GS23B Tetrode Amplifier Project: RF Testing and Tuning
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RF Testing

The equipment needed to tune the amplifier is minimal. I used the 600 Watt dummy load shown above, a Bird 43 Wattmeter with a 2.5 kW slug covering 100-250 MHz, the built-in SWR meter on the Icom-746Pro transceiver, and the three panel meters on the rf deck for plate current, screen current, and control grid current.

I let the WSJT program control rf testing: I arbitrarily selected one of the four FSK tones for USB and set the sequence for a 30 second on-off cycle as is the usual practice for meteor scatter operation. The first step is to get a good SWR between transceiver and rf deck. The transceiver must key PTT on the amplifier and all bias voltages must be in place, including the anode. This may seem obvious but it wasn't to me at the time. In my setup, the match between transceiver and amplifier shows a very sharp resonance.

In the initial search for power gain, I throttled back the transceiver to its minimum output of 5 Watts. A 250 Watt slug was put in the inline wattmeter for increased sensitivity. The standard procedure for tuning a W6PO triode amplifier is to systematically vary the tune and load capacitors while watching the wattmeter. This should guide you toward resonance. When rf was applied the plate current jumped, which was a healthy sign. I was properly driving the tetrode but not yet seeing any gain.

Highest power meter deflection occurred at maximum capacitor separation, which indicated I had too much tune and load capacitance (C5, C6, respectively) available. I addressed the latter by shortening the inductor pipes in the output tank (L5, L6) by loosening their clamps and pushing them as far down into the chassis as possible. This helped a little, so the next step was to physically shorten them by sawing off 1/4 inch each. This gave me a minimal amount of rf gain, but it was clear I was still way off resonance. I removed another 1/4 inch from each pipe and observed about 13 dB gain. I reduced plate tank inductance even more by shortening the straps that connect the pipes to the anode clamp. The tune capacitor responded accordingly, but it would not make any more power. The amplifier wanted lighter loading and I was at maximum capacitor separation at C6.

The next logical step was to reduce the load capacitance area. This was implemented by progressively snipping corners from the capacitor plates. With each reduction in capacitor area, incremental power increases were obtained. I took this to the limit by completely removing the plate attached to the anode ring. The amplifier was now pushing about 200 Watts with 5 Watts drive, but all the modifications to the load capacitor had made it difficult to achieve resonance on the tune side of the circuit. The output power was not stable and I also started getting rf arcs that would usually (but not always) trip the screen current shutdown.

RF arcs are notoriously difficult to locate, but not in this case. I could always trace them to one of the two nylon 1/4-20 screws retaining the moveable capacitor plates to their teflon shafts. Both nylon screw heads are visible in this top view with the GS23B removed. The pitted plastic revealed the presence of an rf arc. I assumed the screws were high field points that served to initiate arcs, presumably because the plates were too close together. Adjusting the inductor lengths could achieve resonance at higher plate separation, but arcing invariably occurred when the power level exceeded 200 Watts. To further complicate things, the SWR read by the transceiver would get progressively worse as amplifier power increased. I was also concerned that the amplifier just couldn't be loaded lightly enough.

After toasting almost a dozen nylon screws, it hit me that the issue was not high-field dielectric breakdown but excessive heat in the screws. In one case I saw that nylon had dripped down the copper capacitor plate like candle wax. This explained the power instability on the wattmeter and fluctuating screen current. The melting problem was solved with (expensive) teflon screws that I purchased from McMaster-Carr.

I also realized that plate load resistance is a function of plate current. The capacitor dimensions were calculated assuming the tube was carrying more than 800 mA at 1.5 kW output, not 200 mA where I was trying to tune. The amplifier started to respond favorably to load adjustments when the drive power increased to 10 Watts with a concomitant jump in plate current. A new capacitor plate was attached to the anode ring to replace the one I had chopped up trying to make the amplifier work at 5 Watts input. For this reason, I do not drive the amplifier with less than 15 Watts. I'm sure the circuit can be tweaked to provide enough dynamic range for efficient operation between 0--30 Watts drive, but the way I have it now is adequate for my use.


A tetrode amplifier tunes with a distinctly different procedure than a triode; it must be re-adjusted for each drive level. Lightening the load causes the screen current to increase along with the rf power. It also increases the plate voltage swing, which can result in rf arcing. The load capacitor is set to control the screen current, not the output power. The goal in tetrode amplifier adjustment is to set the load capacitance for optimum or recommended screen current. This current is likely to be an empirical, approximate value that trades-off gain, distortion, and propensity for rf arcing. A triode amplifier, in contrast, only requires the operator to adjust the tune and load capacitors for maximum output power.

My procedure is as follows: Arbitrarily set the load capacitance (C6). Apply rf and check for good SWR; this rarely needs adjustment. Adjust the tune capacitor (C5) to maximize the screen current while watching the control grid current (plate current is not affected by tuning). Try another value of C6 and peak with C5. As resonance is approached, screen current (Ig2) will first go negative and then swing positive. For a given load, maximum screen current gives maximum rf power.

I also see control grid current (Ig1) become negative as the output power cranks up. This alarmed me at first because it was inconsistent with the operation of KD5HIO's amplifier where Ig1 is always positive. I re-checked and re-calibrated the meter to make sure it was properly wired and determined everything was OK. Then it dawned on me that negative Ig1 is qualitatively consistent with K5GW's 432 MHz GS23B amplifier. The upshot is there exists enough variation between individual tubes to significantly affect the operating characteristics. I would not expect my amplifier to perform the same way if a different GS23B were substituted.

The screen and control grid currents (Ig2 and Ig1) listed on the first page are approximate. Lightening the plate load (ie. decreasing C6) will cause a slight increase in power, but will also dramatically increase Ig2 and force Ig1 even more negative when C5 is brought into resonance. I stop reducing C6 when Ig1 gets to -4 mA and leave it at that.

Low-pass filter

An unfiltered amplifier will produce some power at even and odd harmonics of its operating frequency. The W6PO style output circuit used here is unlikely to have sufficient harmonic reduction to comply with the FCC limits. To address this as well as keep my signal out of the neighbors' home entertainment systems, I place a low-pass filter between the station and antenna. The filter should also help kill some receive noise, although I haven't tested it directly for that purpose. The low-pass filter was the last component of the project and ironically it was also the most expensive. I had no desire to build one, primarily because of the high power levels I was dealing with. I use the DCI-160-LP from DCI Digital Communications in Canada. A much cheaper option recommended by WA6PY is the LPF144-7 that is made in Sweden by SM6FHZ.

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