There are a couple of options available:
1. Fitting semeiconductor technology (which has made some huge advances in regulation since the 50's) offers one solution, it provides a simple linear regulated supply with a very low noise heater voltage and slow rise time during switch on. These linear supplies are capable of providing 5 amps - enough current to comfortably run eight 6386 tubes.
2. Fitting a replacement CVT transformer for the old Sola transformer is now available. Your new transformer has been designed specifically for correct regulation with a 2.8amp load when all eight 6386 tubes are running. The CVT is supplied with a standard capacitor and has been developed for US and EUK applications. It will run at 120vac or 240vac with 60hz or 50hz operation.
The graph below shows a simplified version of a magnetization curve to demonstrate the functional concept of this transformer. In the saturation region of the curve (red), a large change in input voltage results in a small change in output voltage.