[Radio Wrangler]

If you just stick a bridge rectifier in the heater supply, you A) lose some voltage in the diodes and B) still have a waveform with a strong varying component... 100Hz ripple, so you likely still have hum but double the frequency.

You need to smooth and filter the rectified output of a higher voltage secondary. So it's common to add regulation.

It quickly becomes a big project.

BUT other people have 5-10s working fine on AC heating. So something is wrong. You need to find it and fix it. It's likely less work than going DC heating.

Switching off the heaters is a very neat test and saves what might be an awful lot of work which could be for nothing.

David