Uploaded December 2025 | Updated September 2026, 3 weeks ago
Experiment with an old valve (vacuum tube) 4B/603E - pulse modulator tetrode for radar. This tube had gone gassy and was just an ornament. Decided to open the pinch seal and attach a tube to see if I could restore a decent vacuum. Once air has been admitted the oxide coatings on the cathode tend to react and lose their emissive properties.
This valve has 4 indirectly heated cathodes giving a large emissive surface for high current pulse applications. The test circuit was set-up with a working valve, checked working and then substituted with the faulty one. For thermionic emission to work properly as a valve the vacuum needs to be good. Despite using a turbo-molecular pump it proved difficult to achieve an adequate vacuum. Longer pumping times would be required and heating to assist with out-gassing.
The experiment was cut short as the plastic exhaust pipe collapsed due to the heat from the cathodes preventing further pumping. While the vacuum was improving it was still showing discharge emission from the remaining air. This would be preventing proper operation as an amplifier.
A reattempt, managing to reduce the gas discharged, didn’t get the valve into a healthy state. This may be due to the cathode emissive oxides having reacted with the air.
One other reason for this experiment was to check the turbo-molecular pump. It has a small vibration which fortunately only occurs at low speed on spinning down. Letting in air at low-speed assists braking and reduces the vibration duration.
CONTENT:
00:00 - Intro
00:30 - Equipment
00:50 - Test circuit diagram
01:10 – Start vacuum pump
01:25 – Roughing pump on
02:00 – Turbo-molecular pump on
03:00 - Valve testing
03:18 – Issue with test
03:40 – Good tube test
03:55 – More on high vacuum pumps
04:00 - Diffusion pump
04:35 – Turbo-molecular pump
05:56 - Outro
Experiment with an old valve (vacuum tube) 4B/603E - pulse modulator tetrode for radar. This tube had gone gassy and was just an ornament. Decided to open the pinch seal and attach a tube to see if I could restore a decent vacuum. Once air has been admitted the oxide coatings on the cathode tend to react and lose their emissive properties.
This valve has 4 indirectly heated cathodes giving a large emissive surface for high current pulse applications. The test circuit was set-up with a working valve, checked working and then substituted with the faulty one. For thermionic emission to work properly as a valve the vacuum needs to be good. Despite using a turbo-molecular pump it proved difficult to achieve an adequate vacuum. Longer pumping times would be required and heating to assist with out-gassing.
The experiment was cut short as the plastic exhaust pipe collapsed due to the heat from the cathodes preventing further pumping. While the vacuum was improving it was still showing discharge emission from the remaining air. This would be preventing proper operation as an amplifier.
A reattempt, managing to reduce the gas discharged, didn’t get the valve into a healthy state. This may be due to the cathode emissive oxides having reacted with the air.
One other reason for this experiment was to check the turbo-molecular pump. It has a small vibration which fortunately only occurs at low speed on spinning down. Letting in air at low-speed assists braking and reduces the vibration duration.
CONTENT:
00:00 - Intro
00:30 - Equipment
00:50 - Test circuit diagram
01:10 – Start vacuum pump
01:25 – Roughing pump on
02:00 – Turbo-molecular pump on
03:00 - Valve testing
03:18 – Issue with test
03:40 – Good tube test
03:55 – More on high vacuum pumps
04:00 - Diffusion pump
04:35 – Turbo-molecular pump
05:56 - Outro
