The AM Forum

I am playing around with driving the various homebrew rigs with a solid state exciter, and found something odd.
On my 2x 813 rig, I get no swr when I peak the grid circuit, works like normal.
On my 2x 4x150a rig, I get a very high swr, which decreases when the grid circuit is peaked, to a lower but not low value.

Both rigs use a link for the input, cold end twards the grid coil, and I started off with a 3 turn link on the 4x150a rig, then went to a 2 turn link, with only a slight improvement in the swr.

What drives the swr?
The coils in the 4x150 are made of thick enameled wire on a coil form, the 813 rig has thinner wire on pvc pipe pieces.

The antenna tuner in the icom 756 pro 2 will tune the swr out, but its not stable with drive power, changing the drive makes the tuner try and retune.

Very simple setup, very short piece of coax between the antenna input to the 2 turn grid coil.
I do not remember the tube side of the circuit, but it gets fixed and grid leak bias, has a cap across the coil to tune the grid, rf choke in the grid voltage feed.


The home brew vfo/exciter drives the rig fine, 6146, pie net output.

I was thinking of turning the vfo/exciter into a CW rig, plus making room in the rack for another power supply to run the modulator or RF decks.

Where does all the swr come from?

1.  SWR itself is unimportant.  It is merely an alternate way to measure impedance in a circuit, using ratios instead instead of actual figures, and the necessity to perform algebraic calculations.

2. The input impedance of the transmitter is determined by the interface circuit.  

3. Three approaches can be taken in feeding the transmitter
-Ignore impedance matches, accept loss, and with a SS exciter, accept power foldback from protection circuits.
-Match the impedance at the transmitter side
a. Use fixed tuned circuits for each band
b. Provide tuning circuits, either manual or automatic (LDG boards are used in some amplifiers)
-  Again, used fixed or tunable circuits on the exciter side

You have chosen to use the auto tuner on the 756.  That's fine.

Solution:  Let the auto tuner find a match, then turn OFF the "Tune on PTT" or "Tune on Transmit" function, in order to maintain the initial match.

The SWR to some extent is not important as stated. What is key is the grid current. Don't exceed it. I have the same issue with GORT. It's a tuned input but I just watch the grid current and adjust accordingly with the drive of the exciter. I drive GORT with a Viking 2 and try to match for best power transfer with the V2.

The issue is the solid state rig will not create enough drive.
I am not going to run it at higher power out into a high swr, that is asking for trouble.
Besides, other rigs have a very low (or no) swr with the grid circuit peaked.

The output of a 756 pro series rig in AM is 25 watts of carrier.
If the swr was zero, I might need 10 watts drive to get the right grid current, I have a lot of grid leak and a very high bias voltage, so it takes some drive.

Why do you have the cold end of the link towards the grid coil??  I would put the hot end towards the grid coil.  You would have the link at the cold end of the grid coil.  This may have nothing to do the high SWR.

Fred

The grid in the 4CX250B takes nothing in the way of power until it pulls current so drive power changes load impedance to the plastic radio.

The auto tuner tuning at low power is designed to tune into a load that doesn't change with power level.

A resistor from grid to earth through a DC blocking capacitor to keep grid bias intact will help lower the load impedance. The value depends on the impedance stepup of the input tuning network.

A starting point might be 500 ohms. Be sure to rate the dissipation of the resistor for overdrive mistake situations.

I think all rigs were designed that way.
I also think all my rigs were built that way.

Ok, maybe you are on to something.
But if I turn the drive up and get say 5 ma of grid drive and tune the grid cap, the swr will go down to a low of 3 to 1, and not zero (at peak grid drive).

I have to look at the grid circuit, I had to work last night so I did not get a chance to play radio.
There might be some wierd stuff going on with the grid circuit values, I do not remember offhand how I did the bias hookup.

I also need to plug in the other RF deck (4D32) and see how that one acts.

The load presented by the grid tuning network can be out of range of the ricebox tuner. If there's a way to adjust it try a few very small changes and careful SWR readings at typical drive levels. It might take an old fashioned SWR bridge and tube type driver substituted to do this.

OR cut and try. Edison method.

One unrelated question. How did you determine the operating grid current of the 250B in class C?

TIA,

Dave

I look at the tube data for the grid drive of the tube/s in class C plate modulated service.

I then increase the voltage (a lot) to make the tube run way into class C.

I then inject tones and study the intermod and harmonics and adjust things for the best display.


I posted the results here quite some time ago in the clean rig thread.

I could adjust things to make things dirty or quite clean...

My best SWR from the plastic radio driver is when the class C drive is adequate and rated grid current is flowing.
This tells me the power taken by the grid is probably lowering the impedance of the setup to a reasonable value.
It also says I got lucky with the match-up using an MB-40 tank. I did have to fiddle with the link's turns and position.
A 750 Ohm swamping resistor across the grid coil might make it perfect, but it works well so I am happy.
The behavior also means that I can not use something like a miliwatt sized MFJ analyzer to see the Z at the transmitter drive jack.

Still think your link is not correct as to which end is nearer the grid coil,  but I will check it further as you may be right.  In my HB xmtr I'm sure I have the hot end of the link nearer the grid coil.  Although, I don't think it has much to do with your SWR problem.

You should try loading the grid circuit down with some resistance.   As other have said, the grid circuit impedance probably swings over a wide range with tuning the grid circuit.

I'm not sure what your grid circuit looks like, but try a 100 ohms resistor to ground.  This will give half the 50 ohms you need for a match.  The remaining grid circuit will be in parallel with the 100 ohms getting you closer to 50 ohms.

Another thought,  make sure you have the correct by-pass caps in the grid circuit where needed.

Try to post a schematic of the grid circuit if you haven't already, I may have missed it in earlier post.

Fred

I did a bunch of experiments, and nothing seems to help much.
I tried different numbers of link turns, thinner wire, rewound the grid coil, put an adjustable cap in series and parallel with the link, putting it across the link and closing it (maximum capacitance) lowers the swr a bit.

Maybe I should try putting the link over the grid coil.

I suppose there are grid driven amps designed for a 50 ohm input, what does the grid circuit look like on those?
I assume they are broad banded and show a low swr to the radio driving them...

did you calculate the reactance of your link at the freqs you want to operate ? it's probably not 50 ohms. And the tuning cap you used was probably not large enough to resonate the link at the freqs you tested.

Peter

They look like a 1:4 or 1:9 xfmr with a resistor across the output.

 

Well, since the icom does not seem to be selling, I will try some other grid coil setups.

You probably need to adjust the amount of link coupling and if the link isn't mechanically variable, try putting a variable cap in series with the link, as close to the link as possible.  A value of 300 - 500 pf variable should do the trick for 75M.  You may have to add a couple of turns to the link to make this work well. 

I removed the grid coil and link and re did it with B+W coil stock, 3 turn link over slightly smaller coil stock.

I get a very low swr, no swr really, good drive with low power.

However, when I modulate the rig, the swr goes way up.

With the tube driver, I see no change at all in the 6146 plate current, or the RF deck grid current.
This is starting to be a pain in the butt.

I increased the grid drop resistance to get a very high bias voltage with more drive, and the situation got worse.

Brett,

If you go back and ask yourself what the difference between a low power drive where things look good and that same signal with audio applied it may point you in some areas to check.  For 100% modulation source your drive power will  be running 4 times the carrier power on peaks.  Forget for just a minute the frequency domain points.  Take  your driver and increase  the steady state power to 4 times that of what  your unmodulated carrier.  Look at the SWR and make sure the amplifier is stable and the SWR is still low.   By doing that you will be determining how your amplifier is behaving as a steady state amplifier.  It may also help to look at the output on a spectrum analyzer to make sure its a stable as it needs to be.  Repeat the test just mentioned with the 4 times the unmodulated carrier test up and down say 5 KHz since that is what the amplifier will see under modulation peak power wise.    Every time you change something in the grid circuit, go back and check and make sure the amplifier is still neutralized since its a grid driven amplifier.   Trigger parasitic's can show under modulation, but I would not go down that path until you checked the steady state performance of the gain block itself while using the Icom as a driver. 

The output networks for your tube driver are quite different from the broad band SS driver and clearly their are compatibility issues based on the results your reporting.  My point in running the test I mentioned is to characterized the compatibility issues between the amplifier and that particular SS exciter.   

Joe, GMS           

   

I did turn the drive up quite a bit, not 4x normal, but maybe 2x, and still had the problem.
No sign of erratic operation or parasitics that I could see on the spectrum display, and the output swr stays low, so I do not think I have much power going out that is off frequency...

More tests tonight...

I didn't have such mismatch problems in both my single 813 rig and twin 813 larger rig.
Simply wound a few turns of 20 gauge hookup wire over the cold end on B&W mini stock with the windings having the right turn sense and both ground of link and RF ground (less neut. impedances) of grid coil connected together.  Are your coils wound with the same rotational sense?  Best impedance match sensitivity also is when the link is on the cold end of the resonant L/C grid coil.  Less variation in load- change sensitivity too.

In the dual 813 rig I made the ganged input band switch to select 5, 3, and 2 turns on the link along with full, half and quarter inductance ( approximately) for 160, 80, and 40 meters.

SWR is less than 2:1 in all cases (as roughly measured by the Drake TR 7 driver ) and power inputs of 10 to 20 watts is as expected.

In the single 813 rig I simply wound a 3 turns link good for all three bands and placed about 5k swamping across the grid to RF/neut. intermediate ground line.

I'll go to the downstairs computer and pull up a pix of the dual 813 grid input circuit to add here later.
Basically it's physically mounted on plexiglass to isolate a couple of variable caps from DC ground, etc.

That is some nice work.

In my 2x813 rig, I use seperate coils and links switched in for each band, 160 to 10, and do not have an issue with that rig.

There may be something different with the 2x4x150a rig, the 4x150 is different from something like the 813.

If I put a load resistor across the grid coil, that will just lower the Q, no?

I could put a resistor and cap from each grid to ground to load the grid...

I have the 3x4D32 rf deck on the bench, I plan on changing the grid input coils to the B+W coil stock with links over the coil like I did on the 4x150 rig, that got the swr down to zero (with no modulation).

Not sure why modulation would effect the grid so much, it might be a design issue, as the grid stuff is close to the high voltage feedthrough from the plate choke. Maybe some trace of RF gets into the grid circuit from that?

The deck is quite cramped, as it started out as a one tube deal on a chassis that was too small, then got a 2nd tube added...

http://n2dts.smugmug.com/Ham-radio#!/i-c57nxT7/A

Top:

http://n2dts.smugmug.com/Ham-radio#!/i-k3PMKRC/A

It works great as is with a tube driver, but maybe I should rebuild it on a bigger chassis and reuse the front panel.
I could then add 80 meters.
And maybe a 3rd tube....

Your stuff looks pretty darn neat too.  Yeah 5k does lower Q slightly, but this is a transmitter with tons of excess power compared with a receiver. Heath did this trick for years.  Broadens tuning, makes it less critical.  I found that I didn't really need it in the twin 813 rig. So both rigs take about 12 to 15 watts drive, the twin putting out over twice the power of course.  Is your link wound same direction as grid coil?

Grid current is measured but I find screen current is more critical when starting from a gross miss-tuned state.  When returning to previously recorded settings simply watching plate current dip is sufficient for touching up.  I haven't tried for minimum IP3 or some such.  I really should, given that I have the QS1R to watch everything.

For modulation to affect much of anything in indicated plate current usually indicates inadequate grid drive.  I found out early on that drive for CW in class C can be less that that required for AM.  Well that you knew.  So plate current shouldn't change much in the class C stage on a coil/mag. meter when going from 0 to 100% modulation. Since mine has a shared but very beefy power supply for mod. and class C final (3000v @ 1A P. Dahl, running 120 on 240 input) I see a small drop and sometimes increase in Ip as I swing the monkey.

Also you probably don't have complete neutralization if modulation is affecting your grid so much.  You need a big one of these piston or equivalent neut.caps.  ;D

_{The Raytheon fil. xfor is in honor of Derb.}

The 813 and 4x150 rig are not neutralized.
The 813 rig does not seem to care.
The 3x4D32 is, but it does not seem to work right.

All the rigs work fine and do not do anything odd other then the solid state rig drive problem.

The 3x4D32 does 80 and 40 meters, the 4X150 is set up for 40 only.

I think I will rebuild the 4X150 rf deck and make it bigger, better, and neutralized.
80 meters would be easy to add.

That should be an easy fun winter project.

That dual 813 rig is a real gem, Rick!

Let me know if ya ever decide to sell it... ;)   

T

Well, I started redoing the 4D32 deck grid coils, and found I have no good B+W coil stock, nor do I have any good enameled wire to make coils.
I have real thick stuff, and real fine stuff...

I suppose no place local will have any.
 

Well, I got my enameled wire and re did the grid coils in the 3X4D32 rig, and added neutralization and the grid goes down to 1.2-1 swr and does not change with modulation.

Waiting for a chassis to re do the 2x4x150 rf deck.
I will add neutralization and better cooling.

Great!  Knew you could do it.
FB OM.
Where did you find the wire?  RS's three pack of " magnet wire?"

I did not even look at radio shack, I did not know they sold it.
I got it off ebay, $6.00 for 200 feet of #24, should last me a while.

The RF deck worked fine un-neutralized, but the solid state driver did not like it.
Much less change in grid current with plate tuning (almost none) and no change of swr with modulation.

I hooked up the icom 756 pro2 as an exciter and after redoing the other rf deck, will look into turning the VFO-exciter into a CW rig.
A single 6146, about 40 watts out I think.
I need to come up with a plan for converting one of the home brew receivers to CW.
I have a nice product detector circuit prototyped, but need to figure out the bfo/offset/narrow filter things.

Fun with ham radio...

 

For the record, a good source for enameled wire of whatever size you need is your local motor rewind shop. Just about any city with some industry will have at least one shop. I have great luck with the smaller shops. If they don't just hand you a few feet, a small donation to their coffee fund is all it takes.

Steve   ..

But like bearing supply houses, they are getting very rare...

Brett,
It's been a couple of years since I bought some but R.S. had a three pack of #22, #26 and #30 spools.

Still do at "sale" prices. urk.

http://www.radioshack.com/product/index.jsp?productId=2036277&znt_campaign=Category_CMS&znt_source=CAT&znt_medium=RSCOM&znt_content=CT2032227 (http://www.radioshack.com/product/index.jsp?productId=2036277&znt_campaign=Category_CMS&znt_source=CAT&znt_medium=RSCOM&znt_content=CT2032227)

But it's great for winding those toroids for coupling and filters, etc. Will last me a long time.

"I got it off ebay, $6.00 for 200 feet of #24, should last me a while."


I've got several rolls of the rat shack stuff. It is high priced, but thats expected of radio shaft. It's a question of 'Do I need it now?'. I did, so I purchased.

I have a roll of Belden #34 that I bought from Ralph, W2wme, back in the 70's. That roll has and will last a long time. I'm sure my wife will be throwing it out when I become a SK (or actually a dead D10-4).

KLC

New Years Resolution. -  Become a AM gangstra and strap everyone. Then you'll all be sorry.

The 3x4D32 rig is done and working very well, new grid coils, but the big improvement was adding neutralization.
I used the setup K1JJ did on the pair of 813's modulated by 813's, a tuning cap in the grid circuit between the tank ckt and ground, and a copper strap close to the tubes for a pickup.
Worked a treat, easy to adjust, no high voltage to get your hands close to.
Very low and steady SWR on the icom driver.

So I tore apart the 2X 4X150a deck, which started out as one tube on a very small chassis, then had another tube added, which pushed things to the limit.
So close, I found the B+ had jumped to the grid coil at some point.
No space for any neutralization parts, so I tore it apart and started rebuilding it on a bigger chassis.
Cooling will be improved, and with plenty of space, voltage will not be an issue.

Two tubes did 300 watts carrier out (1200 volts, 300 ma) without getting warm, and I think the tubes would be loafing along at 1500 volts and 400ma.

After an old buzzard transmission, I could hold my hand on the tubes and they felt just warm, not hot, from a poor flowing muffin fan mounted inside the chassis, not much air flow really.

I might add 80 meters.
I also might try hooking it up in place of the 2x 813 RF deck and running it at 1500 or 1600 volts, and use the 4x150 modulator deck with it.
That would be a 2x4x150x2x4x150 rig...

One 4x150a has 250 watts of plate dissapation in a small tube, more then an 813.

If I built a modulator using four 4D32 tubes and used it on my 3x4D32 rig, it would be a 3x4D32x4x4D32 rig.

Wow! Lots OF 3's & 4's.  Keep paralleling those babies and you'll be able to run you 250 watt monitor speakers right off the cathodes.  8)

Good going.

Yes, thinking about it, in the past I had built a 4x813 amp for the flex stuff, which ran very hot at 200 watts carrier, but four 4x150a's would have 1000 watts of plate disipation...
Very low filiment power and lower plate voltages as well.

I suppose you could grid modulate it with low power and/or run AB1 for a clean signal.
Two 4x150a's do 600 watts in AB1.

Not that I have any use for an amplifier.

And I am just jumping on the X bandwagon, notice how everything is advertized as 2x whatever..
X is in.

Making progress on the 4x150 rf deck:

http://n2dts.smugmug.com/Ham-radio/i-VCw9F2g/A

And, I scored 3 new 4x150a (Eimac) tubes for $45.00 each.
That is a good price for a tube with 250 watts of plate dissipation.

I have all the holes drilled, and will prime and paint the chassis tomorrow.

I added grid band switching for 80 and 40 meters and neutralization.

The hard part is going to be making a bottom panel with a BIG hole in it for the fan.
Big holes in sheet metal are a pain!

More progress:

http://n2dts.smugmug.com/Ham-radio/i-QJBq4hR/A

About those sockets, they may not provide the kind of "over the base seals" cooling recommended by Eimac.
That whole bottom surface needs to be cooled according to them. It's the heater energy.
I know a lot of folks don't do it, even some military examples, and the 'right' sockets are high cost.
It might be impractical to drill holes and groove the top with a Dremel.
What do you think about the cooling of those tubes?

Anyway, please let me suggest this 12VDC blower instead of a fan. Japan Servo FBDC12H7P-949. It's quieter than most fans. It makes 24CFM at 0.1" static (15CFM@0.6") The tubes need a minimum of 7.2CFM for a pair, and you can use a good intake filter, like a 4" metal mesh filter, and still have enough oomph in the blower (YMMV).

It makes far more airflow that the blower in the SDP-1000L, a 1KW input amp here with two 4X250's. It cost $4-8 depending where. ($4.95 http://www.mpja.com/12VDC-3-3_4in-Japan-Servo-Blower/productinfo/18224%20FN/) It's not Chinese made. It's quiet enough I use it on a 200W power supply that runs 24x7 on my desk. The original Chinese fan communes with its ancestors..

Size / Dimension 97mm x 94mm x 33mm
Rated Voltage 12 VDC
Fan Speed 3500 R.P.M. (Revolutions per minute)
Maximum Air Flow 23 CFM (Cubic feet per minute)
Noise Level 56 dB-A
Power Connector / Function 3pin, Speed Sensor (Tach Out Put)

data sheet attached.

Thanks for the blower tip!

The rig has been running those loctal sockets for a year, plenty of old buzzard transmissions and even with low air flow, the tubes only get warm to the touch.

I do have the normal air system sockets in a modulator I run, along with a real blower.
I also have 2 more air system sockets, 2 different types, they made a lot of different sockets to fit these style tubes...

The way I run these tubes, 300 watts out, 360 watts in, 60 watts of plate dissipation out of a total of 500.

If I ran 400 watts output, the plate dissipation would likely be about 80 watts (40 a tube).

I got the new deck done and tested:

http://n2dts.smugmug.com/Ham-radio/i-cbTgZb3/A

Bigger chassis (taller) so the fan moves a lot more air past the tubes.
I have a few tweaks to do, label things, fine tune the neutralization, and maybe adjust the grid coils so the grid tuning stays the same on 80 and 40 meters, or at least very close.

I ran it a while at 400 watts out, shut it off, shorted the high voltage and felt the tubes, slightly warm to the touch, about 90F I think.

The 4D32 rig puts out a lot more heat at the same power input and output.
They take more filament power though, about 3 amps more.

In the future, I want to make an octal to spade lug adapter and run it in place of the 2x 813 rig, and take the voltage up to 1500, and modulate it with the 4x150a modulator deck, it has its own power supply.
A pair of 4x150's modulated by a pair.

Along with other projects, I put a filament transformer in the 2x 4x150 rf deck, and hooked it up in place of the 2x 813 rf deck. I made a cable to hook it up to the control deck that runs the 813 rig and fired it up on 40 meters.
I ran it up to 1500 volts and 400 watts of carrier output without any problems.
At that power level, the tube temp was 107F.
Modulating it with the 4x150 modulator deck, I got lots of positive modulation, it would do 2kw peaks easy.

I then switched to 80 meters, and the ricebox had a problem driving it (swr) till I added 20 feet of coax between the two.
I tuned it up at 1500 volts, did a yay lo, and the plate tuning cap arc'd.
I turned the voltage down to 1000 volts and it still arc'd.
I think the cap warped from the arcing, its marginal for 1500 volts.

I guess something is off with the tank coil since the thing worked ok on 40 meters.


 

Yep, I found one of the plates on the tuning cap warped, I corrected that and hit the plates with some fine sand paper to smooth things out.
Wish I had a bigger (wider spaced) cap, but this one will do.

If I increase the voltage to the RF deck, that increases the plate impiedance of the thing, so which way do I need to go with the tank coil to lower the voltage on the plate tune cap?

The coil is made from something like number 4 solid wire since an 80 meter coil with tubing would be very large...

I hooked the 2x 4x150 deck up in place of the 2x 813 rf deck again after bending the warped plate tuning cap plates back and sanding them to remove the rough edges.

I set the RF deck voltage at 1000 volts, ran it to 300 ma, got about 250 watts carrier out, then hit it with modulation.
Wow, no wonder I cooked the cap, the 4x150 mod deck can do 600 watts and I am running about 300 watts input on the rf deck. That is 2x the audio power then the rf power, what peaks!
Running 250 watts carrier, and running the modulation at about 85% negative, I was off the scale in the positive direction and doing 1500 watts pep!

I had a short qso with John, N2XD without anything crapping out.

Tube temps got up to 107F.

I really need a wider spaced cap so I can up the voltage on the RF deck some, plus with all that audio power, its easy to hit very high peak voltages.

 

Anyone got a higher voltage cap to trade/sell?
The cap in the rig now is a Johnson 154-2 which is 350 pf at 2000 volts,and something 150 pf and 3000 volts would be good.

Here is a picture:
http://n2dts.smugmug.com/Ham-radio/i-phqTm8g

It can be longer, but not wider or taller.
I suppose they made a cap that was 150pf or 200 pf and 3000 volts in the same size ?

The 2KV cap is a bit close.  3KV would be better.  I have a 3KV cap in my 50 watt rig, about 650 volts B+, plate modulated., never had any arcs.

Fred

That is very nice work on these decks. It is amazing what neutralization can do to settle down the drive situation. Even tubes that "do not need it" NEED IT.

Yes, at 2kv, I am pushing the cap to the limit.
There is no DC across the cap, but the rf voltage is at the limit at about 1200 volts on the plates.
I never had an arc running it with the 811 mod deck with a common power supply because the most I can get out of that is 1250 volts, and the modulator drags down the plate voltage some, reducing the peak power and voltage.
Things really change when you run seperate supplys.

I never used to bother with neutralization, but the rigs tune up much better with it.
The 813 rig still does not have it, and does not seem to mind much at all, but I might add it.
With it, both the 4x150 and 4D32 rigs have a stable grid drive, it does not change with final tuning, and does not change with modulation.

SWR to the rice box is still higher then it should be, but the tuner in the rig tunes it out just fine.
I wonder what could be done to lower the swr enough to not need the tuner.
A resistor across the grid coil? Less turns on the link coil?


With a higher rated cap, I could turn the plate voltage up to 1500 volts and see if anything blows up.

The 4x150/4cx250 tubes make for an interesting RF deck, I could likely get more power out of them then the 2X813 rf deck, and that deck is huge.
The 4x150 runs lower voltages as well, which makes for smaller parts.

Odd that the RCA tube manual says you need to modulate the screens of these tubes to get good modulation, but I seem to get insane peaks with the screens self modulating.

Brett, how about putting 300-400 pF in series with the existing cap?

I thought of that at about the same time you did I think.
On come caps, you can pull every other plate and get 2x the spacing, but not this one.
A door knob in series might do the trick though, at least as a test.

I tried the cap in series, first I used 500pf at 5000 volts, it worked ok, then I tried a 100pf cap, could not load it up right, then I tried a 1200 pf cap and it worked well also, so I left that in.

Turned the voltage up to 1500 volts, got 400 watts out, over 2 kw with modulation, measured the tube temps, 80F, looked the max temps up in the tube manual, 175C for the base, 200C for the plates.

175C is 347F so I have a ways to go.

Good going. Looks like you tried for the optimim.  Of course your split voltage ratings also vary from equal voltage each when stacking capacitors.  Optimum voltage split, of course, is when both capacitors are the same value.

You need to split the capacitance equally at what the resonant capacitance combo will be, and if reasonably calculated still gives some vernier action of the variable cap over a desired freq. range.

So it's a compromise between loss of vernier action with too large or to small a series fixed cap and loss of a decent voltage split whereby one cap is still going to see most of the voltage anyway .  Many mysterious arcs have occurred this way.

Mmm, same thing three ways just as muffled said I.  ;D

Sounds like the 500 pf would be better then 1200 then....

Yes sir, closer.  Voltage split Formula is in most handbooks along with cap. Values from stacking in series.
And nice talking with you on 40 this morning. You were 10 over at times. I like your 4x150 rig. Good tight audio.

Yes, real nice qso today.

The audio is too tight.
I need to look into that next, with the signal generator, the rigs seem to pass higher frequencies ok, but not so much with my voice.