[TV MAN]

6V6's are beefier than 6Y6's are. They can use higher voltages & currents, but are also more pricey to buy. The Russian versions work well and can be had reasonable. A 5AR4 can sub for a 5U4, (it will raise your B+ so be careful) are smaller and use less heater current, but are more expensive to buy. A 5U4 is filament hungry and might be stressing your power trans. A 5Y3 will plug in but IS lighter. It might red plate (fuse). You can check your current draw across the resistor between your main filter caps by measuring your voltage and using ohms law. Then you can look up the specs online and see if it's within tollerance. Also, has this been recapped? If the coupler caps to the 6V6's is leaky, they will be drawing more current than what they should, causing things to run hot. This might be why you are finding heavier tubes in place of the originals - to "fix" an over current situation instead of fixing the problem. You should have either a negative voltage or 0 on the control grid of your output tubes. A + voltage means you have leaky coupling cap. I'd check this 1st. It might be the root of your problem.

Edit: Also a leaky or shorted cathode bypass cap (electrolytic) can mess up your bias on your 6V6s.