Yaesu FL2100Z


The Yaesu FL2100Z is a HF linear amplifier rated for 1200W PEP input, and using two 572B triode tubes.

This article explores the recommended operating conditions, and alternatives.

All references to AM in this article are to 'standard' double sideband AM with peak modulation depth of 100%.

Performance model

A model was constructed to calculate anode current having regard to linearity of the tube transfer characteristics and conduction angle. A Fourier analysis of the anode current waveform yields the magnitude of fundamental current, leading to resonant load impedance,  and anode RF power, and then power input, output and efficiency. For more on the model technique, see RF Power Amplifier Tube Performance Computer.

Fig 1: 572B characteristics and load lines

An approximate load line is laid onto the anode characteristics graph and points taken for anode current vs grid voltage, see Fig 1. The points used for establishing the load line are:

The peak anode current (from the specifications) should be no more than about 0.8A and a saturation voltage of 150V is assumed. The FL2100Z has a supply voltage of about 2200V on load. To achieve full output, it will require about 110V peak drive, so the grounded grid valve is equivalent to a grounded cathode valve operating from a supply of 2310V. The blue load line is for Yaesu's recommended setup and passes through 150V,0.8A and 2310V,0.045A.

Fig 2:

Fig 2 shows the measured data points and a cubic spline interpolation of the transfer characteristic along the Yaesu load line.

Fig 3:

Fig 3 shows the anode current waveform sampled at 64 points on a cycle and using the transfer characteristic interpolation performed earlier. A Fourier analysis of the 64 points yields the DC and fundamental components of the anode current. The fundamental component of anode current is also shown to scale in Fig 3.

Key down constant carrier

Fig 4:

Fig 4 shows the model results. The model is of the FL2100Z tuned and loaded as per the manual for maximum output with least drive and DC anode current equal to 0.5A.

At 3.3V bias, the idle current of 0.09A is achieved. This figure is derived from the valve characteristics, and this figure seems rather high, probably due to a lack of lines on the valve characteristic chart for the quite non-linear region of low anode current. Additonally Yaesu recommend rather high quiescent current for this tube. In practice, a slighly negative bias voltage will usually be required.

At 110V peak drive, the target DC anode current of 0.5A (specified in the FL2100Z manual) is achieved. The peak anode current at 0.75A per tube is within specification, and anode dissipation at 160W per tube is within spec.

Assuming 90% efficiency in the output network, the PA should deliver around 750W at these settings for 1100W DC input.

Note that ability to sustain this power level depends on adequate cooling of the valves, the power supply rating, and other components. The manual specifies this power level for Morse code telegraphy, but not for AM or RTTY for example.

It is good operating practice to further overcouple the PA (lower loading C) and retune, reducing power output by up to 5% in the interest of reduced risk of valve cutoff and attendent distortion products. Since the FL2100Z lacks grid metering, overcoupling is a wise measure to reduce the risk of excessive grid current.

SSB Telephony

Operation under SSB telephony will not sag the power supply as much, and so the PA operates on slightly higher voltage. If it has been tuned as above (as per the manual), little increase in output power will be obtained.

To tune for maximum PEP, a pulsed tone source is needed to provide a low duty cycle, but peak signal that does not sag the power supply any more than telephony modulation.

AM telephony and constant carrier modes

Yaesu recommend tuning the amplifier for key down constant carrier as above, then reducing drive until anode current is 0.2A.

Under those conditions, output carrier power should be about 107W, and anode dissipation 171W per tube. This dissipation level is reaching the limit, and may be why Yaesu recommends this setup. Some 572Bs are rated for more than 200W anode dissipation, Svetlana rates theirs for just 160W.

At this setting, the PA should deliver approximately 107W of carrier, and be comfortably capable of carrying 100% modulation to 430W PEP.

Alternative operating points

Yaesu operates the valves at higher idle current than some datasheets suggest for AB2 SSB telephony, and higher than many other ham grade 572B and 811A PAs.

The green load line in Fig 1 is for operation at idle current of 45mA, half of Yaesu's recommendation.

Table 1: Loading alternatives
Item Yaesu
Lower idle current
Optimised AM 500W PEP
AM carrier
CW (AM PEP) AM carrier CW (AM PEP) AM carrier

Ia idle (A)



Ia DC (A)





Anode dissipation (W)





Resonant load (Ω)

2796 2720 4219

RF Power out (W)

751 107 772 147

Efficiency (%)

65.0 23.1 66.5 29.1

Table 1 shows the input values / assumptions and calculated results for models of the Yaesu and alternative operating points for the FL2100Z.Operation on reduced idle current reduces idle anode dissipation from 108W per valve to 59W per valve, and under key down maximum output CW, from 161W per valve to 152W per valve whilst increasing RF output slightly.

The greatest benefit from reduce idle current is AM operation. Yaesu's recommended operating point for AM results in anode dissipation of 170W per valve. Under reduced idle current, AM carrier power can be increased by 37% from 107W to 147W for the same anode dissipation, and still have more than adequate headroom for 100% modulation to 600W PEP.

The red load line in Fig 1 is for loading for a maximum of 500W key down CW. Table 1 also shows a configuration optimised for AM at 500W PEP, which accomodates the Australian AM power limit of 120W pY. Note the increased resonant load impedance, the amplifier is loaded for reduced output (500W maximum key down), and AM carrier drive set for a quarter of that for a carrier only anode dissipation of just 118W per valve. For strict compliance with the 120W pY limit using unprocessed audio, carrier should be adjusted for 118W.

Fig 5:

Fig 5 shows the model results for the Optimised 500W PEP AM load line, carrier only.

The tuning instructions for the Optimised 500W PEP AM load line are:

  1. tune / load the amplifier for the maximum RF output with the minimum drive power at anode current 320mA, output power should be 500W;
  2. set AM carrier drive until output power is 125W, anode current should be about 160mA.
Under these conditions, DC input power should be about 375W, RF output power 125W, loss in output circuit of about 11% of 125W or 14W, so anode dissipation equal to 365-125-14=236W. Calculate the dissipation for our own settings to confirm correct setup.

It is good practice at Step 1 to finish off by slightly overcoupling the PA (less loading C) and retuning to reduce the risk of saturating anode voltage, and the attendent distortion and risk of excessive grid current.


The FL2100Z is dissipation limited in high duty cycle modes.

Yaesu's recommended setup of the amplifier operates it with higher idle current than some 572B datasheets suggest and higher than many other ham grade 572B and 811A PAs.

Reducing idle current permits operation at higher power AM, and reduces operating temperature of the valves in SSB telephony.

An alternative setup with relatively low anode dissipation is offered for operation on AM at 500W PEP, or 125W carrier.


RF Power Amplifier Tube Performance Computer


Version Date Description
1.01 25/11/2010 Initial.

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.

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