Tuned Grid Drive - Swamping resistor?

[Ed/KB1HYS]

I need a driver for my twin-turbo RF deck. (dual 4-400 in class C). I have a KW TS-520 that should work ok fine, but it puts out ~100 watts. I only need 30 or so to drive the deck in class C. 

I can run the 520 in  "Novice Mode" and pull one final tube to reduce the output, but it is still too mucho for the final.

SO, can I run the 520 into the grid circuit using a big-arsed 50 ohm swamping resistor?

If I do that, can I then be able to switch the Grid Bias to get Class AB and use the deck as a Linear for the TS-520?

The deck is primarily going to be an AM rig, but it would be nice  to get double duty out of it.

Any advice ?

[KB5MD]

I run a TS520 as an exciter for a single 833a transmitter.  I just tune everything up and then decrease the drive control on the TS520.  I'm assuming you mean a Kenwood TS520.
I've tried the swamping resistor on an RCA BTA 1r1 that runs 4-400's and for some reason could never get it to work.

[WQ9E]

Ed,

As KB5MD said you can just reduce the drive from the TS-520 to whatever you need to drive your 4-400's.  If you want to run the 4-400's in linear mode you can reduce the TS-520 mic gain as necessary; again without modification.

If you do decide to go with a swamping resistor take a look at what Heathkit did with the KL-1 (I can send you a PDF of the manual) or Johnson did with the Thunderbolt (manual available on BAMA).

Rodger WQ9E

[Ed/KB1HYS]

great thanks!

[AB2EZ]

Ed

If you ever need a power divider... the following approach allows you to use a standard dummy load. I.e., the input, and the two outputs are all referenced to ground.

The core is the same type that some of us use to construct transformers for Class E rigs: Type 43, 1 inch outer diameter, 1.125 inches long. [If the core gets hot, you either need to use more cores or scale up the number of turns... but one core, and the number of turns given below, should be good for input power levels of at least up to 100 watts]

For a 50-50 power divider (half of the power into the dummy load and half of the power into the rig) use N=3 turns on the primary (one end connected to the "input" SO-239 and the other end connected to the "output to dummy load" SO-239); and M=3 turns on the secondary (one end connected to ground and the other end connected to the "output to rig" SO-239). The input impedance to the power divider will be: 50 ohms + 50 ohms = 100 ohms (provided the dummy load has a 50 ohm impedance, and the input impedance to the rig is 50 ohms)

For a 64-36 power divider (64% of the power into the dummy load and 36% of the power into the rig) use N=3 turns on the primary, and M=4 turns on the secondary. The input impedance to the power divider will be: 50 ohms + [50 ohms x 3/4 x 3/4] = 78 ohms (provided the dummy load has a 50 ohm impedance, and the input impedance to the rig is 50 ohms).

For a 73.5-26.5 power divider (73.5% of the power into the dummy load and 26.5% of the power into the rig) use N=3 turns on the primary, and M=5 turns on the secondary. The input impedance to the power divider will be: 50 ohms + [50 ohms x 3/5 x 3/5] = 68 ohms   (provided the dummy load has a 50 ohm impedance, and the input impedance to the rig is 50 ohms).

Stu

[Pete, WA2CWA]

Why not just insert a variable negative voltage to the ALC terminal (Pin 6 of the Remote Connector) to the TS-520? Then you can adjust the RF output for whatever you want depending on how you plan to use the equipment. This saves detuning the TS-520 with the Drive Control. I always felt "detuning" something was not always a good thing for the equipment.

[KD6VXI]

I always felt "detuning" something was not always a good thing for the equipment.

True that.  Take a look at the efficiency of a tank circuit when it's tuned to full output, and you 'reduce Pout via the drive control'.  It's a touchy balance, back and forth, to get the tubes not going out of emission, as well as get the tank circuit tuned up.

Not to mention, most tank circuits in our equipment that has a pair of 6146s in it (or similiar) wasn't designed for a 10-30 watt carrier.

--Shane

[WA1GFZ]

You could also build a 50 Ohm power attenuator pad. Caddok (SP?) sells cheap noninductive power resistors in TO220 or TO247 package. 3 on a heat sink or chassis could attenuate the drive and provide a stable load on the rig. Also you could reduce the screen voltage on the 6146s to reduce the output so the tubes last for ever.