A No Power Tune-Up and Neutralization Procedure for the Swan 500CX Transceiver

By John Magliacane, KD2BD

Originally Published on QRZ.COM on December 27, 2011

My Swan 500CX has seen little use over the past 20 years due to my construction of an Elecraft K2/100 in 2003, and my general lack of radio activity in recent years. In 2011, I decided to put the rigs side-by-side and compare the RF output of each transceiver.

I assumed the Swan would pounce all over the Elecraft, but was disappointed to find the Swan falling short of the Elecraft's 100 watts output in TUNE. Even more disturbing was the fact the RF output continued to fall to even lower levels as my testing continued. Obviously, the Swan was in trouble, and the time had come to nurse it back to good health.

All switch and relay contacts were cleaned, and all potentiometers were sprayed. Tubes that tested weak were replaced. The 6LQ6 finals were replaced with a pair previously used in TV service that had identical construction characteristics and equally good emission.

Reading about the operating characteristics of the 6LQ6 created some concern in my mind about how they were being treated in a transceiver advertised as having a 550 watt PEP input rating. Once available for several dollars a piece many years ago, 6LQ6s haven't been manufactured in decades, and command a King's Ransom if purchased NOS today.

Tuning Is Dangerous

From what I could gather from the Swan's operating manual, when the transceiver is in TUNE position, the finals are operating with approximately 720 volts on plate and 500 mA of total cathode current. That's 360 watts of input power. Since the finals are biased for Class AB1 operation, 60 percent plate efficiency should yield 220 watts of output power. While each 6LQ6 can safely handle as much as 350 mA of continuous average plate current, they are dissipating 70 watts of heat per tube in TUNE, which is more than twice their 30 watt continuous plate dissipation rating (but below their 200 watt short duration rating).

These figures assume the transmitter's pi-network is properly tuned and loaded to resonance at the operating frequency. Any deviation from the proper tuning and loading conditions, and the plate dissipation will be significantly higher.

TUNE applies the amount of grid drive assumed to be available during the highest crests of SSB modulation. Tuning under this peak power condition is necessary to minimize flat-topping distortion under SSB operating conditions. However, due to the power levels involved, it is likely the greatest contributor to final tube damage than any other operating mode.

In an effort to avoid placing the transceiver in TUNE mode when changing bands, I made up some handwritten charts illustrating the proper tuning and loading control positions for each band of operation. When the final tubes are replaced, the optimum tuning and loading control positions can change. Furthermore, a neutralization procedure must be performed that requires the final amplifier be operated for short periods of time at reduced power levels and at either side of resonance.

I felt really uncomfortable performing these procedures with 6LQ6s being in such short supply, so I came up with a different approach that worked extremely well, and places absolutely no stress on the finals.

A No Power Tune-Up Procedure

The ARRL Handbook provides a simple equation for predicting the optimum plate load impedance given the plate voltage, current, and operating class. For 500 mA at 720 volts in Class AB1, that comes out to 960 ohms, or 1920 ohms per tube. With the power supply disconnected from the transceiver, I grabbed a pair of 2200 ohm resistors, and connected each in parallel with each 6LQ6 (between plate and ground). Next, I connected an old homebrew impedance bridge between the transceiver's output and a signal generator operating at 3850 kHz. The impedance bridge was adjusted to indicate a "null" on its meter when looking into a 50 ohm resistive load. (Modern-day antenna analyzers can easily provide this function in one neat package.) Finally, I adjusted the rig's tuning and loading controls until a pure 50 ohm output impedance was found, and recorded the corresponding knob positions for future reference.

A No Power Neutralization Procedure

The neutralization adjustments were also made without any power being applied to the rig. I removed the 2200 ohm resistors from the 6LQ6s in the previous step, and fed a 14.150 MHz signal from my signal generator directly into the output of the transceiver. I tuned the rig to the 20-meter band, connected an oscilloscope to the plate of the driver tube (6GK6), and tuned the neutralization capacitor for minimum feedthrough (minimum reading on the scope). The 10-meter neutralization was accomplished in a similar manner.

The transmit mixer and driver coils were re-aligned as per the operating manual. This procedure required removing the screen grid voltage from the final amplifier, adjusting the PA bias control for maximum (-110 volt) bias, and maximizing the grid current being drawn by the final amplifier under full grid drive (TUNE) conditions.

There's No ALC in TUNE or CW Modes

What I discovered from this procedure (and later experimentation after normal operating bias was restored), is that final amplifier grid current is drawn under TUNE and higher-powered CW conditions. The ALC circuit that is designed to keep the final in AB1, is audio derived from grid current peaks, making it effective for SSB voice, but completely immune to steady-state conditions present under CW and TUNE modes of operation. The grid current levels I saw could easily exceed the maximum allowable for 6146Bs.

Power Output Measurement

When I built my Elecraft, I calibrated the rig's internal wattmeter by measuring the RF output voltage across a 50 ohm load using a 400 MHz Tektronix oscilloscope. Then I solved for P, knowing E and R, and the relationship between peak-to-peak and RMS for sinusoidal AC voltages.

As a sanity check, I re-tested my Elecraft using an older 100 MHz Tektronix, and came up with very similar results. Then I fired up the Swan using exactly the same test arrangement.

Using the optimum tuning and loading control positions I discovered earlier, I measured an RF output voltage of 325 volts peak-to-peak in TUNE on 75-meters, which equates to 265 watts of output power. The S-meter indicated 600 mA of cathode current being drawn in CW. Assuming 720 volts of plate voltage, that's 432 watts of input power, 60% of which should be 260 watts of RF. There's less than 2% error between these two figures.

Next, I picked up the microphone, and while speaking at a level that brought the average cathode current close to 225 mA on peaks (as recommended in the Operating Manual), I measured very close to 400 volts p-p, which equates to close to 400 watts PEP across 50 ohms.

This is far above what I ever expected!

I noticed what I believe was some minor rounding of the RF envelope peaks at this power level, so it may be necessary to keep the average cathode current peaks below the recommended 225 mA level to avoid splatter. At no time did I detect any grid current being drawn in SSB at this power level.

Incidentally, my power supply is still operating with its original filter capacitors, and they most certainly have lost capacitance over the past 35 years. Some "ripple" was obvious in the scope pattern while in TUNE and CW. I'm sure the supply's dynamic voltage regulation isn't what it could (should) be.

Believe The Hype

Nevertheless, after being re-tubed and re-tuned, getting in excess of 300 watts PEP output on SSB with a 500CX is easy. This level of output power is in very close alignment with Swan's advertised 550 watt PEP input power level.

Plate Dissipation Analysis

If the plate voltage averages 800 volts in SSB, and the average cathode current in SSB peaks to 225 mA, the average peak input power is 180 watts, approximately 40 percent of which will be lost as heat. This equates to 72 watts, or 36 watts per tube, which is 6 watts above the 30 watt maximum average plate dissipation rating of the 6LQ6. Since SSB voice isn't a steady-state condition, and since I'm using a fan to keep the finals cool, I feel somewhat comfortable operating the rig under these conditions.

On The Air

After I put the rig back together, I tuned to 20-meters and immediately answered a CQ from Wolfgang, OE9LWV, in western Austria with 5x7 signals both ways. My antenna is an 80-meter half-wave dipole up 20 feet fed through 35 year old RG8 foam and a homebrew L-network.

Also see my Swan 500CX AGC Modification Page.

telegraphy key John Magliacane
Amateur Radio Operator: KD2BD