The AM Forum

In reference to the 400v screen supply for the K1JJ/Tesla 300/813 rig:

Does this need to be a regulated supply?

I see that it needs to be variable to set the 80mA screen current.  Is this preferably done with a variac, or via a bleeder tap, or some other method?

What filter topology is needed just after the rectifier?  Just a filter cap?

Thanks,
Steve KK7UV

I used a dropping resistor off the modulated B+  to put 400V on the screens..a couple bypass caps on there and it works great

The 400volts is a nominal voltage... in practice there is usually a "sweet spot" for screen voltage.

The K1JJ rig has a schematic posted up in the "AMWindow" section of this place (upper left corner
of the main page). It shows what has been used successfully by others.

Probably no regulation is essential, always nice though.


                     _-_-bear

That schematic does not show how the 400v is derived, which is why I asked.

http://www.amwindow.org/tech/htm/813/813ps.htm

There's a 15H choke in the screen lead so it looks like it's a "self-modulated" screen. The 400V would need to be filtered. Regulating it would probably be overkill.

Alternatively, replacing the choke with a dropping resistor from the modulated B+ supply could be used to modulate the screen. The value of the resistor will determine the screen voltage. If your target is 400V at 80Ma, then subtract the screen voltage from the plate voltage and divide by .08 to find the resistor value. Example: (2000V - 400V)/.08=20000 ohms. This resistor will dissipate 128 watts(!) which would come from the 2KV supply.

Using a self-modulating choke, a separate 400V supply at 80mA would use only 32 watts.  So, there would be 96 watts less heat in the shack and an over-all smaller power supply.

If you use a self-modulating choke, make sure its insulation is rated for 2000V (have fun finding one) or you can mount it insulated from the chassis (ground) as a work-around. The 10H filter choke, used in the 400V power supply, is cleverly mounted in the negative lead so it's not subjected to high voltage. You can't do that with the self-modulating choke.

Nothing is ever easy!


Don

Aha!   Thanks!    I hadn't seen the PS schematic before.

Hi Steve,

Looks like the guys have answered your questions well.

I agree that when going above about 1600-1800 VDC HV, a Variac and separate screen supply is the best way to go with 813s due to the moderate waste of power and heat when tapping the HV supply as Don pointed out. Also, the Variac gives you exquiste control of screen current on the fly as you change HV settings, plate loading and other things that influence screen current.  No need to regulate the RF final screen voltage since the screen current is at steady state (~80mA) when dead carrier or under full modulation.

However, if the modulators are run with screen voltage as tetrodes (instead of triodes as shown) then we definitely need a regulated modulator screen supply.

An RF final screen resistor limits you to one basic optimized setting. A screen resistor HV tap works great in smaller rigs and also eliminates the need for the self-modulation screen choke, so it has its advantages too.  A matter of preference in many cases...

Consider blowing some pressurized air through the 813 bases and upwards. Especially for the modulators running class AB2, this can increase the tube dissipation rating from 125 to maybe 160 watts. I see a difference in color from dull red to gray when I do.  I have also used lantern chimneys, available on the web to do the job. Blown 813s (2X2) is quite the rig and will do 1KW carrier @ 140% modulation at 3KV when pushed to the insane limit.  Running at 600W carrier is a very FB setting.

T  

Thanks very much for the tips.    I will surely consider blowing air into the chassis and exiting up thru the socket holes (I plan to mount the 813s with their bases sunk 1.25 to 1.5 inches into the chassis box).   Will an axial fans provide enough air or should I look into squirrel-cage type blowers?