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Ameritron AL811H tube selection

This article examines the performance of the Ameritron AL-811H amplifier with the supplied 811A tubes and with 572B tubes, and details a procedure for replacement of existing tubes.

Introduction

The Ameritron AL-811H amplifier is designed for four 811A triode tubes. The amplifier operates the tubes beyond tube specifications which delivers an amplifier that is dissipation limited in high duty cycle modes

The 572B was designed as a drop-in replacement for the 811A, it is dimensionally compatible, has similar transfer characteristics and has considerably higher voltage and anode dissipation ratings. (Note the foregoing does not apply fully to the Svetlana 572B.) At the time of writing, 811A tubes cost about US$20 each and 572B tubes cost about US$40 each.

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.

Fig 1: 811A anode characteristics from RCA data sheet

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:

Fig 2:

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

811A

Table 1: 811A
Item SSB telephony, A1 telegraphy FM AM (carrier)

Va peak (v)

1682

1470

764

Vdc (V)

1800

1600

1700

Grid bias (V)

3

3

3

Drive peak (V)

82

70

28

Tubes

4

4

4

Ia peak (A)

2.192

1.830

0.664

Ia DC (A)

0.680

0.572

0.228

Ia idle (A)

0.113

0.113

0.113

RF power at anode (W)

901

659

129

Anode dissipation (W)

324

256

259

Resonant load (Ω)

1570

1640

2265

RF Power out (W)

811 593 116

Efficiency (%)

66.2 64.8 29.9

Table 1 shows the input values / assumptions and calculated results for the models using the recommended 811A tubes. Note that these are derived from average characteristics, and individual tubes may perform differently when new, and over time. Red text denotes ICAS ratings exceeded, though that is not as bad as it might seem. The tube is still well within operating voltage limits specified for plate modulated Class C AM, and average dissipation under SSB telephony is much lower than the notional key down value in the table.

Operating current is similar to the datasheet recommendation for ICAS class AB2 RF linear amplifier for SSB telephony, but the voltages are considerably above recommendation (eg anode to cathode voltage ~1900V against 1500V) and as a result, power output and anode dissipation are higher than datasheet figures.

Grid bias has been set to a value to obtain typical idle current of around 0.1A. Circuit diagrams for the AL811H variously show zero bias, and a string of 6 diodes in the cathode circuit which would result in a bias of about -3.6V.

The efficiency calculation is RF power output / DC power input. Note that this is a grounded grid amplifier and RF drive power is another source of power input to the amplifier, typically around 60W for maximum output.

RF power output assumes an output circuit efficiency of 90%.

SSB telephony, A1 telegraphy

The assumption is that power supply voltage does not sag under short duration signal peaks. That is a good assumption for telephony, but DC voltage does sag a little under telegraphy.

Anode dissipation is calculated as 324W which at 81W per tube exceeds specifications. Under telephony, the low duty cycle keeps average dissipation within ratings. A1 telegraphy with a duty cycle of around 50% would result in average dissipation of around (324+1800*0.1)/2/4 or 63W per tube which is within ratings. Extended tune up on full power unmodulated carrier will exceed ratings and risks tube damage.

Maximum RF PEP output is calculated as 811W.

FM

Under continuous FM carrier, the power supply voltage sags. The operating conditions are varied to achieve maximum power output within anode dissipation ratings. Note the higher resonant load impedance reflects the changed operating conditions.

Maximum RF PEP output is calculated as 593W.

AM carrier

Under continuous AM carrier, the power supply voltage sags. The operating conditions are varied to achieve maximum power output within anode dissipation ratings. Note the higher resonant load impedance reflects the changed operating conditions.

Maximum carrier power is calculated as 116W and maximum RF PEP output is calculated as 464W.

Note that if the amplifier is operated on the load line for SSB telephony, the maximum carrier output within anode dissipation ratings is only 50W.

Summary

The amplifier is capable of around 800W PEP output on SSB telephony (including output circuit losses) provided that the signal does not sag the power supply voltage significantly.

The amplifier is dissipation limited for AM, FM and any high duty cycle modes. In all of these modes, maximum safe output power requires adjustment of the load impedance for optimum efficiency.

572B

Table 2: 572B
Item SSB telephony, A1 telegraphy FM AM (carrier)

Va peak (v)

1688

1489

750

Vdc (V)

1800

1600

1700

Grid bias (V)

-3.4 -3.4 -3.4

Drive peak (V)

88

80

46

Tubes

4

4

4

Ia peak (A)

2.246

2.008

1.021

Ia DC (A)

0.674

0.601

0.305

Ia idle (A)

0.100

0.100

0.100

RF power at anode (W)

911

711

179

Anode dissipation (W)

303

251

340

Resonant load (Ω)

1564

1540

1571

RF Power out (W)

820

640

161

Output efficiency (%)

67.5 66.5 31.1

Table 2 shows the input values / assumptions and calculated results for the models using 572B tubes. Note that these are derived from average characteristics, and individual tubes may perform differently when new, and over time. Note that these characteristics are NOT for Svetlana tubes which have different characteristic near cut-off and will probably not work in this amplifier without modification of the bias circuit.

Operating current is similar to the datasheet recommendation for class AB2 RF linear amplifier for SSB telephony.

Grid bias has been set to a value to obtain typical idle current of around 0.1A. Circuit diagrams for the AL811H variously show zero bias, and a string of 6 diodes in the cathode circuit which would result in a bias of about -3.6V.

The efficiency calculation is RF power output / DC power input. Note that this is a grounded grid amplifier and RF drive power is another source of power input to the amplifier, typically around 70W for maximum output.

RF power output assumes an output circuit efficiency of 90%.

SSB telephony, A1 telegraphy

The assumption is that power supply voltage does not sag under short duration signal peaks. That is a good assumption for telephony, but DC voltage does sag a little under telegraphy.

Anode dissipation is calculated as 303W at PEP which at 76W per tube is well within anode dissipation rating (160W - 225 W depending on maker).

Maximum RF PEP output is calculated as 820W.

FM

Under continuous FM carrier, the power supply voltage sags. The operating conditions are varied to achieve maximum power output. Note the higher resonant load impedance reflects the changed operating conditions.

Anode dissipation is calculated as 251W which at 63W per tube is well within anode dissipation rating (160W - 225 W depending on maker).

Maximum RF PEP output is calculated as 640W.

AM carrier

Under continuous AM carrier, the power supply voltage sags. The operating conditions are varied to achieve maximum power output. Note the higher resonant load impedance reflects the changed operating conditions.

Anode dissipation is calculated as 340W which at 85W per tube is well within anode dissipation rating (160W - 225 W depending on maker).

Maximum carrier power is calculated as 161W and maximum RF PEP output is calculated as 644W.

Note that if the amplifier is operated essentially on the load line for SSB telephony and does not need to be reloaded for optimum AM operation.

Summary

The amplifier is capable of around 800W PEP output on SSB telephony (including output circuit losses) provided that the signal does not sag the power supply voltage significantly.

The amplifier delivers a little lower PEP on high duty cycle modes due to power supply sag. In all of these modes, maximum safe output can be achieved without change of the load impedance and well within rated anode dissipation.

Installation procedure

The following is a procedure for installing new tubes in the amplifier. Note that the procedure is not appropriate to old tubes or NOS tubes that are likely to be gassy.

The procedure provides for a period of filament operation without high voltage to normalise the structure of the thoriated tungsten cathode and to raise the temperature of the envelope to assist getter activity.

Stage 1 - install tubes and burn-in

  1. disconnect AC power;
  2. remove cover;
  3. check that HV filter capacitor have discharged and install a temporary jumper from HV+ to chassis;
  4. remove the anode caps;
  5. remove the existing tubes and store with care;
  6. install new tubes;
  7. tie anode caps safely away from the tube anode caps;
  8. remove jumper installed at 3;
  9. reinstall cover;
  10. connect power;
  11. apply power and allow tubes to operate under normal filament voltage for 10 hours; and
  12. power down.

Stage 2 - connect anodes, neutralise and test

  1. disconnect AC power;
  2. remove cover;
  3. check that HV filter capacitor have discharged and install a temporary jumper from HV+ to chassis;
  4. install the anode caps;
  5. connect a dummy load to amplifier output (this need only be a low power load, 1W rating should be quite adequate);
  6. set the bandswitch, loading control to the recommended value (5.0 / 4.5) for the highest configured operating band (10m / 12m) depending on the model;
  7. connect an RF voltmeter to the centre pin of the RF output jack;
  8. jumper an external 12V DC power supply to the yellow wire on the back of the grid current meter;
  9. set the driving transceiver up to supply a low power carrier on the highest operating band (10m / 12m) and adjust power and amplifier anode tuning to get a suitable deflection on the RF voltmeter connected above;
  10. adjust (by bending) the neutralising capacitor plates for minimum indication on the RF voltmeter;
  11. unkey the transceiver;
  12. remove the external 12V DC supply;
  13. remove the RF voltmeter;
  14. remove jumper installed at 3;
  15. reinstall cover;
  16. connect power;
  17. reinstall the antenna;
  18. apply power and check idle current;
  19. follow normal tune up procedure and test amplifier on all bands;
  20. power down.

In the case of the amplifier tested, the neutralising capacitance needed to be increased by more than available by bending the existing plate. A small piece of aluminium was riveted to the existing plate to provide another small tab that could be bent towards the anode to adjust neutralisation.

Operation of the amplifier on reduced carrier for several hours to raise the envelope to full operating temperature will further assist the getter to clean up some residual gas molecules.

Conclusions

The AL811H is dissipation limited in high duty cycle modes, and is at risk of damage even during extended tuning under continuous unmodulated carrier. The cause is the dissipation rating of the 811A tubes. Further, the 811A are operated at higher voltage than recommended for ICAS conditions, though that isn't necessarily a problem.

The 572B was designed as a drop-in replacement for the 811A, it is dimensionally compatible, has similar transfer characteristics and has considerably higher voltage and anode dissipation ratings. (Note the foregoing does not apply fully to the Svetlana 572B.)

The use of 572Bs will not substantially increase the SSB telephony PEP output of the amplifier due to the available HV supply, but it will allow higher power output on higher duty cycle modes that are dissipation limited using 811As. The power supply may not be adequate to operate the amplifier at high average output power levels for extended periods.

An AL-811H with 572Bs fitted is a much more rugged amplifier, less likely to suffer tube flashover, more tolerant of extended tuning, and capable of higher power on high duty cycle modes without re-adjustment of loading when changing mode.

 At the time of writing, 811A tubes cost about US$20 each and 572B tubes cost about US$40 each. The US$80 additional cost of a set of 572B tubes is less than 10% of the price of the amplifier (US$899 at 10/11/2008).

Update 10/11/2008 - AL-811HD

Since writing this article in March 2008, Ameritron have advertised a new product, the AL-811HD which is an AL-811 fitted with 4 x 572B and rated at 800W. At a price of US$300 higher than the AL-811H, it may / should have more than just a different set of tubes.

Ameritron's rating of the amplifier at 800W is consistent with this article.

Links

RF Power Amplifier Tube Performance Computer

Changes

Version Date Description
1.01 25/03/2008 Initial.
1.02    
1.03    

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


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