About Power Amp Tube - Transmitters . One Question

If the power amplifier tube is said to be gassy, how could you detect that problem by looking at the output carrier?

Is it by considerable reduction of carrier amplitude ? or is it by the presence of the hum ?


Thanks for who ever is going to answer this question.

Going way, way back to my first years in this field, the primary indication of a gassy tube is the milky white color of the glass envelope, if glass of course. This would be indicative of lowered emission, which will impact gain negatively. But this is from my memory of burning my fingers on many a tube too many decades ago. If the tube is all metal such as the 4X150 & 4X250, transmitter finals used in HF up to UHF, then one must take the time to properly troubleshoot an issue and insure the input drive to the fianls is within nominal levels. Or that the final stage is tuned correctly and with a good match to the antenna.

The output gets “mushy”. The transconductance starts to look more like a battery than a signal amplifier. The ultimate gassy amplifier is a thyratron; up to the point of burn out, its transconductance curve will start to migrate in the direction of a thyratron’s behavior.

So if I understand correctly, it would be the reduction of the carrier amplitude, since it would be loosing its gain

Anything that a reduction in gain or bandwidth would cause is possible. Also once it is bad enough, expect it to go into saturation and stay there.

Okay thanks so much, I really appreciate it because I did not see Tubes in my course. Thanks again'

caissie90 said:

Okay thanks so much, I really appreciate it because I did not see Tubes in my course. Thanks again'

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I should think not

I was at college in 1972, and valves weren't been taught even back then.

We did actually do valves one afternoon - but more by accident than anything else. The usual lecturer was away for some reason, and his replacement did valves

Loss of output from an RF amplifier could be due to several things other than gas in the valve (toob)

Loss of emission from the cathode, either due to age or low heater voltage.

Low screen grid volts.

Low anode volts.

If there is gas in the valve envelope, it is often possible to see a blue glow between the anode and cathode when the valve is operating.

As mentioned in another post, if the glass envelope shows a milky white appearance on the inside, a seal has failed and there is a significant amount of air in there - the valve is scrap!

JimB