 times.)
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #41 on: November 29, 2009, 03:06:48 PM »

Cool fan system, Tom!  I have been following this thread with great interest; you have accomplished so much in a short time span!   Just wondering what sort of filter you are using to suppress the unlicensed critters from entering the squirrel cage?
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« Reply #42 on: November 29, 2009, 03:23:05 PM »

heheh  - Brian, thankfully, the nearest neighbor is 1/4 mile away and the next one 1/2 mile..  BTW, how much wud you charge me send send you Fabio to "HRO Beautify?"   Wink


Rick:  I'll bet the Lady Bugs are gonna love this white-color shroud. Perfect spot.  Maybe a fine screen on the bottom is a good idea...

Rodger - I'll try your 2.5mh / .001 filter as a next test - thanks.  Another thing - I just tested it -  I have a fixed, regulated 90V on the grid all the time. When keyed with no plate voltage, there is just 120 ma screen current with RF full drive. So that is no problem for the screen. Unless you can think of another related risk?


 ** BTW, THE RFI/Noise AC LINE FILTER FOR THE SS AMP DID NOT HELP AT ALL. **

Bruce: I'm not sure how much NFB I'm using, but judging from the gain reduction, it must be at least 6-10db. I set it to the point of diminishing returns for the low end sweep.

Still testing and trying things...

T
  
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #43 on: November 29, 2009, 03:53:32 PM »

OK, Brian - I'll have to live with looking at Fabio until you get caught up... Wink

  I don't have a schematic of the SS amp, but will have to open it up.  If it does have that cap, do I just take it out or reduce its size?

**  Rodger:  I tried the 2.5mh / .001 filters on the output of the driver xfmr to the grids. Made no difference.

I'm concerned that the mod meter almost slams against the pin  at over 1A on unkey. I have the mod transformer primary gaps close, but think I'll add some gaps on the mod xfmr secondary too. That's all I'd need to blow this iron in the testing phase when I KNOW there is a problem and can do something about it.  The rig runs so nicely otherwise in all respects.

Think I'll try a bandaid sequencing relay on the SS amp output next. I don't care if IT blows - I'll just build up a tube direct coupled driver... Sad

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #44 on: November 29, 2009, 03:56:36 PM »

Tom,
  If the 200 watt amp is disconnected from the rig, sitting where it would normally be but connected to a speaker or load of some sort, do you get a "thump" out of the speaker when you un-key? If not, then it would be some kind of interaction between the rig and amp when connected. If you do, then it is something getting into the amp.
  Also, I wonder if you have some other impedance taps on the modulator side of the backwards 8 ohm transformer you might try and see if that changes anything.
  Just a thought.....Bill
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« Reply #45 on: November 29, 2009, 04:14:46 PM »

If you are REALLY an OLD BUZZARD, the simple solution is to NEVER UNKEY the rig.  How long can you talk?   Time to set a record!
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Rick / W8KHK  ex WB2HKX, WB4GNR
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"Let's go sailing, Tommy!" - Yaz


« Reply #46 on: November 29, 2009, 04:21:11 PM »

Tom,
  If the 200 watt amp is disconnected from the rig, sitting where it would normally be but connected to a speaker or load of some sort, do you get a "thump" out of the speaker when you un-key? If not, then it would be some kind of interaction between the rig and amp when connected. If you do, then it is something getting into the amp.
  Also, I wonder if you have some other impedance taps on the modulator side of the backwards 8 ohm transformer you might try and see if that changes anything.
  Just a thought.....Bill

Very good idea, Bill. The "disconnect / divide and conquer" technique works well. Will do.

Brian - The Tron always told me to use the primary for gaps- though there has been a debate about that for years. This 1KW RCA BC iron has the gaps STOCK on the primary. But Tron always told me to protect both pri and sec anyway. The 10KW iron I used to have had gaps on both. In fact, I'm making up TWO more arc gaps - one for the mod sec and another for the 70H Heising reactor. That shud cover all bases... Grin


Yep, never unkey - something like the Roach Motel... Wink
T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #47 on: November 29, 2009, 04:35:08 PM »

Tom,

Another possibility is to look for an AC line voltage transient upon unkeying that may be on the AC line that feeds the SS audio driver amplifier.

If that amplifier is DC-coupled (a real possibility), it's low frequency response can be down to DC in the event no effort was made by the designer to limit the low-end for speaker driver protection, etc. Any switching transient that appears on the 120 VAC input to the amplifier may be amplified and is being seen at the amplifier output.

Many solid state audio amplifiers can be unstable at very low frequencies when driving an extremely reactive or inductive load, such as a driver transformer. I have a Crown D-75; it is a high quality amplifier, DC-coupled from input to output. Crown does not recommend that this amplifier be used to directly drive an audio transformer at the output; if a transformer is required at the amplifier output, for driving a 70.7 volt line for example, a 600 uf non-polar capacitor shunted with a 4 ohm/20 watt resistor is required in series with one side of the audio output. Crown was emphatic about the use of a non-polar cap. As far as I know, such a capacitor is not a commonly available item, due to the value of capacitance.

By the same token, you may be switching so much energy in that transmitter between transmit and receive that the switching transient may be appearing at the amplifier audio input and is being amplified accordingly. That is why I asked earlier if you tried to short the amplfier audio input and to observe the result.

Regardless, I really believe the issue is in the solid-state audio driver amplfier you are using in this application. A scope looking at that amp will be your best friend at this point.

73,

Bruce
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Real transmitters are homebrewed with a ratchet wrench, and you have to stand up to tune them!

Arthur C. Clarke's Third Law: "Any sufficiently advanced technology is indistinguishable from magic".
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« Reply #48 on: November 29, 2009, 04:42:18 PM »

Tom,
  If the 200 watt amp is disconnected from the rig, sitting where it would normally be but connected to a speaker or load of some sort, do you get a "thump" out of the speaker when you un-key? If not, then it would be some kind of interaction between the rig and amp when connected. If you do, then it is something getting into the amp.
  Also, I wonder if you have some other impedance taps on the modulator side of the backwards 8 ohm transformer you might try and see if that changes anything.
  Just a thought.....Bill

Very good idea, Bill. The "disconnect / divide and conquer" technique works well. Will do.

Brian - The Tron always told me to use the primary for gaps- though there has been a debate about that for years. This 1KW RCA BC iron has the gaps STOCK on the primary. But Tron always told me to protect both pri and sec anyway. The 10KW iron I used to have had gaps on both. In fact, I'm making up TWO more arc gaps - one for the mod sec and another for the 70H Heising reactor. That shud cover all bases... Grin


Yep, never unkey - something like the Roach Motel... Wink
T

Yes, I protect both the primary and secondary windings of my modulation transformer with spark gaps. I also protect the swinging choke for the HV plate supply with a spark gap.

Now that you mention it, I should add a spark gap across the modulation reactor as well.

73,

Bruce
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Real transmitters are homebrewed with a ratchet wrench, and you have to stand up to tune them!

Arthur C. Clarke's Third Law: "Any sufficiently advanced technology is indistinguishable from magic".
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« Reply #49 on: November 29, 2009, 05:10:58 PM »


If that amplifier is DC-coupled (a real possibility), it's low frequency response can be down to DC in the event no effort was made by the designer to limit the low-end for speaker driver protection, etc. Any switching transient that appears on the 120 VAC input to the amplifier may be amplified and is being seen at the amplifier output.

Many solid state audio amplifiers can be unstable at very low frequencies when driving an extremely reactive or inductive load, such as a driver transformer. I have a Crown D-75; it is a high quality amplifier, DC-coupled from input to output. Crown does not recommend that this amplifier be used to directly drive an audio transformer at the output; if a transformer is required at the amplifier output, for driving a 70.7 volt line for example, a 600 uf non-polar capacitor shunted with a 4 ohm/20 watt resistor is required in series with one side of the audio output. Crown was emphatic about the use of a non-polar cap. As far as I know, such a capacitor is not a commonly available item, due to the value of capacitance.

  I was just reading in my Crown M600 manual very similar language. The M600 is an old DC coupled 600 watt @ 8 ohms brute of an amp. A place I worked moons ago used these in a bridge to drive a HV transformer to make 40,000 volts at 50 ma (yes 2 KW). The HV transformer was a DC short, so we HAD TO use a series NON-POLAR capacitor between the AMP one side and the transformer. We used dual 1600Uf @ 450v electrolytic caps in series with the plus signs together, making a poor mans AC capacitor. These large caps act like a short when reverse biased. The machines ran this way for years 24/7. Adding a power diode across  each cap made NO difference.

  I used the M600 as a modulator once, and did the same thing with two caps in series. It worked fantastic. So Tom, maybe your SS driver in the presence of RF is generating an offset voltage in the output leading to a large DC current into the transformer. Then when you unkey, the current goes to zero almost instantly causing a pulse to come through. If so, a series AC cap combo between the SS AMP and the driver transformer might just be the ticket.

Jim
WD5JKO
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