Is rectifier tube arcing a problem?

BTW, it seems the arcing can developed over time.  Both the Svetlana 5C3S started arcing after several weeks.  Both the RCA arc right the way.  I just got the TungSol.  One of them started arcing after a couple hrs of use.  The other one doesn’t arc (yet).  
 

I also read I should wait 2 or 3 mins before restarting the amp.  I swap cables and restart in less than a min.  Can this be a reason of causing the arcing?  Once the tube starts to arc, it arc every time afterwards.  

My first tube amp was a Cary SLA 70 which used a pair of CV729s/5R4GYs/CV378s and I remember when one of them started doing something weird (this was almost 30 years ago, but maybe it arced or glowed or flashed, I don’t remember precisely) and when I called Cray (this was when Cray offered much better customer service than they do now) I was told that it wasn’t anything to be afraid of, but that the tube was probably on its way out & I should probably keep one on hand.

Since that time, I did have a couple of rectifier tubes fail with a flash or an arc on startup, and that would blow the AC power fuse, but no other damage.

I am not an electrician or a tech so this is not advice, only my experience with a somewhat similar situation.

Can this be a reason of causing the arcing? 

No.

as long as your amp is fused correctly you shouldn‘t suffer catastrophic consequences due to rectifier failure. The fact that they are arcing does tend to suggest that they are on the way out. Sounds like the Tung Sol you bought was a dud. All assuming your tech is competent.

The rectifiers you note are not "slow start" and I would wait a minimum of 5+++ minutes (depends upon the circuit design) before re-powering the amp.

The power supply caps need to drain stored energy in order for the circuit/design to withstand the inrush current of re-powering the unit.

This said, arcing tubes are not a good thing.

DeKay

psu. caps should have bleeder resistor that only takes few seconds to lower charge

Should...

Not the case with my Audion and Bottlehead amps (I’ve blown fuses on both).

Also had the crap shocked out of me by various vintage amps that had been powered down for 8-24 hours (just for fun - because I knew better;-).

Better safe than sorry, plus what’s the hurry?

DeKay

Thanks for the reply.  Actually, my question is, do I need to buy another TungSol.  

And if I buy another one, is there a way to prevent it from arcing.  For example, wait a bit longer (5+ mins) before restarting.  Or this arcing is caused by the design of the amp?  Or due to incompatibility?  I read another thread about Elrog 300B failing on Coincident Frankenstein 300B, while EML 300B works just fine.  But since the TungSol are not that expensive, I think I will try one more time.  

Where are you buying your tubes from?

Check with your manufacturer exactly which type of rectifier you amp is designed for.

Brands don’t matter.

Maybe you have a manual?

The technician said that its OK to continue using rectifiers that are arcing? Is this the wild west?

Throw those tubes out and get new ones of the specified type, and if it keeps arcing what would your own assessment be?

get new ones of the specified type

5U4‘s are definitely on the upper spectrum of the ruggedness stakes for commercial rectifiers, so I don‘t think they are the problem..

A dirty tube socket may be your problem.

@gte357s 

One thing that can cause rectifier tubes to arc is too much capacitance at the input of the power supply. What amp are you using?

I've seen a lot more arcing failure with Russian made rectifier tubes. Depending on the amp you may have to stick with NOS tubes to get reliable service.

In some cases its possible to run solid state rectifiers. A bit of attention has to be paid to the amp circuit since doing so will raise the B+ voltage which may not be a good thing. But if you have good rectifiers (like HEXFREDs) they can be nice and quiet. Amps that use solid state rectifiers usually sound more neutral on account of less power supply sag when more power is required. Tube rectifiers are certainly romantic but other than noise they really don't have a place in a hifi power amp; they are great for blues guitar though.

It is a Audio Note Kit Interstage 300B monoblock.   Here is the assembling instructions menu.  I have it built by someone and have another technician to go through it, so, I believe it is not a built issue.  It is design for 5U4G.  However, the company sells the kit, and doesn’t have much detail about the design.  Also, they make their own C-Core transformer.  I read that one possible reason is the transformer is “too good” and has a much lower resistance than others?  Anyway, I think this is more a compatibility issue.  
Any suggestion is appreciated, like adding resistor or capacitor somewhere.

Instruction manual

A 5U4 should handle that load just fine. I think you've had bad luck with tubes... Is this happening in both channels? Do both channels sound the same?

If this is "only" happening when you power down one of the amps and then restart it within a very short period of time, just wait longer (5-10 minutes).

If it happening @ other times, then it needs to be looked into.

I would not reuse a rectifier that I observed arcing, but I've always had plenty of spares.

I found a schematic of the power supply section @ the Steve Hoffman Forums, but could not link it.

I Googled your amp + rectifier and it came up on the first page of hits.

DeKay

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.

Short answer is Yes, the arcing happens in both channel.  
 

for the 5C3S, at the beginning, I don’t think they arc.  Then after a month, both tubes arc.  For the RCA, they both arc right the way.  For the TungSol, so far, only one arc.  And when I switch the tube, same behaviour.  
 

maybe as you say I have bad luck, but the percentage seems too high, 50% or 3 out of 6 pairs arc.  That’s why I think could it be a me doing something wrong which cause this.  But there is only one button, I can’t think of anything except I didn’t wait long enough when restarting the amp.  
 

is it very common for 5U4G to arc?  I only have two amps that use 5U4G, and I didn’t roll the tube in my previous amp.  

@atmasphere and others

BTW, is there other rectifier can be used for this amp?  Such as a soft start 5AR4?

From what I gathered surfing the WWW 5U4G is the only rectifier recommended for your amps in stock form.

I've only come across them with guitar amps of which a few local guys preferred the 5U4G (over the GB version).

This is of course apples to oranges as far as your AN's go.

Try and find the thread (threads) @ the Steve Hoffman Forums pertaining to your specific amps as there were gobs of posts/pics/info that I did not read through in detail (including power supply/slow start mods).

As they are kits I doubt that well thought out modifications would decrease their resale value down the road.

From the few pics I viewed they looked easy to work on (unlike my old Audion 300B amp that resembled the innards of my TV7 tube tester;-).

I would start by confirming that your units are actually built to the original AN specifications and then go from there.

Yes, it's possible that your old production tubes are duds as I've always found rectifier tubes difficult to test with my old TV7/Hickok 539 testers in that they always tested good until they didn't (same thing with 12ax7 tubes on the same testers).

DeKay

@dekay 

I found that thread at the Steve Hoffman Forum.  Honestly, what being explained there is beyond my technical knowledge.  I like the idea about adding a soft start, but I need to take it to my tech to do that.  But do you think adding a soft start will eliminate the arcing?

I have been running tube gear.. at first a couple components… more and more… now all tube. I have never had a piece of equipment that arched. If the second tube arched I would get rid of it. Maybe this Is just me, I have about 50 tubes in the two systems I use constantly, and no arching… never.

Some/most of it is beyond my tech knowledge as well, but I do understand the basics of it.

I've relied upon techs/friends to update my tube gear in the past and I gleaned a tiny bit of understanding from doing so

This would be good info for your local repair person to read/consider.

Until 10 years ago I did not know what the tiny transformers a friend installed on my Dynaco ST70's, being used as mono blocks, were for (power supply chokes).

He performed the modifications in 1979, and said that the stock mono/stereo switch/circuit was a bad joke.

The improvement in sound quality from the mod was not a slight one.

DeKay

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.

@imhififan thank you. This seems something small and won’t change the sound.

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

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.

Finally, some real help for this problem. Thanks for helping to solve problems that the wannabees simply cannot.

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

Yes.

I agree something is up in the amps- the 5U4 should easily handle the load all on its own. It sounds like something is causing an excessive load during warmup, which is a classic symptom of the filter capacitance at the output of the rectifier being too large. Both doing the same thing is not coincidence! What is the capacitor value at the output of the rectifier tube? There is no schematic in the manual...

@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?

And the amp section schematic

If it is, that 220uF value seems really high to me! An SET is a class A device so the load on the power supply is constant. If you cut all those values in half I think you’ll find that the rectifiers hold up just fine.

This appears to me to be a classic case of too much filtering (the filter caps need to have a time constant lower than that of the amp and in an SET that isn’t hard...)- when the amp is off, the voltages in the power supply are drained off. When you turn the amp on, as the rectifier warms up current flows to charge the caps. There is a certain point on the exponential charging curve (this is the charging curve of any capacitor) where the maximum current of the 5U4 is exceeded. In due time the tube, which is otherwise pretty forgiving, arcs. If I am right about this the arcing happens as the amp is warming up.

+1 atmashpere too. I'm not a tube guy, but atmasphere has a point about the caps being too large. I ran into this problem while servicing a Sony V-fet amp. Both of you guys know your stuff. 

@atmasphere @imhififan 

Yes, mine also has 220uF capacitor, and yes, the arcing only happens when the amp is turn on. After than, and if the fuse doesn’t blow, the amps sound fine.

here is the wiring diagram of the capacitor and resistors.

Wiring Diagram

Besides adding diodes, is there other recommended options about changing capacitors or resistors?  But I guess changing capacitors and resistors will have a bigger impact to the sound, right?

Yes, mine also has 220uF capacitor, and yes, the arcing only happens when the amp is turn on. After than, and if the fuse doesn’t blow, the amps sound fine.

Nailed it!

I would replace the capacitors after the choke with 50uF devices rather than the 220uf units. If anything you might hear a slight improvement since the smaller cap might perform better at high frequencies, if you install ones of similar quality. If you do this there may be no need to install the solid state rectifiers, although the caps in succeeding legs of the power supply are gross overkill (contributing to rectifier failure) as well. You could run 1/5th the value and be fine.

@atmasphere

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?

@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.

fiesta:

"Finally, some real help for this problem. Thanks for helping to solve problems that the wannabees simply cannot."

You have contributed nothing useful to this thread (instead you have performed as a cheerleader with your +/- opinion of those who have contributed useful info/avenues of thought).

DeKay

@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.

@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?

@gte357s 

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?

A few years back I built an ANK Kit-1 300b stereo power amp.

The rectifier tube arked upon power up. I contacted ANK’s builder at the time and he suggest trying a new EH 5u4gb. Told me to buy two or three cause one might ark even new, so throw it away (and any other rectifier tube that arks at startup).

I bought three and only one arked at startup. I marked it and some others but never threw them away lol.

It’s been working fine ever since although I don’t use it constantly.

I did notice shortly after that ANK started including the EH 5u4gb with their kits. But that was then.

Say from 220uf to 100 instead of going all the way down to 47uf.

@fiesta75 

You work out the timing constants. The amp likely does not have a timing constant in the audio circuit that is less than 5Hz since the output transformer won't go down anywhere near that. So it will not be able to modulate the power supply if the power supply has timing constants of below 5Hz. 220uF is way overkill; I don't know of a tube rectifier that will survive that.

Not only that and more importantly, the load on the power supply is constant owing to the circuit is class A- it draws the same power at idle as it does at full power. This makes the power supply requirements a lot simpler. That is why 47uf will be fine. I have an amp that makes the same power and it has only 30uF in this position. The power supply voltage dominates the equation when you're talking about energy storage to do work (like driving a loudspeaker). The formula is:

Work = 1/2 (C x Vsquared)

That is why tube amps don't need nearly the power supply capacitance as solid state amps do.

atmasphere - Thanks for taking the time to educate, me anyway. I have always appreciated and respect the knowledge you and imhififan have brought to the AG forum.👍

That’s my amp on the Steve Hoffman forum. I haven’t had any issues with arcing but as you can see in my thread, I used a soft start before the mains transformer, and also have a lot less capacitance hanging on the output since I used a Maida regulator for the driver tube. 
 

Hope you come up with a good solution.