4x3 screen modulation questions...

[AMLOVER]

Hi, to all of you and many thanks for the amazing infos you offered to us about am modulation that is my hobby.
Reading carefully your topics about screen modulation I am balancing between the droping resistor from HV plate voltage and the famous 10H choke with individual supply.
My question mostly has to do with the protection of my tetrode sweet lady (4x3) rather than the perfect modulation. You see, worse modulation it doesn't hurt that much as the "goodbye" of a sweet tetrode lady does....
I understand that in case of the droping resistor I'll need a HV relay to keep HV away of screen grid when excitation is off and that in choke case I'll need a very fast overcurrent protection system to protect screen if plate's HV fails.

Case A - using the droping resistor :

Question a) Where is better to put the HV relay after or before the reactor?
One side of the iron is grounded and the other is connected to the reactor through a 4uf/10kv capacitor.

Question b) Should I put a second relay to cut off the AF amp (solid state mono 8ohm/500w) when no excitation or  is it enough the inside it protection from missing load?

Case B - using the 10H choke and separate 500v screen supply :

Question A) What is the most reliable and easier method to protect the screen in case of plate HV failure?

Excuse me for that long topic but those questions have become my last month's nightmare.

Thank you, Apostolos from Greece.

[N2DTS]

What I do is a seperate screen supply, with a dropping resistor (drop about 20% of voltage) and an overload relay (relay and wirewound resistor, reset button/light).
It self modulates, it is forgiving, and its fool proof ( I ought to know!)

If you make the dropping resistor a panel mount pot, you can use the deck as a linear amplifier.

Brett

[WA1GFZ]

AH 4X3 I Love them I've only run them GG linear though. I would be interested in information about your rig. I bet KB3AHE method would work just fine.

[AMLOVER]

Thank you for your helpfull answers.

For WA1GFZ I should have been better explained that I modulate high level by swinging the plate voltage with the help of a reactor 40H, a modulation transformer 8ohm/3500ohm and a capacitor 4uf/10kv (modified Heising). I studied the KB3AHE's modulation method but it is for only screen am modulation. Thank you for response.
 
I am very interested for N2DTS protection method. Could you please give me some infos about the relay, reset button/light and the wirewound resistor rates? The dropping resistor must cut 20% of the nominal 500v or 20% of a bit higher voltage so that the tube finally gets 500v as its requirement is? I use 500vdc and I wish to protect the screen for currents over 200ma as far the tube consumes 170ma in full power.
If you don't mind, I would appreciate a small schematic with the right position and rates of all materials, thank you for your help.

Apostolos

[N2DTS]

Well, I have a variac power supply for the screen, and a heavy bleeder resistor, then a relay that is used to key the screen voltage, after that I go to a wirewound panel mount pot in series with the screen voltage.
After that, I go to a relay coil in series with the screen voltage, and a resistor across that coil (sets trip point).
The relay is wired so that excessive current opens the relay, the contacts are wired so that when the relay closes, it connects the screen voltage to a resistor to ground holding it closed.
There is a momentary reset button in line with the screen voltage that opens up the path through the resistor to ground. The button I used has a light in it, so I wired that to another set of contacts on the overload relay to supply 6 volts (filiment voltage) to the light when the relay pulls in.

Very easy to set up, I short the screen output to ground, then turn the variac up slowly while watching the screen current meter, when it gets to the current limit point I want, I adjust the pot across the relay coil till the relay trips. On my 813 pair, I set it just over the recomended screen current for 2 tubes, and usualy run the current well below that point.

You can place an electrolytic cap across the relay coil to stop chatter, but I dont, as it gives an audible warning you are in the danger zone, the relay will chatter a bit close to the trip point.

Almost any relay will work, you just need to pick the resistance value across the coil that gives the current you want.

On a side note, I have done experiments on the amount of voltage dropped in the screen resistor, dropping a lot of voltage and turning up the variac, and found you can drop a lot of voltage and have the screen current very self regulate, you can go crazy with the tuning and loading, plate voltage, and not trip the screen current overload.

I have had the same pair of 813's in the rig for the last 25 years.....
That is somewhat amazing for $5.00 hamfest specials....

Brett

[W3SLK]

Brett said:

I have had the same pair of 813's in the rig for the last 25 years.....
That is somewhat amazing for $5.00 hamfest specials....

I have the same pair of Marconi's in my HoseNose-500 also. They survived a 250 mile trip from VA.

[WA1GFZ]

Plate modulated 4X3 we need pictures OM.

[AMLOVER]

Dear Brett,

I attach a schematic that is about what I have understood from your detailed description, if somewhere is wrong please let me know. Is 10k bleeder OK or should be heavier as it consumes 50ma at 500v?
As 4x3 consumes 170ma at screen grid I'll set the trip wirewound resistor for 200ma max current to a 26.5vdc/900ohm coil vacuum relay. What could it be an aproximate resistor value to start or what is the formula to find it out?
As long I have a screen separate mameter I think that I don't need an extra indicator/reset button because I can reset the system with lowering the variac after fixing the screen overcurrent problem and come slowly up again.

WA1GFZ of course I'll publish here many photos with the 4 very heavy irons and the huge plate tank. The "monster" is a totally homebrew construction in a 7' heavy duty server rack.

W3SLK I hope that my sweet tetrode lady will be the same protected and long life as your ones.

Thank you all, Apostolos

[ke7trp]

That is very close to how mine is done.  Variac, Screen supply 0 to 1000 volts.  Trip relay, momentary push button to reset.  If no HV, It trips, If to much current on screen, It trips. 

Clark

[AMLOVER]

Hi Clark,
Could you please send me a schematic of your trip relay setting including rates?
I want to be very sure bebore I operate the screen department of my "lady".
She seems to me very sensitive and as far I am in love    with her I wouldn't like to hurt her ...screen! 
A little mistake and ... 
Thanks

[ke7trp]

I will try to find time to scan it all in today.

Clark

[AMLOVER]

Thank you so much Clark.
First thing tomorrow morning to check your schematic and then run to the shack for digging.......

Apostolos