Testing for Stability
Home Test for Amplifier Stability
For homebrew amplifiers or modified suppressors in transmitters or transmitters, this is the simplest possible nearly 100% reliable test procedure. This procedure applies to any adjustable network amplifier stage requiring parasitic suppression. Testing for stability always involves varying load impedances presented to the output device.
Operating the amplifier stage with the highest safe voltage, terminate the output in a dummy load.
Use the smallest possible line fuses! Know where the "off" switch is so you can turn it off if the amplifier starts oscillating.
Starting on the highest band:
With open input port
1.) Set the loading control at 50% capacitance or more
2.) Engage the amplifier so it shows normal quiescent current on CW mode (if applicable)
3.) Rotate plate control through its range while carefully watching grid current and plate current. There should be no sign of movement. If there is any meter movement, be careful. You have an unstable amplifier.
4.) Repeat in SSB mode (if applicable)
5.) Change band to next lower band
I.) Short input and repeat and repeat 1-5
II.) Terminate input with 50 ohms (can be very small resistor) and repeat 1-5
In grounded grid amplifiers, be sure all grid pins are grounded with the lowest RF impedance path possible. If the amplifier is unstable at VHF, inductance of the parasitic suppressor should be increased or resistor value changed until the amplifier is fully stable.
Going back and forth between resistance value and number of turns, some resistance value requiring the least parasitic inductance will be optimum. After finding the best compromise values of resistance and inductance, add about 20% more inductance. If the resistor overheats on ten meters, increase power rating of the resistor. HF resistor heating will peak on the highest band, generally ten meters in an HF amp.
Here is the general procedure for finding optimum suppressor values, if you really want to optimize the system. It requires several suitable resistor values and some time:
1.) Run HV and filament at maximum allowable voltage. Start with much more inductance than required, and approximately 100 ohms of non-inductive resistance from a metal composition or metal film resistor.
2.) Activate the amplifier with normal resting current, but do not apply RF drive power
3.) Working and down one band at a time through all bandswitch positions, tune the PLATE capacitor watching for an increase in plate current or grid current (no RF excitation)
4.) If stable, remove high voltage and bridge suppressor turns with solder. Repeat until the amplifier just becomes unstable at VHF.
5.) With the amplifier on this band with the PLATE control setting, adjust the resistance up or down. Find a value that makes the stage stable. In general longer and thinner anode path connections require more resistance. If you don't want to go through this bother, use 50-100 ohms on average length anode connections, and 100-250 ohms on exceptionally long or thin anode leads. Both the required inductance and required resistance increase with long anode leads.
6.) Remove more turns by bridging them with solder. Retest again by bridging in other resistances.
7.) When satisfied resistance is in the right range, find the inductance where stability appears perfect. Add about 20% more inductance after that.
8.) Retest thoroughly. If the resistor overheats when running full power on the highest band, use a higher wattage resistor.
When I design commercial or amateur amplifiers, a series of rigorous tests are made. This is the commercial protocol:
Most Reliable Spurious Test Protocol
Starting on highest band with dummy load on input, while watching all PA current meters and spectrum analyzer, these steps are followed for each operating frequency range:
Starting on highest band with exciter on input, while watching all PA current meters and spectrum analyzer, these steps are followed for one central frequency in each operating frequency range: