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Gen 3+1 tubes are hybrid tubes.
One of the issues with high gain tubes ( eg, multiple MCPs or high-voltage over the MCP ) is that they introduce noise, and can have a higher EBI level. Hybrids are a special class of cascade tube that uses either a Gen2 or Gen3 tube for one stage and a Gen1 tube for the other. Some have Gen3 tubes at the front and Gen1 at the back. Others have a diode-tube at the front and a Gen2 at the back. There are possibly also GaAs Gen1 to Gen3 tubes out there, and I know of at least one record of a Gen1 diode tube with a GaAs photocathode.
The hybrid tubes were developed as the "ultimate" form of tube, and had special power supplies and very high gains, often in excess of 100,000x and with S/N way above normal tubes. They can produce very low noise images that lend themselves to NV photography with longer exposures.
The last I heard of hybrid tubes was around the turn of the century - and they were Gen3+1 and were used in some of th Stinger Night Sights, and used a 25mm tube.
Generally, they have fantastic resolution, very low noise, high signal and can outperform most Gen3 tubes.
However, they don't easily support features like autogating in some configurations, and unless a diode-tube is used, there will be some pincushion distortion. Power supplies are quite expensive to make for them, and although they are complicated, they were made in both 25mm and 18mm tube formats, so they could be retrofitted to existing housings.
They make excellent astronomy tubes, and the information on them is a little sketchy - something that was only realized when a "known" model of 3+1 tube failed, but was found to be a Gen3 tube with an extended fiber plate.
A modern high-performance PVS-14 would give similar results to a Gen3+1 - but they are still collectors items.
Regards
David.
Yes, it was quite sad to hear... The owner of the tube had been helping me classify each of the model numbers for the PVS-4 Wikipedia entry and all the information that came out of the factory pegged it at Gen3+1 for that model number, but they had a tube fail and when they took it apart, they discovered the long FO tube and informed me that the records I had were inaccurate - So it looks like some of the SNS records are incorrect. Weight used to be how we classified them, but even this does not help with certainly anymore. It makes finding these tubes even harder now.
Murtis - The issue is that when an electron is amplified at the MCP, the MCP add's noise, and other factors such as the MCP-PC voltage also add's noise ( and noise that get's amplified ) -
MCPs contribute significant amounts of noise to an image when turned up to very high levels, and this is why only single-layer MCPs are used in modern military tubes and multi-MCP tubes are purely for research purposes.
A cascade is just another form of amplification, but the cascade does not add noise in the same way that a MCP does - and the noise from the subsequent image tubes is a hundred times lower than natural noise int he primary photocathode, so is negligible - as a result, cascades have very low noise for the amount of gain they produce, although they do have other losses at each stage.
This is how a 40 year of cascade tube such as you can get for a few hundred dollars ( see the DIY cascade thread ) can perform similarly to an older Gen3 tube, and has more gain, typically, than a PVS-14.
A hybrid tube takes advantage of this and uses a second electro-optical amplifier to amplify the image from the prior tube, rather than use a MCP which is in-tube but adds additional noise.
However...
At some point, a Gen2 tube photocathode ( or a Gen1 cascade photocathode ) lacks sensitivity sufficient that the image, even if amplified over 100,000x , will start to break up - This is the only area a Gen3 tube will show an image where a cascade will start to fail - and of course, the Gen3 single-stage benefits from a higher resolution at lower light levels also.
Usually this is under overcast starlight conditions - where the light levels matter enough that the PC sensitivity is the main limiting factor. ( Under about NL6 - night level 6 ).
Also, modern tubes have much lower MCP noise levels than you might imagine from reading the above - this is one area in which technology keeps improving. Photonis have phenomenal low-noise MCPs in their tubes, which allow tubes with around half of the sensitivity of Gen3 tubes to compete in terms of functionality, so you can read between the lines and guess that their MCPs probably generate about half the noise of US based MCPs. After all, with the gain levels and noise levels being equivalent, they have to make up for the shortfall in input signal somewhere - and in their older tubes, it was with higher low-noise amplification. In their newer tubes, they've done something else with the photocathode also -
Regards
David