Boots for my KX3?

My effort to build a relay box so that I can use my grounded grid 572B amplifier with my KX3 has hit a major bump. A few seconds of transmitting destroys the KX3 power amp / final transistors. Did I say major bump?, obviously this is a killer.

My design is straight forward and I use PTT rather than VOX for both CW and SSB so I'm not transmitting when I switch. There doesn't appear to be any shorts between the relay contacts. The problem, I believe, must be the open frame DPDT relay I'm using for bypassing the amplifier. On transmit the amplifier output and the KX3 output are both routed through the relay armature. The two signals are only about an 1" apart for 1.5". I suspect there is a lot of coupling between them. Some of the 90 watts meant for the antenna is feeding back into my KX3. I can imagine this extra power, if it doesn't directly destroy the KX3 finals, could screw up the KX3 final mismatch detection circuitry. I may be running the finals into a load they aren't designed for while the extra RF from the amplifier is causing the mismatch detection circuit to say everything is cool.

The possible solution: a relay designed for RF switching, namely back-to-back Dow-Key relays. Now if I could just tell if the KX3 finals are being stressed I'd be comfortable. I'd hate to blow another set.

I'm afraid this project is going back on the shelf. I'm having plenty of fun with my KX3 barefoot. No need to risk the finals a third time.

Boots for a KX3

I finally got the relay box finished so that I can use my grounded grid 572B amplifier with my KX3. On receive it bypasses the amplifier and delivers -100VDC cutoff bias. On transmit the 572B grid is grounded and the KX3 output is routed through the amplifier.

Lacking a triple pole-double throw relay I used a double pole-double throw relay to transfer the antenna and a single pole-double throw relay to switch the cutoff grid bias. The two relays run off of 12 VDC but together require about 1/2 amp, way more than the KX3 Keyline Out line can handle. A "Keyall" board from Jackson Harbor Press/WB9KZY solved that problem. Ideally I would have used a single transformer that delivered 12VAC and 100VAC for the two supplies. Lacking the ideal I used back to back 12V transformers. One toggle switch was added to allow manual send/receive control without the Keyline Out signal from my KX3. Another toggle switch was needed to power on/off the 12V/100V power supply.

When I use the relay box and my amp, the KX3 antenna tuner has to stay in the circuit in order to match the transceiver to the uncontrolled input impedance of the amplifier. In addition the antenna relay can not follow break-in CW keying without losing the first dit so I must use the KX3 in PTT mode. On the plus side I run about 80 watts to the antenna instead of 4, maybe not a big deal for CW but real help for SSB.

KX3 and a Boat Anchor Amp

A year ago, in a rare foreshadowing of radios to come, I acquired a linear amplifier built around a single 572B running grounded grid. It matched my Central Electronics 20A and the seller
had done really nice job of replacing the four original/modified 1625s in the P&H Electronics LA-400B with a 572B.

Now, seeing it in my shack, I realize that I have a potential May-December wedding here. Sure enough, my KX3 with KXAT3 easily drives the 572B to 100 watts out. The next step is to build a relay box that bypasses the amplifier and delivers -100VDC cutoff bias on receive. It can be controlled via the KX3 Keyline Out signal and a "Keyall" board from Jackson Harbor Press/WB9KZY.

At 35 pounds this one will be a heavy weight. I doubt it will see any picnic table operating but it will certainly help warm up the shack next winter.