Screen Inductor minimum value for clean self modulation?

[KL7OF]

I have a couple mod tx's from Gates Bc1.  They have another winding on the secondary to modulate the B+ for the driver tubes (807's)......I seem to remember either K4kyv or Tron telling me that the ratio is wrong and wouldn't work for screen modulation... Has anyone tried this? 

[Bacon, WA3WDR]

If you took a screen-choke circuit and added a fairly high-value resistor from modulated B+ to the screen (and probably a good sized capacitor in series with the resistor to block DC), then you could push a little more modulation to the screen.  The resistor would be higher in value than a dropping resistor would be - probably a lot higher, maybe 50K or 100K for a 4-1000.  It's just to push a little more audio onto the screen than the screen generates by itself.

You would want the capacitor reactance to be equal to the resistance down around 10 or 20 Hz or so, so it passes audio ok.  For 50-100K, that would be about 0.15uF.  You could leave the capacitor out, but then the resistor will waste power because of the DC through it.

[K1JJ]

I may try that on both rigs, Bacon.  I like the blocking cap to eliminate power loss.

Good idea. TNX, OM.

T

[Bacon, WA3WDR]

Tom, I think the series resistor would be that big - but it may want to be as little as 20K with maybe 0.5uF in series.  You'll have to see what the trapezoid says.

Also if the curve is bent, it might be possible to compensate with some form of diode-resistor loading.  Ho BOY, is this getting tricky. heh heh

Steve - because the plate voltage of a driver stage would be fairly high, I suspect there would be too much audio on that extra winding in your Gates transformer to use it for the screen.  If that winding is on the outside of the core, you might be able to pull some of it off, but I'm not sure you'd want to do that.  Too bad it doesn't have taps.

Then again if you have a screen choke, that winding might be good as a source of some extra audio... just put it in series with a resistor (maybe 10K / 10W or so), and hang that across the screen choke, with the right audio polarity.  No capacitor needed!

[KL7OF]

Bacon.....those gates mod txs do have taps on the driver winding.......I'll have to dig out the book and see what the specs are.....

[Bacon, WA3WDR]

Hey - it might work then, Steve.  With some luck, you can find a couple of taps that have the right number of turns between them.

I was thinking about the DC issue.  It isn't very much current though, and I recall Tom talking about the screen current on one big tube being about zero because of the secondary emission.  Screen current may not be a problem.

[mario_puzos]

What are the guidelines for calculating the minimum inductance for a screen circuit using plate modulation? [For a decent low end]

I'm wondering if my 8H @ 300ma choke is barely enuff inductance for 80 cycles and a 140ma @ 500V,  4-1000A screen circuit.

I seem to remember a rough rule of thumb like X Henries for every X thousand ohms of screen impedance?? And screen impedance  =  screen voltage/ screen current ?


Thanks.


T

IN RELATION TO FEED THE SCREEN THROUGH AN INDUCTOR wanted to know if the modulation level INCREASES COMPARED TO FEED THROUGH THE SCREENS OF RESISTANCE?

[flintstone mop]

Wow, Big G always takes such great photos, Paul. Love that 4X1 - 833A's. Why did he ever get rid of it, I dunno.   In fact I heard Tom/KBW on yesterday using it and sounded great.

Jim, I understand what you're saying about the tube not caring about the inductance, as long as it has the collasping field to handle the lowest particular frequency. That's a good thing, cuz it means that the varying impedance will not affect modulation as the frequency goes higher - great.  I was worried about that. 

Well, figgering the min inductance will be simple... I'll just try 8H at first, which is easier to fit in physically, and jumper in another 8H and see what effect it has.  My guess is it will extend the low end down for a flatter response.  I will do it first in the 813 rig and then in the 4X1 rig to see what different current/screen loads make too. Will report back here. BTW, I do remember about a 10% increase in ability to hit 100% when adding a choke in the past. My first 6146 rig in 1969 did not have a screen resistor tied to mod B+ and I couldn't figure out why it would not modulate heavily until someone told me...

Though, what stands out to me is the Tron/HLR said it takes 20H for a broadcash type low end - for a 4X1 screen..  I've axed him questions for over 35 years and it's hard to believe, but I've never found him to be off the mark. Not once.     

I'll run some tests in a month or so. Got all of the rigs now laid out with finished brackets and starting to mount the smaller parts. Then the wiring... Then the testing and arcing.

T

Tom
I have done similar strange things, getting  rid of really nice transmitters...............

The 4CX1500B rig I bought from Chuck had a 20Hy in the screen of that pube.

[mario_puzos]

IN RELATION TO FEED THE SCREEN THROUGH AN INDUCTOR wanted to know if the modulation level INCREASES COMPARED TO FEED THROUGH THE SCREENS OF RESISTANCE?

[K1JJ]

Mario,

That's a good question.  (and good Necro.. )

I would have to say that the power resistor connected from the modulator HV to the screen would give more available voltage swing to the screen. Of course, this would depend on the resistor value. Too low of a resistance would burn out the screen from simple DC current, however. But more audio swing is not always best.

The screen choke method is more of a fixed situation that integrates its modulating voltage from the rising and collasping field induced by audio.

That said, both systems work very well in practice.   The idea here is to find a balance in screen modulating audio so that the final result is LINEAR. If there is too much screen audio added, you would think that this might induce a non-linear effect, or at least screen saturation.

I prefer the screen choke connected to a variable Variac-controoled LV supply. This wastes little power and also lets us adjust the screen DC current to compensate for various plate loads - that affect the screen current greatly.   When using the HV plate-screen resistor method, we are stuck with a particular screen voltage unless we have an adjustable slider on the resistor.

Good luck with the project.

Tom, K1JJ

[mario_puzos]

HELLO TOM I AGREE WITH YOU THAT YOU CAN PUT IN THE SUPPLY OF THE SCREEN TO PROTECT THE VALVE IN CASE OF FAILURE OF THE EXCITEMENT IN THE VALVE IS DAMAGE,

[Opcom]

The 4-1000 KW rig here runs 3000V on the plate and 400V on the screen and has a Chicago RC8300 reactor, rated at 8H, 300mA, 65 ohms, 3500V insulation for the screen modulating element.

[mario_puzos]

HELLO THANK YOU FOR YOUR DATA OPCOM, 4-400 for 2 valves with a voltage of 500V and 0056 DISPLAY OF CONSUMPTION AMPER INDUCTANCE SHOULD HAVE THE REACTOR FOR CUTTING FREQUENCY 60HZ?

[KM1H]

FWIW the Champion 300 uses 7H @ 50ma which seems to be adequate once changes are made in the low level stages. Ive left it alone in the 813 conversion.

I might play with the 2 resistor idea in one of my own rigs.

Carl

[WA5VGO]

For the 4-250A, Eimac says 10HY divided by the number of tubes. The current rating should be 2-3 times the operating screen current.

73,
Darrell, WA5VGO

[mario_puzos]

carl and darrell ok if I read that according 10HY Eimac is divided the number of valves, valves for 2 5HY serious? or 10HY?