Did My Output Transformer Go Bad - Dyna/VTA 70

I've been running a pair of Dynaco 70s with the VTA boards in a vertical bi-amp configuration for about 15 months now  (love the sound BTW).  Today, I heard a slight fizzing sound from the left side for a second or two, then it seemed to settle down and play ok.  Within a few minutes, the left side was notably distorting, and eventually stopped making sound.  I've narrowed the issue to one channel of the left side amp, and am wondering if I've lost an output transformer.  I've switched tubes around, and the issue stays with that same channel.  I've removed the tubes from the bad channel, and reconnected all the speakers to the other working channel of the suspected "bad" amp, and am getting good sound from it.  

Any suggestions for what might be wrong, or logical next steps to help trace the problem?

I just had a similar experience and I had to order an output transformer ( I actually ordered 2).  Triode electronics has them ( http://www.triodeelectronics.com/noname.html ) or I found them overseas in Italy ( https://www.laboratoriomusicale.net/en/output-transformer-dynaco-a470-for-st-70-4300-ct-4-8-ohm-replica-new ).  If you get them from Italy then you will have to do a bank transfer once you receive the invoice.  Mine are Processing now.

In another post Bob mentioned that the left channel transformer is more susceptible to failure because of the heat from the rectifier tube.  In my case there was a small hum that started then got louder (only in the left channel) until it was unbearable.  It would increase with the volume.  You could hear the hum coming from the transformer and out of the left channel very loudly until my rectifier gave out in a spectacular show of lightning in a bottle.

The output transformer is usually the last thing to go out. There are over a dozen resistors and capacitors in the circuit of just one channel that are more likely to go out before the transformer does. Transformer problems usually start with cyclic humming. What you describe sounds more like a capacitor or resistor.

With the amp completely disconnected, get your nose close to the board (without touching it!!) and take a big whiff. If you smell burning, it's most likely a resistor or smaller capacitor. Start looking for burn marks. If you don't smell or see anything, it could be a larger capacitor.

If you have the tools and experience in hunting down voltages on a circuit board, that'll be your next task.

MechEngVic wrote:The output transformer is usually the last thing to go out.

Totally agree with MechEngVic, it is HIGHLY likely that some other component or tube has gone bad. Or a bad solder connection. If in doubt, Ohm out the output transformer.

Everyone's input is much appreciated.  It'll likely be weeks before I get this task near the top of the list, but will definitely be checking the circuit components for issues.