J306, J307 & J308 GM Tube Background Comparison

I got these tubes just to see what they were like. I was a little surprised to learn that they are black due to a sleeve of heat shrink over the tube. I had previously thought they were black due to them being an opaque glass, but after peeling a little of the sleeve off it revealed tin oxide coated clear glass, much like the J305. I accidentally rubbed the markings from the J306 when cleaning it, as I have done on a J305. The markings come off very easy with these tubes.

 

     

Going from top to bottom - J306, J307 then J308.

Specs from Alibaba:

 

The background comparison result. Each tube was left to run for around 30 days, although the first (J306) was short by a few days.

 

I think the graph says it all. This was a simple background test for each tube. I simply threw it on a counter and left if for days and let Radmon log the results. The J308 seems a little underwhelming at around double that of a J305. The J307, despite it's length (it's the longest) was kind of miserable too. I initially expected something around 90-100 cpm. It seems circumference matters with these tubes. That makes sense, larger circumference, larger capture area. The J306 came out on top and gave similar results to a similar sized metal wall SI-22G. Prices were (all including tax and shipping): J306 - £24.59, J307 - £24.59 and J308 - £25.88. The J308 was bought separately, so the cost was slightly higher with shipping. The J306, J307 & J308 are all the same price from the seller, and I have to wonder why. Aliexpress I guess!   Bought from here: https://vi.aliexpress.com/item/1005005407840365.html

They are pretty big (sensitive) tubes. J306 is 200mm long and 19mm diameter. Lots of surface area to catch background 'stuff'. I have an even larger SI-21G (257mm x 18mm) on test and that is ~125 cpm background.