Limit AL-811 Amplifier HF Digital Communication Transmit Power and Message Duration

[jimfraziernm]

LESSON LEARNED – Limit AL-811 Amplifier HF Digital Communication Transmit Power and Message Duration

After going through a couple of sets of AL-811As I was finally enlighten to the fact that these triodes were not designed to support long duration, 100% duty cycle communication modes. 

I discovered why I was blowing away all these AL-811A tubes. It appears I am burning the tubes up.  Other than the fact that the Al-811As are poorly manufactured in China, I believe the tubes are failing because my exclusive use of the Ameritron Amplifier is to support Amateur Digital Modes of Communications that entail 100% duty factor for durations greater than 30 seconds. For example, I almost exclusively use the Ameritron Amplifier to support  JT65 and JT9 communications which is 100% duty factor communications and each message transmission lasts approximately 47 seconds. To make that DX QSO into Europe, Australia, New Zealand, my simple vertical antenna system requires I pump up the transmit power to 100 watts to complete the QSO. 
According to the following link, https://www.w8ji.com/al811h_schematic.htm, "The most common cause of operational or customer induced tube failure is excessive anode dissipation over time. Heat is a function of duty cycle and short term average dissipation. The 30-second or longer time-period dissipation (dissipation is not the same as output power) must be kept below 60 watts. "  Further, "The single largest problem with the AL811 amps is tubes. Most tube failures, as mentioned above, are caused by poor tube manufacturing and operators who don't quite tune fast enough or well enough, or operators who use high duty cycle modes. " 

I am a high duty  cycle mode operator.  Hence, I am convinced the high failure rate of the Al-811As I have been experiencing is not due to the amplifier but my > 60 watts, high duty cycle operation for message durations exceeding 30 seconds.  So, it appears I need to place limits on the transmit power (below 60 watts) and message duration (when possible) when I am using the amplifier to support BPSK31, JT65, or JT9 digital communications.

Revised KC5RUO Ameritron AL-811 Amplifier Operations for HF Digital Communications

Due to the 100% duty factor associated with HF digital communications:
BPSK31:  transmit < 50 watts maximum, keep message transmission times < 30 seconds
Ameritron engineer recommended keeping 100% DUTY CYCLE DIGITAL   COMMUNICATIONS transmit power less than 60 watts and for no longer than 30 seconds
JT65/JT9:  < 32 watts because transmission time ≈ 47 seconds
JT65/JT9 message transmission protocol requires a message duration of almost 47 sec
Hence, transmit power for JT65/JT9 must be reduced to something less than 60 watts
To be on the safe side I calculated a maximum transmit power of 32 watts, it is based upon the following:
The JT65/JT9 message transmission duration is longer by a factor of
(47 sec/ 30 sec = 1.5666)
Hence, a safe maximum transmit power:  50 watts/ 1.5666 =  32 watts

KC5RUO

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