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

[Gito]

Hi Tom.

 Tom,but  when You idle the plate current,doesn't the transmitter works in different class ( class AB2?) and a expense in the tube efficiency?
It different when You" delay" the driver when You unkeyed.The RF transmitter is working for it was design like the data's book.(pure class C)
The spike is caused when the plate current instantly  disappear ,not because the high voltage instantly disappear.( I think the B+ graduadlly disappeared).

When the drive is still"on" the the plate current slowly disappear ,at the same time The B+ plate " slowly" disappear.
And I think the Spike can be elimatted.

Gito

[W2PFY]

Hey Tom heard you on and you sounded good. I wonder if the cap is to ground or is one end of the mod transformer secondary to ground? I think its ok for BC rigs that are more or less always on. In the Westinghouse they have it across the B+ feeding the bottom of the mod transformer to the bottom end of the modulation transformer therefor no cap to charge up like a cap in series with an inductor. In my Westinghouse I have it across the top of the secondary of the mod transformer to the top of the reactor. The bottom of the reactor is at B+ as well as the bottom of the secondary of the mod transformer. No arc overs of any type in my arrangement.

No common point to ground I feel is the answer.

[K1JJ]

Gito:

The final operates in class C all the time when drive is applied because it uses a grid leak resistor. When I unkey, for an instant the drive disappears  but the fixed bias lets in run in class AB for a second so the modulation iron will still have a load to work into.  The idea is to keep the tube on for an instant while the iron dumps its power and that's what is happening now.


Terry: The heising coupling cap is located on the high side.  The Tron seems to think the whole problem is related to the final dropping out into cutoff too quickly, thus the iron was left hanging without a load, so it sparked instead. So far, his info seems correct. (as usual)     OK on the Westinghouse arrangement. I wud try that, but I think the problem is solved for now, TNX.  BTW, heard youse guys on 160M tonight. Good signal with your generator running.  I have no ant for 160M right now nor does the 4X1 have a tank coil for it. I may have to do some winter climbing, dunno.

T

[Opcom]

good that it's fixed. This thread has taught me a few things!

[Gito]

Hi Tom

I get the conclusion,that the culprit is the cutting of the drive instantly.
When the drive stop ,the current of the plate Rf is instantly stop,making a flywheel effect ,just like ignition coil work,when there's current flowing in the coil,the core stay magnetize ,when the current collapse ,there a flywheel effect that makes a great big emf voltage in the coil ,causing an ark jumps a cross the spark plug.
In my opinion when You idle the Transmitter.it does not completely remove the spike,the Spike is still there but with a reduced amplitude/lower voltage,So there's no ark cross the spark gap(not enough peak voltage).

Gito

[Gito]

Hi again

Tom when You idle the Rf tube,there's a magnetizing current flowing in  modulator transformer,for instance there's a 100 ma current flowing there's it means there's 2500 v X .1A = 250 watt power dissipated in the modulator transformer.,makes it a " little" saturated ,so the back EMF is smaller than no "saturated" trans former.
I mean "saturated" is an example ,meaning the Modulation Transformer is partly magnetize.

Gito

[WA1GFZ]

There is 100 ma of current going through the transformer. The only dissipation is due the wire resistance....But you have a nice big electro magnet. So when the current stops a large voltage is induced back into the winding since there is no load across it. This is why you generate a big spark.
All dressed up with no place to go.

[K1JJ]

Then what's the correct solution?

Right now I have the final conducting for about 1 second after unkey. It appears to absorb the mag collapse OK.

Gito suggested to keep the drive going. This will keep the tube on and conducting plate current - just the same as putting it in class AB for an instant to keep plate currrent going. I don't see any difference in the end result.

But I do see the wisdom in Gito's sugestion to let the plate voltage and current die slowly into a load.  My transceiver drive drops instantly and will have to stay that way. Maybe there's another PRACTICAL way to slowly drop off the HV so there will be an EASIER collapse of the field. Right now I keep the HV on all the time and key both cathodes of the final and modulators using a 56K biasing resistor that gets shorted by a relay.   I could add a keying relay if needed for the HV.

BTW, I added a 100uf cap across the 56K keying resistor and that helped to ramp down the bias (thus plate current) in the brief AB mode. Maybe that is the right approach.  Taken too far, I could see the RF plate get red for a second...  


Opcom:  Yes, this has been a learning experience for many of us. I post most of this stuff (some trivial) for two reasons. One is for the Newbies to learn and get confidence that they too, can build a big rig. And second, for we OT's that need to be reminded of stuff we once knew - or never knew at all..   This particular plate modulated project has been especially tough compared to other projects, but that's a good thing in ways to get the problems ironed out early rather than popping up later one at a time.  Plus, it's been stressed so much that the weak parts have been kicked out by now.  I've gotten a bunch of emails form various guys offering parts, help or just questions themselves. It's a good blog.

T

[WA1GFZ]

Tom I think the first action should be to bias the modulators off slowly. The cap across the resistor sounds like a KISS way to go. Once you dissipate the mod transformer energy then you can drop the drive then last change the antenna.

[K1JJ]

Tom I think the first action should be to bias the modulators off slowly. The cap across the resistor sounds like a KISS way to go. Once you dissipate the mod transformer energy then you can drop the drive then last change the antenna.

Frank,

Surprisingly, the cap on the mod keying resistor had no effect. The spark was there regardless of when the mods turned on and off. But the RF final had the big effect.  It seems the mod iron is dropping the field even with the mod tubes uncapped at the plates.   I figgered since they seem to affect the loading at times (their idle current) it wud help to delay them, but no cigar.

T

[WA1GFZ]

Well if there is any secondary current, it also stores energy in the mod core.
You want to use the final to suck the well dry before you remove the drive and antenna as a load. so if there is any final idle current with the modulators off you still could generate a spark. I bet the inductance is pretty high at low current. Maybe this is why guys drop the HV and let the system discharge before the load and drive drops out. maybe you also need a cap on the final CT also. 

[W1RKW]

Yes,  you're right, RF drive. Sorry. Memory is getting fuzzy.

Tom,
I had a similar situation with the CVM-4 and 813 rig and not driving enough snot into the rig. For some reason running it light, it did not like it. It arced all over the place but when it was run under the specs of your plan it was FB.  Maybe you haven't found the sweet spot yet?

Bob,

Do you mean RF drive?  Then yes, low RF drive will not give the RF final enuff current to load the mod xfmr to dump the power. (Assuming you are using too much fixed bias too) I've seen it written before to let it idle like a linear, but didn't realize this was one of the reasons.  The downside is the final is now more sensitive to taking off in the time it is running linear due to higher gain.

 In my case, I have enuff drive, but the tube is being cut off anothere way - by too much fixed bias.  I have about -90v fixed and -140v grid leak bias. Should be more like -60 v fixed or so.

[K1JJ]

Well if there is any secondary current, it also stores energy in the mod core.
You want to use the final to suck the well dry before you remove the drive and antenna as a load. so if there is any final idle current with the modulators off you still could generate a spark. I bet the inductance is pretty high at low current. Maybe this is why guys drop the HV and let the system discharge before the load and drive drops out. maybe you also need a cap on the final CT also. 

The final CT is where the 59K resistor and cap is now. All set.

As I mentioned, the drive comes out of the transceiver and I key the transceiver, so its drive drops first on unkey. Not sure I want to time the transceiver keying externally just yet.  I've gotten other plate modulated systems to work this way, so can do it again.

I used to key the HV DC until a short wiped out the arms of the relay. I think I'll put another HV relay in there and drop it out early on unkey. That may help a lot to slowly bleed off the power. At least it won't be supplying additional  power during unkey as it does now.

T

[Gito]

Hi

I think the spark is caused by the instantly stopping current flowing from B+ to the Mod. Transformer to the RF  final tube Plate,because of the stopping of the drive.

I think idle current is different ,it does not stop instantly.it decay in phase with the decaying B+,because it takes "a part" of a second to completely discharge the Capacitor. So the magnetizing Core is decaying in phase with the decaying current( the current is flowing till the B+ is Zero).

 This gives me an idea ,make the smoothing capacitor capacity(for the B+) bigger,so it needs more time to discharge .
Can the power supply of the driver discharge A part off a second slower than the power supply of the RF final,by making a bigger capacitor as the smoothing part of it's B+.
Hoping the driver stays on for part off a second after the B+ off the RF final tube decay.

Gito

[K1JJ]

I see what you are saying now, Gito.

This is based on the HV being shut off before the filter caps so the caps can discharge slowly, otherwise the current will remain the same regardless of how long the drive is kept on.  Right now my HV is always on.


I'll have to think more on it.

T

[K1JJ]

May, 2013 ---

I see this old topic has almost 18,000 hits over 3 years.  This is most likely due to outside general web search engine readers.

UPDATE:  Since this thread, I've retired Fabio I  -   and built a new Fabio II. Might as well have some continuity here.

I look back at Fabio I construction and cringe at some things, so hopefully the newer version will give new readers better ideas for their own construction.

Fabio II:  (not completed yet)

http://amfone.net/Amforum/index.php?topic=33816.0

73,
Tom, K1JJ