The AM Forum

The stuff I am building will be the first non-triode transmitter I've made.

I could use some discussion on screen voltage and the various methods of obtaining same.

a: screen supply only

b: screen supply with dropping rx from plate supply,

c: adding protective bias via cathode rx or other means.

I guess what I am after is what's the best no fuss no muss way of having protective bias on the tube in case teh screen supply craps out? Any law against having part of your voltage coming from one source and part from another? I might be cornfusing protective bias with screen concerns and not seperating the 2 concerns properly.

bear wih me....training wheels not off yet.  :P

Tim,

If the screen supply craps out the only thing that happens is reduced power output so that is probably the "best" failure mode.  Many transmitters reduce the screen voltage to provide a low power tune-up mode.

The greatest screen concern is what happens to the screen if the plate supply is lost, your tube will quickly be turned into a permanent, non-functional triode.  The screen grid is fairly fragile in all tubes and extremely fragile in many of the later very high gain tetrodes.  The screen needs protection in case of loss of plate supply (which could be obtained by supplying the screen supply via the HV supply) but also in case of over drive and/or reduced loading and improper tuning. 

If you are using a fairly rugged "old school" glass tetrode you could power the screen via a dropping resistor from the plate supply.  If this is a modulated class C final then you will want to use a choke to allow the screen to "self-modulate" for full modulation.  Take a look at the Johnson Desk or Collins KW-1 schematic for ideas.  A fast acting fuse or over current relay in the screen supply will provide limited protection for the screen and its supply. 

If this is going to be a linear final stage for a low power AM exciter, then you might want to take the easy way out like I did with my homebrew triple 4CX800 amp.  I bought the tetrode boards from G3SEK which provide both protection and regulation for the screen along with switching and other protection functions.  They were reasonably priced and once I got past the UK method of resistor code marking assembly was fast and easy.  One idea used in the tetrode board that could be borrowed for other supplies is the concept of using a sufficiently high value dropping resistor that when the the screen tries to draw excessive current then the voltage drops rapidly so that the screen can never draw enough current X voltage to exceed its safe dissipation rating.  Under normal conditions the screen voltage remains in regulation but it drops out of regulation if the current greatly exceeds normal conditions.  For calculation purposes, assume that all screen input will be dissipated in the screen element.

Definitely put in a screen current meter; in a tetrode tube this is the best indicator of proper tuning.  If screen current is too high at the proper control grid drive then the loading is too light.  If you build in a method to reduce the screen voltage then you will have a handy low power tune mode which will help avoid pyrotechnics during initial tuning.

In any case, strongly consider adding an over current trip to protect your new rig in case something unexpected happens (antenna damage, etc.).  Take a look at a Johnson 500 or Desk KW schematic for ideas.

Rodger WQ9E

How big does the choke have to be?  have to be able to handle the screen current of course, but any other concerns?

The tubes are the HK257B.

Tim,

Typically used values are usually 10 to 12 henrys.  A standard power supply choke should be fine (not a swinging choke) if you can liberate one from an old transmitter.  Enough current to handle the expected screen needs and voltage should not be a problem although keep in mind that if you use a non-regulated screen supply the screen voltage could rise quite a bit when the screen isn't drawing current.  So a "receiver" type choke might be questionable but any of those used for plate supplies (like in an Apache or Viking for example) should have no problem.

Are the HK257B tubes rated for 75 watts dissipation like the earlier HK257/4E27 or were they bumped up to 125 watts like the 4E27A?  I will be interested to see how your project turns out.  I picked up a box with a number of HK257, 4E27, and 24G tubes and sockets at a recent hamfest and they are sitting on a shelf waiting to be put into use.

Rodger WQ9E

no, still 75 watts. I got 4 of them new.

I'm thinking I'd like to use HK's where ever I can. let me know if you wanna part with any of those HK's.

I'd like to use 24G's for  the modulator tubes in the MiniStrap. 110 watts of audio would match up with the S-21 Xfmr pretty good.

if you have surplus 24G's N sockets lets play make a deal.   ;)

Hi Derb,

In my homebrew rig, which consists of a pair of 4-400As plate & screen modulated by a pair of class B 833As, I use a 30 hy choke as my screen reactor. It is mounted above ground on four ceramic stand-offs, to reduce the distributed capacitance losses to ground, thereby improving the audio high-frequency response of the screen circuit. Based upon the screen impedance (ratio of screen voltage to screen current) when the final is loaded to normal plate input, the predicted/calculated -3 dB low-end response with this value of inductance is about 30 hz. This is more than adequate for really good high-fidelity quality and solid low-frequency response.

I interlock the screen supply enable relay with the HV/plate supply contactor in my rig. I use a spare set of contacts on the contactor for this purpose. This way, if the plate contactor drops out due to a plate supply overload, etc., the screen voltage is simultaneously and immediately removed from the 4-400As as well. The screen supply enable relay is also controlled by the transmitter transmit/receive sequencing system I utilize; the screen voltage is the very last voltage that is applied to the 4-400As, and is applied after the plate supply soft-start relay has transferred. At that point, the transmitter turn-on procedure is completed when going from receive to transmit. For this reason, my rig is a little slow on the trigger, and not ideally suited for rapid fire break-in operation, but I believe it is unconditionally protected against nearly all potential failure modes that could conceivably arise. I also have included a screen overload protection circuit as well; it is designed to trip in the event the total screen current exceeds 120 ma. for the two 4-400As. There are a number of simple, tried-and-true designs for achieving this. The screen supply is variac controlled so I can vary the screen voltage continously from zero to 750 VDC, making tune-up and fine adjustments to the HPA very simple, etc.

I see no point in regulating the screen supply when plate modulating tetrodes or pentodes. It is not done in commercial broadcast transmitters, as there is no technical advantage in doing so, and will only serve to drive the cost, complexity, and reduce the reliability of the design. Besides, once you stick the screen reactor at the output of a regulated screen supply, the series impedance and DCR of the reactor will negate any power supply regulation benefits.

Additionally, you do not want to have a stiff or regulated bias supply for the grid bias for the final amplifier, when utilizing plate modulation. A portion of the bias voltage (about one-third of the required operating bias for class-C plate modulated service) must come from the fixed bias supply (for protection of the final amplifier tube or tubes in the absence of grid drive), and the remainder must be derived from a grid-leak resistor that is in series with the bias supply and grid circuit. This arrangement provides for much more linear modulation of the final amplifier, and further details can be found in any the ARRL Handbooks from the golden age of AM.

Good luck with the new rig. There is nothing in the world like running a high power transmitter that you designed and built yourself. Well maybe I'm somewhat overstating this............let's say almost nothing in the world.

73,

Bruce

yep. I was on top of the world when I built my first HB rig.

you dont know what you have until you dont have it anymore.  :'(

Gonna get it back. with interest.

YUP Tim,
It was BIG
It was BLACK
It was HEAVY

And great TX audio to boot.
Phred

That's right (that's right).

Derb, Here is the schematic of my Globe 500C. It uses a self modulatiing Screen supply that is seperate. You can see the choke on the schematic.

I am going to bypass this and use a big resistor to fire the screen from the modulated HV line. I am mainly doing this as I am going to use the ultra modulation unit and they do not want you to run seperate Screen supply of this type with the UM unit.

Clark

I went with the BAR  (Big Arsed Resistor). Actualy a tap off of the bleeder. 
Mostly cause it's simple, loose Ep and Escreen is off too.  The trick was to get enough bleed current for the supply, but limit the current that the screen could draw in case of trouble.  Worked out pretty good though, I gots 400 V and 40mA with the screen supply shorted to ground. 

Freaky how good that Ohms law thing works  ;D

In my 2x 813 rig, I use a variac power supply, a dropping resistor which is a big wirewound pot on the front panel, overload protection, and full metering.

The series resistor will allow self modulation, and makes the screen self protect up to a point.
I run about 2.5 k ohms and run the voltage up 100 volts or more past the recomended value to get the correct current.
If I wanted to use it as an amplifier, I can turn the dropping resistor to zero ohms and adjust the screen voltage to the correct value.

The overload is just a relay wired so excess current pulls it closed, when closed, it puts a resistor from the screen supply to ground to hold it closed, and a reset button lights up.
Pushing the reset button opens the circuit and resets the overload.
The current set point is set by a pot across the relay coil.

I always use some fixed bias, and some grid leak bias, you cant hurt anything that way short of overloading the plate current, which you can get away with for a bit.

For 2X tubes, I meter each tubes screen current plus both.

Grid leak bias and a screen dropping resistor off the plate supply is likely the reason a lot of tubes went in the trash I suspect, while I have had the same 813 tubes in for 25 years!

Brett
N2DTS

Derb,
My 4x1 has separate adjustable screen supply with a ckt breaker to kill it on overload. I have a 20 henry choke in series with the screen for self modulation and it works great. The rig will modulate to 130-140 % with no problems.

Bill

let me correct myself. they increased it from 75 watts to 125 watts ICAS and 100 watts CCS. So there was a material difference between he B version and the earlier non B.

Also, the screen can dissipate 25 watts by it's own bad self. its a pretty beefy tube for how small it is.

There's 2 things I dont like bout em : they want LAARGE plate voltages and screen voltages to make em work right.

1500 volts is nothing for these things - 2kv is the minimum they really want. and they want 600 screen joltage at that level.  :o

hey Derb ... 2 mo' have shown up on epay ...awooooo ...73 ....John

10-4, good buddy CUM AWN  :D ::)

my 4-1000 set has a variac-controlled screen supply with a high current bleed and an overcurrent relay that opens the DC screen supply and also the HV primary relays for the PA and MOD HVPS's.

beefus, I got dem thar tubes for 10 bucks each. Tnx OM for the heads up.  ;)

Brett -- I'd be most interested in what kind of relay you used and it's associated circuitry

Al

Yes Please share that? I'd like to compare and learn something as well.

I used a plug in relay, I just experimented with the amount of current it needed to close, voltage does not matter since you shunt it anyway to get the current adjustment.

So say my pair of 813 tubes likes 80 ma of screen current, I find a relay that will close with anything under 80 ma through it, that might be a 12 volt relay, or something else.

Once a relay is found, I pass the screen current through the coil to the contacts, wire it up in such a way that when the relay gets pulled closed, it connects the screen supply voltage to a resistor to ground that will maintain the current through the coil and hold it closed (normaly open contact).
I have a lit button on the front panel that lights on relay closure, and the button is a momentary contact type thing, pushing it opens up the path of the current through the resistor, no current through the relay coil, relay opens and passes the voltage to the screen again.
The relay also connects power to light the light in the reset button.


The actual relay I used is made by vanguard, mdp-5001  3pdt 12 volts dc, it draws 100 ma at 12 volts, but it closes at 7 volts and 50 ma.
I got a crap load of them someplace.

The pot across the coil sets the trip point, a wide range of resistance will work, but I like something that gives a close to center position.
I used a small wirewound pot, you could use a fixed resistor if you never plan on adjusting the current trip point.

With some relays, they may buzz or chatter as you get close to the pull in point, which might be wanted as a warning, if you dont like that, a small electrolytic cap across the coil eliminates that.

I like it though, as it lets you know WHY your power went away without looking at every meter and so on...

Brett
N2DTS

It's similar, except Mr. Tucker used a latching relay. not too easy to find cheap today. I do not know the coil volts, but it is essentially the same setup, Ig2 passes through the coil with a variable shunt resistor across it. Too much current pulls the coil in and the relay latches. Note the relay is in the negative lead so as to avoid having high voltage on the coil to GND.

My experience with buzzing is from variable DC current (modulation or ripple) or AC volts. Could it be in your case due to the latching contact circuit?

Hy

In my opinion ,when we used dropping resistor for the screen voltage ,we must have the right drive /grid current to obtain the needed  screen current for the  right screen voltage  =, B+ - ( R x screen current) ,to much drive the screen takes more current ,the screen voltage drops,to little drive ,the screen takes less current,the voltage soars up,it also depended on the B+(plate voltage),
So me must have two protection circuit ,one to protect from high screen current,because to much drive,or the there 's no B+(plate voltage),second from high voltage when there's no drive , the screen will soars up to the B+(plate voltage).
Also the tuning is depended on the tube gain (new tube ,or used tube)

I preferred using fix screen voltage,when we have fix screen voltage ,its' not depended  on the drive (grid 1 current) so it's easier to tune,we can drive with small drive and load/tune the transmitter ,so the chance of the transmitter to overload ( to high plate current) can be avoided,and don't worry about the screen voltage since it's fix
When we have tuned /load it , we can increase the drive level ,and re tuned it till the wanted output is reached.

It"s more easier to get the power output we want,since the power  output is also depended on the screen voltage(not the screen current),since it's fix,it's voltage is not depended on the screen current.

We need only need one protection circuit,it's for the screen current when there's no plate voltage,with grid drive on, the screen current get,s high.

Maybe I'm wrong

Thanks

Gito

I don't think you're wrong.  Both of my previous 813 rigs used fixed screen voltage (one an Eldico TR-1 and the other one HB) and I NEVER had a problem.  I think I'm going to go with a fixed SG supply and use a 20 HY inductor in the screen for self-modulation.

Al

That is the way my GK 500 is now. I am going to bypass this and use a fixed resistor off the plate supply so the screen is modulated. If it does not work out, I will switch it back

C

Clark,

   With a single screen dropping resistor off the HV B+, the screen voltage will try to go to full plate B+ in the absence of screen current, such as with no drive, or CW key up. So can the screen bypass capacitor take 2000 volts? Can the wiring? Can the Final Tube take 2000v on the screen? Using grid block keying, would the G1 cut off bias actually cut off the tube with HV on, and no drive?

   I understand what you are trying to do. Maybe you can figure a voltage divider to limit the CW key up G2 voltage instead of a single dropping resistor. Then think about what percentage of modulation you actually need at G2; It will be less than 100% even if the plate is modulated upward 130%. Also keeping the G2 modulation in phase with the plate modulation at the upper audio frequencies will need some thought.

Good luck with your project.

Jim
WD5JKO 

Jim. Thanks alot for the comments and Guidance!

I am doing this so I can run my Ultra modulation unit. You cant run it now with the Screen Self regulating. Robert is helping me with this project and has supplied the resistors, caps ect..  When I get the parts and his instructions, I will know more about how this will work.  We talked about this issue of the voltage and his system will track the Screen volts. I just dont know how at this point. We are changing the Bypass caps and such to larger broadcast types.  Its not just one resistor being installed.  As it is right now, The GK has no protection.. If you loose B+, The tube is probably toast.

If it does not work,  I will put it back to the way it was.. But it will be a learning experience.

Clark