Is rectifier tube arcing a problem?

@imhififan good point.  I also have considered those points, especially I am not that technically inclined.  

@imhififan 

also to make sure, the diodes should be added at pin 4 and 6 as shown in the second diagram above, with the line on the diode closer to the pin, am I correct?

Yes, I found a picture on the web that show the details

You can solder the diodes on the socket and move the wires accordingly. you can use 1N4007 diode, but UF4007 is better (lower noise) and just cost a few cents more.

BTW, if you interested add on a NTC soft start device please let me know, it just cost less than $20 in parts. Since it will be bypassed in 10 second after power up, it will not affect sound quality. 

@imhififan 

wow, thank you very much, that’s a very clear diagram, now I won’t be worried of making a mistake.

yes, I always want to learn more.  Does the soft start and diode compliment to each other, or I only need to pick one to do?

Just like seat belt and air bags, more protection is better, IMO.

However, some might think that put a silicon diode in a tube power supply is a sin! If that’s the case, you can install soft start alone.

The soft start device is a CL-70 NTC thermistor in series between the fuse and the "hot" wire to power transformer primary winding. 10 seconds after power up, a time delay relay which connect to the power transformer 5V winding will turn on and bypass the thermistor. Both items are available on Amazon.com

CL-70 thermistor

Timer Relay Switch Module

Please bear in mind you also have a 3rd option:- capacitor mods.