Is rectifier tube arcing a problem?

I did some research and couldn’t find a definite answer.  I have an amp that I tried 6 different pairs of 5U4G and 5U4GB. 3 pairs has arcing (RCA 5U4G, TungSol, Svetlana 5C3S) and 3 pairs don’t (RCA 5U4G with hanging filament, EH 5U4GB, Sylvania 5931).  I took the amp to a technician and he checked everything, he can’t find anything wrong.  The problem is, I like the sound of the TungSol and Svetlana which both have arcing.  The technician said it is ok to keep using them, but honestly I am not too comfortable.  But I like their sound.  Is it really ok to keep using the arcing tubes?  Will it damage the amp?




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

CL-70 thermistor


Timer Relay Switch Module

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


@imhififan i am in the the-lesser-the-better camp.  So, I think I will try adding the diodes first.  Thanks for the info though.

thank you very much. A couple more questions:

1) is it able to tell from the diagram if there other rectifier that can be used and handle the current drawn by the 220uF capacitor?

2) is it necessary or recommended to change the other two capacitors? If yes, what values should I use?

3) comparing to adding diodes, in your opinion, which way will yield better sound?


There won't be another tube rectifier that will work better. The problem is that this circuit is designed to expect the voltage drop that exists with the 5U4 at the current at which its operating.

I gave you a range of capacitor values. But to be clear:

replace the 220uF units with 47uF. Replace the 100uF units with 22uF.

Once this is done you'll find additional diodes to be moot. They may also cause commutation noise by interacting with the inductance of the power transformer winding, which can manifest as an annoying low level buzz. If it were me I wouldn't bother and instead work on correcting the problem.


@atmasphere Thanks for the info.  I think so will experiment the diodes approach first, because it is cheap and easy enough to do it myself.  It is also a good learning experience for me.  At the same time, investigate on changing the capacitors.  It seems it is not too difficult to do as well, if the size are the same.

decay - Are you offended? Your multiple comments here were of no value. I was not commenting on you, as I do see that you do make valid contributions occasionally. My comments were meant to encourage imhififan and atmasphere to contribute more often. I have a lot of respect for both of them.

My 2cents - Seems clear that this an inrush current problem, which reducing the capacitor values would help to reduce. It has been my experience that reducing power supply capacitance does usually negatively affect low frequency capabilities. If I were going to reduce the cap values, I would not reduce more than necessary. Say from 220uf to 100 instead of going all the way down to 47uf.  The diode idea was excellent. How about doing both, diodes and just the 220uf caps to start with?