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?

 

 

gte357s

Showing 9 responses by imhififan

Add a negative temperature coefficient (NTC) thermistor on the primary side of the power transformer and bypass it with a time delay relay may help.

 

Soft start will help, you also can add an UF4007 to the plate of rectifier tube to removes negative half cycle voltage and it halves the total time of voltage that feeding the arc.

I need to add two diodes per tube, am I correct?

Yes.

BTW, do not reuse the arced tube! Usually once a tube has arced It will continue to arc at that same spot on power up.

 

@atmasphere

This is the power supply schematic at Steve Hoffman Forums, not sure if OP’s kit has the same power transformer (C core) and same component value?

 

@gte357s

IMO, reduce the capacitor value will has a bigger impact in sound quality, those 220uF capacitors are mounted into a 35mm clamp with point to point soldering. You need to find a 50uF cap with similar quality and diameter to fit into those clamps or you need to find a way to mount them into the chassis if the caps has different dimension. And the modification cannot be easily reverted!

OTH, install two UF4007 on the rectifier tube socket is a very simple job ( it takes less than 10 minutes ) and those diodes only cost a few dollars. Its also very easy to reverted.

Those diodes are functioned as a protection device to reduce the chances of rectifier tube arcing and also can prolong the life of those rectifier tube. BTW, the effect in sound is negligible.

My suggestion is go ahead install the diodes first, put in some new rectifier tube and see if it solve the arcing problem. If you don’t like the result, then go for the capacitor mods or install soft start device.

BTW, always wait as least 5 minutes to let the tubes cool down before you switch the amplifiers on again as @dekay suggested.

 

 

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. 

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.

 

Thanks for the update, @gte357s 

You mentioned one of your 5931 dies after adding diodes that lead me to restudy the schematics and found that the peak inverse voltage(PIV) of a single 1N4007/UF4007 is not high enough for the job. If in the future you decide to retry the diode mods please use two 1N4007/UF4007 in series to double the PIV or use a diode with higher PIV rating.