Let's start with the T-26. A number of vehicles received this treatment both before and after the war. A large number of photographs of these tanks can be found here. A number of projects to increase the armour of the T-26 were developed in the late 1930s, but none were implemented until the start of the Winter War. Compared to the T-26's initial armour, the screens were impressively thick: up to 40 mm. The tank could withstand shots from 45 mm guns from 400-500 meters. This additional armour was attached to both the older model 1933 T-26es and newer model 1939 ones by welding and bolts. Later models used only welds to hold the armour. The additional armour of some tanks gave their turret a more polygonal shape, making them more reminiscent of later light tanks, such as the T-70.
While a number of tanks were up-armoured at the factory to server in the Winter War (89 tanks, according to this source), a larger number received a similar treatment at Leningrad, during the siege. Here, the tank's greater mass of 12 tons did not significantly impact the performance, as the ability to resist higher caliber anti-tank artillery was more important than the ability to move quickly or over a great distance. Leningrad and field modifications of the up-armoured T-26 tank can be distinguished by a lack of movable gun mantlet. Since time was of the essence, the additional mantlet armour was stationary, and had holes cut in it for the gun, unlike the pre-war tanks, where the additional armour was attached to the mantlet.
The BT-7 also received new armour. The new screens gave it 50 mm of front armour (hull and turret), 45 mm of turret side armour, and 30 mm of hull side armour. This gave it immunity from 37-45 mm AT guns and 75-76 mm field guns. The mass increased to 20 tons, reducing the maximum speed to 45 kph. The driver's hatch had to be sacrificed. At least one vehicle was built, and took part in the parade in November of 1941.
A BT-5 also received armour screens, including the interesting sloped side armour. Not much information is available on this vehicle, aside from a quote from the order "On strengthening the armour of T-26, BT, and T-28 tanks with armour screens", where 300 T-26es, 350 BT-5s, and 350 BT-7s were to be equipped with additional armour.
Speaking of the T-28, the T-28 with armour screens is also a pretty well known modification. Initially, all T-28s were to get armour screens, but only 111 tanks actually did. Lots of photographs of these tanks are available here.
I have written about T-34s with additional armour before, but a different armouring scheme was also proposed in November of 1942:
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Here is how Lieutenant-Colonel Shahverdyan describes his invention:
"Technical description of modernization of KV and T-34 tank armour
Modernization of a tank's armour achieves the following:
- Strengthens the tank's armour to resist penetrations.
- Increases the chance of a ricochet.
- Protects the tank's turret from jamming.
- Protects the tank's suspension.
- Strengthens the tank's front for ramming."
The Lieutenant-Colonel proposed corrugated armour, 30 mm thick, with the wave's peak 150 mm away from the tank's initial armour.
Another, much larger, up-armouring was planned for the ISU-122.
"To the People's Commissar of Tank Manufacturing, Comrade Malyshev
Self propelled artillery guns ISU-152 and ISU-122, manufactured at the Kirov factory, differ in mass by 1 ton. The ISU-152 weighs 45.5 tons, and the ISU-122 weighs 44.5 tons. Despite the larger mass, the performance of the ISU-152 is identical to the ISU-122.
As such, it is possible to increase the armour of the ISU-122 within the limits of 45.5 tons.
Preliminary calculations suggest that front armour 150 mm thick and a gun mantlet 160 mm thick will not weigh more than 1 additional ton. This front armour will be able to resist 75 and 88 millimeter enemy shells from a distance of 500 meters and higher.
In order to determine the practicality of this solution, and the necessary amount of work connected with re-balancing the D-25S gun with this additional weight, I deem it reasonable for the Kirov factory to complete the blueprints for this SPG by January 15th, 1945.
After completing that task, a prototype ISU-122 with increased hull armour must be built, in order to carry out trials.
Colonel-General of the Tank Forces, Korobkov"
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150 mm at 30 degrees is effectively 173 mm of armour! That's quite an amount, giving you a sizeable chance to resist even 88L/71 shells at nearly point blank range.