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

[K1JJ]

New Problem:

I have the the modulators connected as tetrodes with 400 volts on the screens with a current meter monitoring screen current.

All is working well and I see about 30 ma of fluctuating positve SCREEN current on audio peaks.  But when I unkey, I get a huge NEGATIVE spike of screen current measured at least 80 ma.  It lasts about a 1/8 second and goes away.  This is also with no audio connected to the modulators.

I am keying a 49K resistor in the CT fil as usual for mod on/off..

What could be causing this?  I have a 680 uf cap across the screen pin to ground along with some smaller .01 types.

T

[AB2EZ]

Tom

Sounds like the large screen bypass capacitor is discharging through the screen power supply (or some other path to ground that you are introducing)... and therefore through the meter in the direction of "negative screen current".

Stu

[WA1GFZ]

put a heavy bleeder on the big cap so it doesn't freak out the meter. Then you could go lighter at the power supply. Keep something at the power supply for safety.

[N2DTS]

I relay the screen voltage on my stuff.
The supply is on all the time, and regulated, and a relay applies the voltage and is keyed by the step start, when the step start goes to full power, the screen voltage gets to the mod and rf tube grids.

The screen voltage cap is not big, but I do choke input on the power supply.

Brett

[K1JJ]

Thanks for the info and suggestions, guys.

Frank, I can't bleed the supply heavily cuz it's using a SS regulator that is almost at the limit now at 400V @100ma.  So I decided to remove that big 680 uf cap at the screen pin and replace it wid a 4uf 1000V oil filled. That will keep the initial charging which drives the meter nuts - and we'll soon see if it helps the screen spike problem or not. 

BTW, I found the spike is in the positive meter direction, just like normal current. I thought it was negative before.

I noticed it only happens when the RF amp is unkeyed with the modulators. With no drive to the RF deck, there's no screen spike in the mod tubes.

Brett - I may try keying the screen suppy as a last resort. But since the screen spike happens only on unkey, I wud need to put the relay on the screen supply's output as a HV relay.  We'll see.

To add to the problem, that big cap held a charge so long when I went to measure the resistance of the circuit I blew out my Asian  VOM. Gotta go down to RS now.... cheezz..   I hate those little meters that can't take abuse. Shud get an old Simpson again.

T

[KC2IFR]

Tom,
When I unkey my 4x1....the screen voltages spikes to about 100 volts above the operating voltage...Jamie said this is because the screen supply no longer has a load on it and  the filter caps  are discharging. Dont forget this is  the screen voltage on the rf 4x1......not modulators.......

[KC2IFR]

Tom,
I forgot to mention........the screen supply for the 4x1 is a separate power supply. I have a 20 henry choke in series with the screen to self modulate it. I do NOT tap the screen voltage off of the modulated b+.

Also I can vary the screen voltage with a variac.......

Bill  

[K1JJ]

OK Bill -

Yes, I do the same thing with my RF final screen using a self modulating choke.  The RF final is FB.

But since I changed the modulators to tetrode configued with 400V screen voltage, the screen meter on the modulators spike. Maybe it was there all the time before and I din't see it cuz I didn't have a mod screen meter, dunno. Might even be an instability of the modulators taking off.. and is now enhanced cuz of a tetrode's greater sensitivity.


 I'm still working on the problem and will post the solution - soon I hope...



T

[K1JJ]

Screen spike problem solved rig is getting very tame and reliable.  Almost there.

Tetrode config on the modulator 4X1's -  update:

Well, I took out the 680 uf bypass cap on the 4x1 modulator screen grids and replaced it with a 4 uf.  The screens now have regulated 400V and the bias is about 90 diodes in the fil CT (-45V) for a stiff bias.  The screen spike problem is gone - fixed! I ran some tests with the audio sweep gen and also some voice checks. I think it's the best config so far!  It's also easy to drive with about 10 watts of audio or so.    The screens are drawing only about 15ma on peaks, while the Eimac specs say 95 ma is for full power - this tells me there's plenty of headroom left.  The grid is drawing no current at all. I'm idling them at about 300ma total and the plate meter barely move under voice peaks.  This tells me we are running close to class A, but really about AB1. That's a great mode for cleanliness.  This is on medium power tap, so it will increase the modulator requirements once I bump it up to full strap.

I'm real pleased the tetrode connection so far.

Still have some minor issues to iron out with the input grid tuning, but almost finished with the rig for now. (until I get the next modification bug up my ass... )

More later -

T

[K1JJ]

I ran some tests tonight. I swept the audio from 30 hz to 8KC and found it looked very good. The only problem was above  5kc when going above 70% modulation.  The driver transformer is causing some minor distortion.  So I'll be building up a transformerless driver for sure.  Otherwise, I think the rig is working pretty well.

I also put the scope probe on the RF final screen and saw the clean audio from the self-modulating choke. However, on the RF grid I looked for audio after the grid leak resistor and saw very little to none. Is the grid leak supposed to have a reasonable audio voltage swinging there or is it just supplying the additional DC bias only?  I always thought it helped to modulate the final too.  

The grid leak circuit consists of a 2500 ohm resistor in series with the fixed bias supply, .001 to ground, in series with a  2.5MH RF choke, and to the RF final grid.  The coupling cap and RF also connect to the grid pin.

Also, I measured the audio voltage at the grid of one modulator tube and saw about 90V peak. So I would need 180 peak to peak to drive two tubes in push pull, right?   I need this info to deteremine the power supply voltage needed for a new audio driver.  Eimac suggests 140V per tube to make 3KW of audio, but my requirements are substantially less.

T

[N2DTS]

Since you only need 10 watts of audio drive now, a good 8 ohm to 10kct 20 watt transformer would likely fix you up nicely.

Are you sure the high frequency distortion is in the driver transformer and not the rf screen bypass, rf plate bypass, mod trans, etc?

My 813 rig is flat up to 10k then rolls off very quickly, some bypass I suppose, since the audio amp and driver transformer are good to past 20kc. Or it could be the mod iron.

Its bad news to go beyond 5kc with a high power rig anyway...

Some guys sound really good and are not real wide, others sound good and take up the entire window from below 3870 to above 3890....

I hear/see all sorts of combinations, guys who sound somewhat rough and take up 25kc, something not right in the trans or the way they are running it, other guys just dont limit the highs at all, and pass audio well past 10kc, as if anyone could hear it at prime time when they operate mostly....
How often can you use the 20Kc filter in prime time, or how many people even have a 20kc filter?

Just poor or sloppy practice...fuel for the anti AM crowd.
Last nite I heard a group on 3868 moaning about AM, which they enjoy, since they snuggle up to 3870 so they can listen to the splatter from 3870 to 3890. They really seem to enjoy this as they do it every nite...





Brett


 

[WA1GFZ]

Tom,
I wonder if you could configure the driver transformer to use 1/2 the secondary? (tube side) to reduce winding inductance? Then feed grid bias in through a pair of load resistors into each grid. Bypass the heck out of the bias side and give it a good load like a power Zener

[K1JJ]

Brett:  The distortion starts directly at the audio transformer's output. I don't see it on the input cuz the SS amp is holding firm.   I only have one transformer left here. A few years ago I sold them off thinking I'd never use low level audio transformers again...

Frank:  The sec is just one winding with a CT, 7,000 ohms, so no way to alter that. I need the CT.

Looks like the only way is to build up a transformerless driver.


I'm still wondering if I should be seeing AUDIO after the RF final  grid leak resistor?  I see plenty on the final's screen, but little to none on the grid leak.

T

[WA1GFZ]

My point was use 1/2 the winding and do a simulated CT with a pair of resistors to a common point. They also serve as a load on the transformer.

[KD6VXI]

Frank:  The sec is just one winding with a CT, 7,000 ohms, so no way to alter that. I need the CT.

Looks like the only way is to build up a transformerless driver.

Tom,

Could you use the CT to one side of the secondary, and then use the trick of a pair of resistors to "simulate" the center tap?  Like the way you get one with no center tap on a filament xformer.

Not sure if it would work at all, but might be worth a shot, if you need the different ratio.

--Shane

I see Frank and I said the same thing.  That IS scary

[K1JJ]

I had a feeling that's what you meant, Frank.

I'll give it a try.

As you said, it might reduce the high freq inductance causing the problem - might be worth a try. The step up will be reduced of course.  What values would you suggest from the winding to CT for the resistors?  It will now be a 1750 ohm winding end-to- end. (7000 ohms /4)    (265 ohms pri > 1750 ohms sec) 


T

[WA1GFZ]

1 or 2 K each big enough to handle the voltage. You may have to play with the value to get the best response out of the transformer. you need to bypass the CT well for all audio frequencies so a number of caps in parallel.

[K1JJ]

Well, after spending the day on Fabio putting in a new grid L/C circuit into the sub-chassis, adding a rotary switch to select the diode stack for modulator bias and running some more tests, I'd say he's about ready for prime time.

The Huzman is sending me a hi-fi driver transformer that will work FB until I get a new direct coupled driver built. The rig is quite stable now for both RF and modulator arcs. The only remaining problem is when I unkey and run less than 200ma of idle current in the modulators, I get a spark gap arc on the mod xfmr secondary. It's simply a matter of the iron needing to dump its power somewhere. Gotta figure that out and work on it.

Here's a pic of the new grid circuit I relocated from a remote outside chassis. I decided to locate it right at the tubes pins like it shud be. The air flow is still superb, so it worked out FB.

The 4X1 modulators definately play well as tetrodes, so I'll leave them that way.

Hope to be on the air with it this weekend.

There's a pic of Fabio in operation tonight running in low tap. The final shows no color at all, while the modulators show a faint orange blush. Fabio is shown in standby. I'll get some plate color shots next time.

T

[K1JJ]

Well, I ran the rig today on the air and everything is basically working FB.  Had a big arc over, but that was just the 4X1 RF tube's plate cap braid arcing to the cabinet, about 1.5" away.  Seems RF can jump huges distances that DC can't. Easy fix.

Anyway here's the last problem:  When I UNKEY the rig, I get a snap - an arc across the secondary modulation transformer arc gaps only.  This happens ONLY if the modulator tubes are idled less than 150ma or so. If I idle them higher there is no snap, but I still want to locate the problem.

I tried all kinds of relay delay combinations... I put a delay on the modulator key ups and unkeys - antenna relay keyup/downs, RF final key up/downs, etc. Nothing seems to help.

One strange thing: If I ground the modulator grids, I always get the snap on unkey even with the modulator idling high, but when I turn on the SS amp and have it running, the arc happens only when the modulators are idling less than 150ma.

My question is WHERE is the power from the mod iron and heising reactor supposed to be dumped on Unkey?   I thought is was into the final, but holding the final on longer after key up doesn't help. 

Any suggestions for a fix?

T

[W1AEX]

That is a very strange event. My thoughts are the same as yours, that when you unkey, the energy would softly drop as the tubes pulled down the supply. I wonder how it would behave if you sequenced your "step-start" to "step-down" when you unkeyed. Still, it would be nice to find where the transient is originating.

The rig sounds very nice. For those who have not heard it, here is a very short clip of Fabio, featuring the moment of arc-eology, courtesy of WB2CAU Eric's recording studios. A scary moment in time, but Mr. Vu brought Fabio back to life after a glance at the inside of the top of rack. Hmmmm... was that black zorch mark always there???

[W1RKW]

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?

[K1JJ]

"uh oh"   HA!  Pretty good catch.

Yep, the plate cap lead arced 1.5" to the rack cabinet top. RF is strange stuff.

I was talking to Tron tonight about the arcing problem. As usual, he made a good suggestion. He said the spark gap arcing on unkey was cuz the RF final was shutting off and resulting in the mod iron not having anywhere to dump the power. But I said I already sequenced the relays so that the RF final stayed on after the mod relay. He axed if I was using fixed bias on the RF final too. I was and the tube was cut off completely once drive from the exciter was gone - regardless of if the RF final's relay was still on... bingo!  I still have yet to reduce the fixed bias so that the RF final idles at a small current when the drive is gone, but I'll bet that fixes it. We'll see.

So remember this one, guys: Use just enuff fixed bias so that the class C final tube idles in class A or somewhere like that with no RF drive. A complete cutoff gives the modulator no where to dump.  (At least I hope that solves it)

T

[K1JJ]

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.

[Gito]

hi

Can it be because the drive is gone instantly when the B+ of the transmitter is unkeyed?
I think You must keep the drive for a part of a second after the B+ and Screen Supply of the RF tube is unkeyed.
So when the B+ " slowly,in part of  a second" fading away  it still gets an Rf Output/ used to dump /to bleed the B+ because of the RF drive.

Gito

[K1JJ]

All set - no more arcing on unkey.

It took TWO things.

1) The fixed bias had to be reduced so that the RF final continued to idle when drive was taken off. (Keeping drive to the final would have done the same thing, thanks Gito)   However, since everything in the shack is being keyed by the transceiver PTT, the exciter drive is shut off as a first step, so the RF tube fixed bias fix was easier.

2) Added a  100 uf cap across the fil keying resistor to keep the cathode bias on longer (and the tube idling) did the trick.   I happen to use cathode keying, but if you use something else, then simply keep the tube idling whatever way you choose.

Now when I unkey, no matter what the modulators are doing or idling current they are at, there is no spark gap arc.  I do see a slight movement UP in the RF plate meter showing the energy is going into the RF final circuit and to the antenna. I unkey the antenna relay last in the sequence.



T