screen modulation again...

[N2DTS]

I rebuilt the quad of 4x150's into a quad of 4-125's and gave it a try.
It seems to work very well, those tubes seem to screen modulate very cleanly with the W2IMX modulator.
2500 volts on the plates, about 200 ma plate current total, and about 150 watts out, 600+ watts pep, no carrier shift, tubes showing a dull orange color.
I also ran it at 200 watts carrier out and it worked well, orange plates.

[WB3JOK]

I'm screen modulating at the exciter level (ARC-5's) and using my quad 4-125A amp in AB1 as you recall from the other thread... but of course it could be done this way too.

Do you know how much screen voltage swing you needed to modulate the '125's at 200W carrier? How far negative is the negative peak?

[w4bfs]

ok Brett ... tnx for reporting yer results ... 100 ohms in each screen lead is more parasite stopper than anything else ... do you have individual grid leak resistors  as well as overall variable res and protective bias ?  did you match your tubes ?

the individual variable bias in the linear amp (JOK) is a good idea but require more rf chokes and coupling caps, I think

[WB3JOK]

the individual variable bias in the linear amp (JOK) is a good idea but require more rf chokes and coupling caps, I think

Well yeah, four of each instead of one of each... the chokes don't have to handle any current, so they are very small and cheap (about the size of 1-watt resistors) and not a budget buster 
Actually the four trimpots were the most expensive part, but well worth it for the ability to "match" any four tubes.

[N2DTS]

One single grid leak bias resistor (big wire wound pot in the control deck), just 4 tubes (the only 4) I had on hand, they all show even plate color.
I do not know if that is just lucky or what.

I spent a while using the deck today at 250 to 300 watts carrier output and it worked very well.
Orange plates at that level, long transmissions.
Put tone into it and it looked very good, a slight kink (at 40 Hz anyway) at about 95% negative modulation, the 40 Hz waveform looked very good otherwise.
800 volts input to the modulator, higher screen voltage then the 4x150 tubes, I am unsure of the swing on the grids, the resting screen voltage at 250 watts carrier is about 200 volts I think, and much more current then the 4x150's at about 40 ma for four tubes.
I removed the 100K resistor on the output to ground of the modulator and everything was still good.

Tried the rf deck on 80 meters as well, and it worked the same there, it sounds very clean and it seems very stable.
Nothing got hot, burned up, or arced over.



 

[N2DTS]

At 200 watts out:

[WB3JOK]

I also notice that "kink" as the envelope approaches zero output (at any modulating frequency). Not sure where it's coming from but as distortion goes it's fairly minor...

Nice looking RF deck with the glowing 4-125A's 

I'd thought of making my tank coil from copper tubing but decided to try toroids instead. It's three T240-2 cores stacked and covered with Kapton tape, then wound with about 16 turns of #12 copper wire. It does not seem to get hot after ragchewing on 40m at 200 watts carrier (also a nice orange plate color).

[N2DTS]

The coils plug in with banana jacks for changing bands.
I guess you could do that with toroids as well somehow.

I have been searching for the source of very low hum when I turn the grid drive on.

If I just key the exciter, there is a hum in the receiver, and garbage around the carrier on the sdr display.
On transmitting, its very low on the carrier but its there. Not sure you would hear anything at the far end and it does not show on the mod monitor.
I tried all sorts of things, adding filiment bypass caps, grounding various points, adding a cap to the bias, grounding the center tap and running the filiments in series, nothing made any difference at all.
I also tried driving the grids with my little ten tec argonaut 5 and it did the same thing.

Where is this hum coming from?

[N2DTS]

Blew it up yesterday running plate modulation tests at 800 watts carrier output.
Fried something or the plate choke did not like something as the first one melted down to slag and the 2nd one cooked the first pie section real good.
Time for a redesign...

[w4bfs]

At 200 watts out:

magnificent rf p0rn ... just an observation .... mounting the tubes so close together might have the plates running hotter than they would somewhat further apart due to intercepted infrared energy .... its a tradeoff between heating and vhf stability

[N2DTS]

It also precludes putting four 4-400's in as well.
That was not the plan, that chassis started out as a quad of 813's in grounded grid, kind of tall narrow tubes, then went to four little 4x150/4xc250b tubes, then went to four 4-125's.
It has turned into a real hack job, but a good experimental deck for little outlay in cash.
I had the tubes and sockets except for a few 4x150's and their sockets, everything else was on hand.
There is a lack of space in the center for all the stuff needed as well...

I just might build a pair or three 4-400 rf deck.
A pair would do 300 watts out with screen modulation and could be quite small, three would do 400 watts at lower voltages.

Something was not up to the job of 700 or 800 watts carrier and an atomic yea low and let loose.
Or the old chokes I am using are no good at that power level.
I have a B+W plate choke but its kind of tall for the space...I will see if it can be made to fit.


It may have been a bad winter, but I have had loads of fun playing with radio.

[WB3JOK]

I built mine on a 17x17x3 aluminum chassis, the most expensive individual part of the whole amp project, and always intended it to use four 4-125A's due to their low cost, ready availability, and inexpensive sockets. It turns out that there is room to plug in a pair of 4-400's instead (although they'd be getting close to the plate choke), in either pair of sockets on the diagonal, and the filament transformers are also just right. But those bottles aren't cheap and then you need a bigger plate transformer to take advantage of the 800W dissipation vs. 500W... maybe the plate tuning cap spacing isn't enough now, etc. For some reason I'm thinking of Tim Allen grunting, "More Power" 

What kind of plate choke did you use? I bought mine, don't remember where or what brand, but it may have been from RFParts.com... anyway it's not showing any signs of "zorching" at 200W carrier.

[N2DTS]

I got it working again at 250/300 watts carrier out (screen modulation).
I put an old B+W plate choke in, its a bit tall, but it fit and it works.
I did not test it with plate modulation, and I might not...

The tubes are cooled, there is a fan under the chassis directly under the tubes blowing up into the chassis and out through the bases.

The choke I had in at first was out of an FT102 I think, maybe not up to the current of plate modulation, but it had held up with the four 4x150's at 1600 volts and 600 ma.

It went when I ran it at 2000 volts and 600 or 700 ma.

The 2nd choke was a 4 pie wound on a big ceramic base, but it was somewhat damaged.
The B+W choke seems fine so far.


Are you screen modulating your quad?
Plate spacing only comes into play with the voltage you run, not the power output.
If you put in a pair of 4-400's in place of the 4-125's and ran the same plate voltage, you would just be able to run more current.
Four 4-125's at 2500 volts can do 600 ma, and under screen mod run at 300 ma for 250 to 300 watts out.
750 watts in, 250 out, 500 plate dis. (really hifi sounding audio though)

Two 4-400's can do 2500 volts and 600 ma, unless they can do more current there is no gain, but they will not be glowing their plates any.
The 4-250's and 400,s like more plate voltage, 3 or 4 KV.

[N2DTS]

Just went back and read that you are using it as an amplifier.
What does that 5th tube do, screen voltage regulation or some sort of drive amp?

I like the way the thing works, and like the lower voltage the 4-125's run, 2500 volts in screen modulation and 2000 volts in plate modulation.

My pair of 813's can run 2000 volts and 400 ma. the 4-125's can do 2000 volts (or more) and 600 ma for 1200 watts input..

I may get a new chassis and rebuild it to actually be what it is, with a bigger space for more coil (its a bit low on 80 meters), more space between tubes and a lot less extra holes and so on.

[N2DTS]

I ordered a new chassis, bottom plate and two filament transformers (Hammond) to clean things up.
I also got a nice 1.5 amp plate choke off ebay.

I might do as you did and raise the tube sockets off the chassis a little but keep the big hole below each tube and have air blow up through them.

[KB2WIG]

FWIW, any thought of using Coleman lantern globes??

Better than nothin?

klc

[Steve - K4HX]

We've seen this before.  

http://amfone.net/Amforum/index.php?topic=18616.msg130398#msg130398

[N2DTS]

4-125's do not need chimneys, and I do not plan to run them so hard as to need them.
If in CCS service at rated output, it would likely be a good thing to have them.

And, yes, 4-125's are wimpy tubes, but I do not see the need for super high power on the ham bands.
300 to 400 watts carrier without going to the ICAS ratings is a good place to be.

[WB3JOK]

What does that 5th tube do, screen voltage regulation or some sort of drive amp?

It is a  4-65A, used as a shunt regulator for the screen supply. Cheaper than many other tubes that can handle high plate voltages and currents! You can't see it in the previous photo but there's a large 100 watt wirewound resistor mounted on the chassis, which takes the "economy" supply (plate trans CT) voltage and applies it to the plate of the 4-65A, which in turn feeds the screens of the quad 4-125A's through a metering resistor. (The grid of the 4-65A is biased via zener diodes to give the desired voltage output).

I don't think I've ever seen negative screen current, and usually about 4-6 ma positive at 420 volts screen, 3 kv plate.

[N2DTS]

That is a cool way to do it, with the 4-65.

I never had reverse screen current on anything before, but the solid state modulator looks mostly like an open circuit I guess, and when used on 4x150's can result in the screen voltage going up on its own and the screen current meter going backwards against its pin.
The only way to stop it was to remove the plate voltage or grid drive.
With brand new 4x150's you could do 2000 plate volts and enough screen voltage to give a 300 watt carrier out of 4 tubes, with a 100K resistor from the screens to ground.

I see no sign of it at any voltage/power output with the 4-125's.
I also see no sign of it with the dx60/qix tube modulator, that transmitter runs 1300 volts and 100 watts carrier out and shows no sign of problems. Maybe it would at higher plate voltages...
I would need to cap input the power supply to get more voltage.
I might try that as a test just to see what happens....

[w4bfs]

.... And, yes, 4-125's are wimpy tubes, but I do not see the need for super high power on the ham bands.
300 to 400 watts carrier without going to the ICAS ratings is a good place to be.

I suppose you mean it has limited plate dissipation .... well maybe shifting from class C screen mod to class F might help ,,, I've just been looking at Georgios work on class F in the transmitters section .... a boost of plate efficiency would definitely help reduce heat production and could only require one more variable cap as a minimum implementation

[N2DTS]

Yes, limited plate dissipation.
The deck runs well at 250 watts carrier which is plenty on 40 meters.
I should try running it at 2750 or 3000 volts and see how it works.

I also have other rigs that can run more then twice that power output, so I am good with it the way it is.
Has anyone tried the class F stuff?
How do you tune it, how do you qsy?

[N2DTS]

Tried 3000 volts, worked fine, more peak power I think.

[w4bfs]

Brett you are doing a fb job in refining this transmitter .... I am surprised that you don't have instability, esp at 3kV ... did you neutralize ? iffn not, what did you wind up with for grid circuit ?  a terminating resistance ?

[N2DTS]

Its semi neutralized in the regular way.
I made sure to ground the metal rings around the 4-125 bases with spring clips.
I should add some resistance across the grid tuned circuit to damp out any nasty stuff (no sign of that yet).

The thing is really working well as a screen modulated RF deck, and I am sure it would work fine with the plate modulator at least up to 2000 volts.
2000 volts and 600 ma would be CCS ratings, 2500 and 600 ma would be ICAS ratings I think.
2500 volts might be pushing the plate tuning cap a bit.

With screen modulation, it runs fine at 2500 or 3000 volts and about 300 ma cathode current and 250 watts output, 1KW pep.
Dull orange plate color.

I have a shorter 1.5 amp plate choke and a new chassis coming.

I do not have the tubes or sockets, but six tubes in a circle would be cool....