New 4-1000A modulated by 4-1000As AM Project - Fabio II Reborn

[K1JJ]

Tom,
I got a kick out of the SS audio driver!  Its big enough to be a tube driver with a pair of 2A3's!  Looks great and can't want to hear it on the air.  Make sure you get on the air before you tear it down to build something different.  

And yes, junk boxes are wonderful things to have!

Joe, GMS  

Yeah, Joe, the SS driver turned out bigger than I expected due to the already-built power supply with the tall board.  Now I'm having a problem locating it close to the modulator deck to keep the audio input and NFB leads as short as possible.

But there's a lot of room in there to add stuff. If the QIX negative peak limiter circuit works out, I will probably mount it in there, since it feeds the SS driver anyway.


Well, I just built a diode stack for the modulator bias. It uses 140 forward biased diodes on a Plexi board and a big rotary switch.  I could use ALL bias from the SS driver board, but wanted the diodes as a failsafe in case the SS driver shorted out.  Nothing like zero bias on a tetrode.... bam!

I started the station interconnections process last night and might be ready to fire up the RF deck followed by the modulator and assorted audio stuff soon.

Picked up some aluminum sheet at the scrap yard yesterday. I'm tempted to build the outer cabinets before doing any testing.  It's scary with everything exposed, even for a day of so.


Fred, I figgered you, especially,  homebrewed all your parts for the 51 watt rig.     Reminds me of a story from 1964. I was a little JN standing at the counter at Hatry's in Hartford, a Collins ham dealer.  An older, well dressed  ham came in who had just bought a brand new Collins S line.  He said he didn't currently have the money to buy the matching power supply for the 32S3.   Corky (salesman) suggested he build it. The guy got POed and told Corky that he didn't build his own refrigerator or washing machine, so why shud he build his own power supply?       (appliance operator circa 1964)

T

[W3GMS]

If the QIX negative peak limiter circuit works out, I will probably mount it in there, since it feeds the SS driver anyway.

Well, I just built a diode stack for the modulator bias. It uses 140 forward biased diodes on a Plexi board and a big rotary switch.  I could use ALL bias from the SS driver board, but wanted the diodes as a failsafe in case the SS driver shorted out.  Nothing like zero bias on a tetrode.... bam!

It's scary with everything exposed, even for a day of so.
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Now when doesn't one of Steve's circuits not work out!  If its his design I am sure it will do the deed!

Wow, that's a lot of diodes but I understand why your doing it.  I believe your using the 4/1 in a triode configuration.  If you were using them as tetrodes, you could dump the screen voltage if you had excessive plate current.  A DC referenced resistor in the negative lead of plate supply ground referenced would be a great reference to use to set a trip point circuit which in turn would drive a FET to disable the screen voltage.  

Your lucky to have a good source for sheet metal!  The old buzzard rigs of the 20 and 30's were commonly open frame construction.  Its amazing how many of the OT op's survived.  Just have to pay attention and mind your P's and Q's!

Joe, GMS
      

[K1JJ]

Joe,

The tubes are in tetrode config with regulated screen voltage.


That trip circuit is a good idea.  I already use it for the linear amps using a negative lead resistor sample.  I might later add it to the modulators too.


The QIX NPL already works FB on the PDM board. I'm simply adapting it for 4X1 modulator use. It's a pair of clipping diodes followed by an 8 pole low pass filter.

Later -

T

[steve_qix]

Just a note    That low-level negative peak limiter is not as effective in A.C. coupled modulators (like this one) as it is in D.C. coupled modulators  (like a pulse width modulator).

However, it will still have an effect.  The key with an A.C. coupled system is to absolutely minimize waveform tilt and/or overshoot.  This is best accomplished by ensuring the frequency response at the low end is very good, and the response throughout the audio range is flat up to the cutoff point, and then rolls off, etc.  If the transmitter will modulate a 30hZ square wave, it should be reasonable.

The N.P.L. will cause a DC shift to occur, and in a D.C. coupled modulator, the DC shift is propagated through the system, and the reference point does not change.  With A.C. coupling, the reference will shift down as the D.C. shift is mathematically integrated.  The better the low frequency response, the more time it takes for the integration to occur.

By the way, the rig looks great.  It'll be interesting to hear it on the air, that's for sure!

I'm building a 16 MOSFET rig for Rattlesnake Island, mostly to improve the signal on 160 meters for the Grey Hair Net.  Should be on the air soon.

Regards,

Steve

[K1JJ]

Steve,

Cool on the 16 pill project for Rattlesnake.  That should get you back up to par when away...

As far as audio respone... the last Fabio generated very close to e-rig shark fins once I had it dialed in. I'm using the same mod iron, SS audio driver and NFB, so hopefully the NPL will like it.

Well, other than building the cabinets for Fabio II, I've finished the internal wiring and interfacing it to the shack. Ready to do some dummyload testing next.  It's always scary when we must apply the HV for the first time.  It's hard to imagine, but technically, if everything worked and was dialed in correctly, the rig could have a QSO right now. But we all know that could never happen except in the Bizzaro Whirl...

Just finished installing the panel lettering labels - my favorite part of the whole project! It really brings the rigs to life.  The band switch lettering still needs exact 160 - 40M labels once tested and found.


Check it out:

T

[K1JJ]

Notice the solid state audio driver to the left, the sequencer to the right and the T-Matcher in the center. All parts of the project.


You'd think I'd break down and buy some matching knobs...

T

Pic #3:  It all started on April 4th, a little over 3 weeks ago.  My goal was May 4th, to be completed before Deerfield.   But testing and debugging usually takes as long or longer than it took to build.

[flintstone mop]

It's amazing the progress on Fabio......from cut front panels to a living breathing machine.
The RF output will be kept secret and we calculate from plate volts and current what's gonna come out to the aerial?

[W3GMS]

Looks GREAT Tom and can't wait to hear it on the air.  Should be very effective at puncturing absorption !!
Joe, GMS

[K1JJ]

Started checking wiring and firing things up.

Found two wiring mistakes in the modulation transformer / Heising choke area. Fixed. What a space shot.

Disconnected the modulator from the RF deck.

Into a dummyload, fired up the RF deck for the first time. At 2KV only 50 watts out and the plates were getting bright red. No way to resonate.  Felt the plate choke and it was HOT.  I had wound my own and it either had a resonance or the wire I used might have been nichrome... by mistake. It had a resistance of about 40 ohms, though I wouldn't think 40 ohms in the plate circuit was that much.   Rewound a new choke with Teflon wire. Less than an ohm.  Fired it up and the plates were gray and put out reasonable power.  Nice, sharp plate dip, screen peak and power output peak. Neutralized.

Then the screen shorted out. Turns out a .001 1KV disc shorted. I forgot that after the screen self-modulation choke there are large EMF collapses when unkeyed.  I replaced the .001 with a 5KV and all was FB.  I put my VOM on the screen lead to monitor and blew out the VOM. Again, forgot it doesn't like spikes. The real problem was that I should key the driver on near to last using the sequencer. I had the driver turning on before the amplifier. Doing so cured all spike action.

Getting late so shut down for the night.   I can see it will be slow ploughing, but that's how it goes.


Question:  I added Plexiglas windows to the 4X1 viewing holes.  If the tubes are 2" away will 1/8" Plexi eventually deform?  It doesn't seem to absorb heat, just like glass, but I seem to remember having a problem once...?

Ques #2:  Does anyone use spark gaps across their screen modulation choke?


T

[steve_qix]

Ques #2:  Does anyone use spark gaps across their screen modulation choke?

Hi Tom,  back when I used modulation transformers, etc.  I had a spark gap across the modulation reactor.  In fact, I seem to recall it came that way, but my memory is a little foggy there.  Wow, that was a long time ago (we're talking 35 years ago or so!).

[n2bc]

Hi Tom,  A good source for high temp capable and RF blocking "view ports" are defunct microwave ovens.

Here's the RF deck /w a microwave door panel. It is a re-package of an RCA BTA-1R1 I did a while back:

[K1JJ]

Hi Tom,  A good source for high temp capable and RF blocking "view ports" are defunct microwave ovens.

Here's the RF deck /w a microwave door panel. It is a re-package of an RCA BTA-1R1 I did a while back:

Hi Bill,

That rig is quite a piece of art!  Is that part of a plate modulated BC AM xmitter that now covers all the bands?  Or maybe it was modified into a linear?

OK on the microwave glass.  I will monitor the Plexiglas and if it starts to distort, I will look into the microwave stuff.


Steve, OK on the Heising choke spark gap. Yes, I use gaps on both the mod xfmr and Heising reactor, but was wondering if anyone used a gap on the screen self-modulation choke.  When the keying sequencer is wrong the screen circuit lets out quite a snap which could potentially damage tubes and parts. The gap wud be just a precaution in case of trouble again.

I have the 4X1 running DEEP into class C right now. Normally -200V of total fixed and grid leak bias is required, but I have stepped it up to -285 volts. I see no plate color, though it is a little harder to drive, like 25 watts vs: 15 watts.   I wonder where it crosses the bridge to become class D?


T

[K5IIA]

Hi Tom,  A good source for high temp capable and RF blocking "view ports" are defunct microwave ovens.

Here's the RF deck /w a microwave door panel. It is a re-package of an RCA BTA-1R1 I did a while back:

Hi Bill,

That rig is quite a piece of art!  Is that part of a plate modulated BC AM xmitter that now covers all the bands?  Or maybe it was modified into a linear?

OK on the microwave glass.  I will monitor the Plexiglas and if it starts to distort, I will look into the microwave stuff.


Steve, OK on the Heising choke spark gap. Yes, I use gaps on both the mod xfmr and Heising reactor, but was wondering if anyone used a gap on the screen self-modulation choke.  When the keying sequencer is wrong the screen circuit lets out quite a snap which could potentially damage tubes and parts. The gap wud be just a precaution in case of trouble again.

I have the 4X1 running DEEP into class C right now. Normally -200V of total fixed and grid leak bias is required, but I have stepped it up to -285 volts. I see no plate color, though it is a little harder to drive, like 25 watts vs: 15 watts.   I wonder where it crosses the bridge to become class D?


T

you would have to cross way more then a bias bridge to get there. would have to take a detour down input lane and get all squared up.  then do some road construction after the first bridge on your left.

awesome on the transmitter. did you play any with just using minimum fixed bias and getting ya mo volts from addition grid leak resistance? and see how it does on the clean tests?

[n2bc]

Tom, thanks for the kind words.  It is a 'repackaged', frequency agile BTA-1R1.  The boss wouldn't let me take a chainsaw to the doorways so I brought it inside one piece at a time and rebuilt it into a standard 5' rack cabinet.

[flintstone mop]

I would rather have a re-packaged BC TX than the huge cabinet and chrome. That is a nice maul.
Fred

[K1JJ]

You're quite a craftsman, Bill.  What a novel idea - to take a FB BC transmitter and convert it to a smaller package to fit into a 19" rack.  Amazing you got it all in that cabinet. Does it include the mod iron and HV PS too?  


Brandon -  I have barely started testing. Just able to get a dead carrier out and slowly fixing little problems and making mods as I go along.  This is possibly the most difficult part of building a rig, so taking my time with it.  I want Fabio II to perform perfectly and cleanly when done. It may well be my best rig ever, if it comes out right.  Blowing out expensive parts along the way ain't on the agenda, hopefully.  

T

[K5IIA]

You're quite a craftsman, Bill.  What a novel idea - to take a FB BC transmitter and convert it to a smaller package to fit into a 19" rack.  Amazing you got it all in that cabinet. Does it include the mod iron and HV PS too?  


Brandon -  I have barely started testing. Just able to get a dead carrier out and slowly fixing little problems and making mods as I go along.  This is possibly the most difficult part of building a rig, so taking my time with it.  I want Fabio II to perform perfectly and cleanly when done. It may well be my best rig ever, if it comes out right.  Blowing out expensive parts along the way ain't on the agenda, hopefully.  

T

I hear ya man. It will be awesome to hear it on the air this fall/winter. I always love and hate that nervous feeling of running something you just put together. It seems like every little smell or sound can send you into a panic looking for something getting hot or not doing what it is supposed to do.

Thanks for the pictures and updates.

[n2bc]

Hi Tom,   everything fit in the 5' rack.

Bottom level has HV supply and mod transformer,  middle level has mod reactor, LV and bias supplies, overload circuits, etc. 

Most of the heavy stuff on the bottom.  The angled cable is to correct a 'twang' in the cabinet - apparently the thing was dropped on it's way to the dumpster and the backside was quite out of square. I tried to 'squish' it back to square, NG, so used the cable & turnbuckles.  Less of a mess than cutting and welding - and hidden inside.


Mid Level - mounting the mod reactor here required some bracing of the mid level plate (1/4" aluminum). 3/8" steel threaded rod runs from mid level aluminum plate to the bottom rack plate which is 1/4" steel.


All the layouts were done with a CAD program (DeltaCAD).  Lots easier to move electrons around than 100# hunks of iron!  Created full sized drilling templates for the interior plates, the RF and Mod chassis, and the front panels.   

[K1JJ]

Hi Bill,

Pretty cool using Cad to lay out the optimum fit for your rack. You sure have a nice organized rig there.

Well, here's the update on Fabio II.  I have run some THD linearity tests. I hope to posts some spec analyzer pics soon. Seems the audio negative feedback is doing its job and with some tweaking I am seeing -35 dB second harmonic and about -50 third harmonic numbers. This is quite good.  Just like my linear amp project, by adjusting parameters I can dial in better cleanliness.  Screen voltage and current, plate tuning and loading , drive levels and NFB all have an effect on THD.

The problem now is I am getting a spark on the mod xfmr secondary arc gaps whenever the rig is unkeyed.  I don't dare raise the HV above 2KV until I solve this.  I've tried a lot of things to cure it, since  I've had these problems before with many rigs. So far the sequencing, delayed relays and all that have not helped. I also tried less fixed bias on the RF final so that it doesn't fully cut off. No diff.  This worked on Fabio I before, but not this time.    

This sequencing is a bear cuz if I play around too much I get catastrophic crap-outs.  When I added too much driver delay, the RF final 4X1's flashed over and made a light show of blue and yellow. I thought I wasted the tubes, but after replacing a shorted .01 filament bypass cap,  everything worked FB.  

I have to hand it to the guys who have built big plate modulated rigs and got them tamed. It's not an easy thing to do.  A rig will tend to beat us up in areas where our knowledge is lacking. It somehow finds these areas. As much time as I have spent in the past with proper sequencing, it always haunts me.

Right now here's the sequence of events to key:

HV always on.

Key on:

1) RF antenna relay switched on (vac relay)
2) RF amplifier turned on via fil CT ground.  (I've tried caps here and use a 56K resistor across the contacts.)
3) Driver on
4) Modulator CT grounded on.  
5) Low level audio turned on

It works FB on keyup.

I reverse the above to unkey and all FB until the end and I get an arc across the mod xfmr gaps.  The Heising reactor is dumping the power, but not all - then it arcs to dump the remaining stored power.  

I will look into it further, to see exactly in what step this arc is occurring. But hopefully I will not need to key the HV to soften things.  As a last resort I will put a longer delay on the ant relay release.  I've tried most everything else.

BTW, the arc still occurs when the low level audio is completely disconnected from the modulator.

T

[W2NBC]

T,

Vacuum relays for antenna switching are FAST mechanically as opposed to open frame, etc. I have been going through the same issues on an old buzzard homebrew here . (pair of 4-400's by 849's).. what cured similar arcing was delaying both the antenna relay and rf drive on release.. The difference is that the plate primary is also part of the sequence for HV.. it all drove me nuts!  BUT this is the fun stuff! enjoy

[K1JJ]

OK Jeff -

Yes, it is a tough game to get the sequencing so right that there are no arc gap snaps.

Exactly where does the power from the Heising reactor supposed to go during unkey?    Do the modulator tubes absorb it or does it go thru the RF final that needs to conduct current to in turn  dump it into the antenna?

I just ran some more tests and still stumped.  I made the ant relay longer, but no change - still arcs at the mod sec gap.   I delayed the RF final CT keying relay.... no change.

I disconnected the SS audio driver and grounded the modulator tube grids - no change.

I also toggled the modulator tube keying off so they did not key up at all - same thing, arc.

Already tried caps across the cathode bias resistors on both mod and final. No change.


Tried keeping the RF final on with no fixed bias. I could see the tube plate current spike on unkey, indicating the power was being dumped into the final -  BUT there is still an arc.  I opened the arc gap a little more, but it is at the limit for even double the working voltage. Something not right.

T

[KA2DZT]

Tom

Probably always better to key on and off the HV power supply.  Opening the CT on filament xfmrs leads to sudden current drop-out in the wrong place in the overall current path.  Doesn't surprise me that you shorted a filament by-pass cap. The back emf from the big reactor will cause arcing.  You avoid these problems by allowing the current to decay more slowly by the time constant from the HV caps in the power supply.  You'll need a step-start for the HV power supply.

I think you mentioned previously something about the screen by-pass cap shorting.  Probably just another problem caused by your method of keying the xmtr.  I also think you're using a reactor in the screen circuit, sudden collapse of current in a reactor causes big back emf.

You still need proper timing other parts on the xmtr.  Especially the antenna relay, it should be first on and last off.  Drive should be up to full drive slightly ahead of the full HV.  A delayed step-start HV supply does this.  I think audio drive to the modulators can arrive any time.

Just a few of my thoughts,

Fred

[W2NBC]

Sequencing theory ..all goes to pot with the BIG mauls in practice  .  hopefully everything should get dumped to the antenna, ( load) .. but what value Heising cap are you using, and what configuration of modified Heising is it? and what circuit changes have been made vs the old Fabio ? baby steps OM 

[K1JJ]

Sequencing theory ..all goes to pot with the BIG mauls in practice  .  hopefully everything should get dumped to the antenna, ( load) .. but what value Heising cap are you using, and what configuration of modified Heising is it? and what circuit changes have been made vs the old Fabio ? baby steps OM  

JJ,  (Jeffie James)

OK on the antenna taking the collapsing power.

I'm using a 1uf Heising cap at 10KV.    Heising config?  The mod xfmr bottom sec connects to the Heising cap and then the cap to the Heising reactor bottom. The top of the mod xfmr sec directly to the Heising reactor.

I am using a standard 1 ohm to ground floating negative lead back to the HV supply to monitor grid current when using the linears. Hope that is no a problem. I didn't use it with Fab I.

With the old Fabio, I did not use a sequencer, rather dicked around with delay caps on the various relays. It took some time but using CT keying I got it right.  But I thought this sequencer wud make it easy.

Just ran some more tests. Nothing worked.  Tried making the driver stay on longer. Tried the ant relay on longer.  Made the RF final draw current all the time by reducing the CT resistor from 50K to 200 ohms.    Tried the modulator tube idle from zero to 400 ma.... no difference.

And I tried a combination of all these things.  The only thing that seems to reduce the spark to near nothing is to reduce the power down to 1/4.  The collapsing field is smaller and does not bang the gaps.

Other than do a step start HV thang, I'm about out of ideas.  

The thang that bothers me about the step start is I am using about 100 uf of HV filter capacitance.  I wonder if that is too high to drop real fast in a second?   I do like the idea of going back to a step start
though. It's safer and I cud also use it with the linear amps...

Fred, yeah there is a lot of bapping going on with the CT keying. Worked with Fab I and great wid linears, but maybe I need to change to HV keying for the plate modulated rig.

T

[K1JJ]

Where would you put the two clicks of HV step start into this key up sequence?   (The initial start and the final voltage)


Key on:

1) RF antenna relay switched on (vac relay)
2) RF amplifier turned on via fil CT ground.  (I've tried caps here and use a 56K resistor across the contacts.)
3) Driver on
4) Modulator CT grounded on.  
5) Low level audio turned on


Key off: Opposite of above


I assume  #2 and #4 would always be keyed on when using HV step start?


T