Transformer open circuit output voltage and current limiting resistor at gate of Triac
One thing that jumps out is that you are using a transistor opto-isolator to turn on the triac. This won't end well because


It could, at best, only turn on when the collector is positive so it would only switch on positive half-cycles of the transformer output.
The LTV-8X7 datasheet, section 4 gives absolute maximum ratings as


Collector - Emitter Voltage, VCEO: 35 V
Emitter - Collector Voltage, VECO: 6 V 



The first parameter means it will be destroyed by your 50 V supply on the first positive half-cycle it sees.

The second means that it will be destroyed on the first negative half-cycle it sees.

Either way you're doomed.



What you need is an opto-triac. 



Figure 1. A sample opto-triac controlled circuit (for mains voltage). Source: Triac Switching circuit with Optocoupler.

You may need to modify the circuit a little to work with your lower transformer voltages.

For more reading on the matter have a look at my article Opto-triacs, solid-state relays (SSR), zero-cross and how they work which may be of help. You may decide that a zero-cross opto-triac means that you don't need to monitor the zero-cross of the transformer.