Modulator Testing under load.

[KB2WIG]

I am almost at the stage of testing my 810 modulator system.

I want to test the modulator output by itself. ( 'caus  the rf deck aint done and the wife sayz i got to clean up this place. )...   If I had (6) 1K /100W resistors, I would  use them...   I'm thinking of using  (1) 200 W and (2) 100 W lamps in parallel with a 75K /(500 Watt??). The scope goes accross  the 100 W. I'll spark gap the pri. and sec.

Comment, better ideas and even criticism will be accepted........    klc

[Rick K5IAR]

Do you happen to have a high wattage speaker or two to throw into the mix?
Rick/K5IAR

[WA1GFZ]

Bulb resistance is very unstable. Usually you can count on a 10:1 change in resistance cold to hot (cold is low resistance). Also make sure you have a probe rated for high voltage. I wiped out a nice 10:1 probe hitting with high voltage.
Get a stable load or wait till you have a final. Wire wound reisitors have a high inductance that could reflect a weird Xl and make the modulator take off.
plastic Pail of water and a bit of salt if you are crazy.

[AB2EZ]

Light bulbs would work if you put DC on the bottom of the secondary of the mod transformer to

A. Keep the bulbs lit to some reasonable level... so that you avoid most of the cold-to-hot resistance change problem. With modulation, the average current through the light bulbs won't change... but because filaments heat up in response to current-squared, the bulbs will get 50% hotter under 100% sinoisoidal modulation

B. Produce the proper average current through the secondary of the transformer (unless you are using a modified Heising configuration, in which case you can still use the light bulbs if you run the DC from the Heising choke through the light bulbs along with the modulated current).

Use the bulbs at roughly their rated power. I.e., if you use 60 watt bulbs, their resistance will be 120 volts x 120 volts /  60 watts = 240 ohms each. You will need 25 of them in series to produce a dummy load with 6000 ohms of resistance, and 1500 watts of power dissipation capability. If you use 25 watt bulbs, the resistance of each bulb will be 576 ohms. You will need 10 of them in series to produce a 5760 ohm load... but the power handling capability of the load will only be 250 watts.

Naturally, you have to be very careful putting the load together in a safe fashion.

Best regards
Stu

[KB2WIG]

The speaker method is a good idea.

The lamp idea .. well ... In high skool lectricity class we did a lot of work with lamp loads and the screw in heater coils. Interesting stuff ( now it is, back then--? what the hell is this for??). I wish I had kept the lab experiments; we had to plot Volts v. Amps on the lamps, generate rating curves....  shunt fed  over & under compound motors, gens, etc.  Gack, that was a long time ago.....

Plastic and salt water....  we had a few water resistors which became objects of amusement. What can be sunk in them, how long before they boil, etc... fun times.  The lab had 2 pieces of dc  rotary equipment.  Ac motors feeding dc gens... If I can remember---  They were rated for something like 180V, 50 A a piece. I still remember the green lightning...

 The 1k/100w comes from Orr's The Radio Handbook  c 195??

God, this stuff is heavy.

klc

I've got a HV probe for the VTVM. I'll try this, maybe the 10:1 probe with a few 1megs across the output. I'v got a few 15KV caps that may be of use keeping the B+ off things .

[flintstone mop]

The resistors seems like a better load. The others are revealing the unsteadiness of light bulbs. A high voltage speaker? Must be April First.

Fred

[AB2EZ]

A danger in using a high voltage speaker as a load is that ... if you hook it up in the wrong phase, it will suck all of the oxygen out of the room.

Stu

[KA2PYQ]

The "cold resistors" approach is fine, however if you
can`t find enough room heaters to put in series,
try borrowing a gfkghdlkm of toasters and coffeepots.
remember to shunt their switches and thermostats
for the purpose. Later you can temporarily remove
their power cords and use them in series- parallel
instead of series, to get the medium impedances
you`ll need for RF dummy loading. It isn`t necessary
to heat coils to luminance.