[Dracule1]

I wanted to try out a different rectifier tube on my tube output stage of my DAC.  I decided to try out the Sophia Electric Princess 274B.  I contacted my DAC designer and asked if this tube is compatible because of the warning Sophia Electric has on their website:

"Note(*): 274B is direct heating tube. The indirect heating 5AR4 may turn on slower, however, people who buy 274B is for its superior sonic performance. To fully utilize its benefits, it may requires first stage capacitor (capacitor input) to be less than 10MFD, or would shorten the 274B life span (very much like smoking is bad for your health). It would not be a problem with common choke input. Please consult to your amplifier maker or refer to a tube manual. This 274B is an update of the NOS WE 274B, so both share with same characters. In 1930s, WE 274B was the rectifier choice for 300B single ended theater amplifier in the US."

My DAC designer said it's fine because the DAC has choke input.  So I got the rigid plate 274B (because of its higher current capility) and installed it.  As soon as I turned the power on, sparks were flying inside the lower portion of the tube which also was buzzing pretty loudly.  I turned the power off after 3 seconds of seeing this display in amazement.  I contacted Sophia Electric (SE) who said they tested the tube in working order before sending it off to me.

So what do you think happened?  SE offered a refund but wouldn't send me a replacement tube -- probably because they think the same thing will a happen implying my DAC was not designed for this tube despite what the designer said.  I was really looking to forward to seeing what this tube can do.

[mitch stl]

Hard to say what happened without knowing the circuit design, but I do have one question: why would you need a higher current capability on a DAC? Their output is voltage into a typically high impedance line level load that draws very little current. I would think the original rectifier tube already had far more current capability than was needed for the circuit.

[richidoo]

I've seen arcs in rectifier tubes before, but only for 1/4 second and then they're fine (5U4). My tech said it is more likely when they are already hot, as when doing tube swaps, etc.

[Dracule1]

Dunno about the circuit too much but my DAC has huge capacitor bank after the choke (I counted thirty seven 22,000 pF caps).  I talked to the designer who thought the inrush of current from the capacitor bank is too much for this tube.

[Dracule1]

Richido, funny you mention that about the 5U4. It's what I use now, and I get arching for about 1/2 second when I turn on the DAC.   It didn't do it for the first 10 hours, but now I get a little light show every time I turn it on.  I hope it's not slowly damaging the tube.

[Steve]

Dunno about the circuit too much but my DAC has huge capacitor bank after the choke (I counted thirty seven 22,000 pF caps).  I talked to the designer who thought the inrush of current from the capacitor bank is too much for this tube.

Hi Dracula1,

I agree it sounds like the problem is too large repetitive peak plate current in nature. Although a choke input, the capacitor bank is simply so large (the choke may also be too small) that the repetitive peak plate current is exceeded, and probably time wise as well.

When the power is turned on, a large current inrush occurs as the capacitors are initially 0 volts, basically a dead short. The inductor also has minimal inductance as well, which allows more current to charge the capacitors. We are talking milliseconds of time for each charge cycle.

Unfortunately, the capacitor bank is so huge that the initial peak current charges the caps insufficiently. When the next charge cycle occurs, more peak current flows. The rectifier tube has a rating of how many huge cycles and the time frame involved. Evidently one or both of the ratings are exceeded.

Lastly, the 5ar4 is an indirectly heated tube, so a couple of differences occur.

1) The tube is simply heavier duty.

2) Being indirectly heated, the charge of the capacitor bank occurs gradually, over several/many seconds. This puts less stress on the tube.

This is somewhat simplistic explanation but I hope I am clear enough for understanding. 

Cheers.

[Quiet Earth]

After the spark and light show occurred, did you try to turn the DAC back on again? I was just wondering if maybe once the caps had charged up a little, the next turn on cycle might be a success. Or, you could run the DAC with the original rectifier for a few hours to get the caps all good and charged up, turn it off, and then immediately install and turn on the 274B (which you may have already sent back  ).

Which brings me to another question ( because I am still learning too  ). If you have sparking issues with the original rectifier that the designer recommends, why would you ever turn the unit off? I would leave it running 24/7 unless I went out of town for an extended amount of time. Rectifiers and line level tubes can run for years and they may even last longer than being switched on and off - especially in your case.

BTW, can you tell us the name of your DAC? Inquiring minds want to know. . . .

[Dracule1]

Thanks Steve,  I understand the problem better now.  It can be difficult understand the designer of my DAC because English is not his first language.  So are you saying the  5AR4 is a better choice for my DAC than the 5U4 or 274B? 

[Dracule1]

Swapping the stock rectifier tube for the 274B immediately after the DAC is fully charged up is NOT recommended by the designer.  He specifically states the DAC needs to be turned off for at least 5 minutes before any tube is swapped.  So I can't turn the DAC off and immediately change the tube.  Don't want to risk damaging the DAC or voiding warranty.

[Quiet Earth]

 

my DAC has huge capacitor bank after the choke (I counted thirty seven 22,000 pF caps).

Just noticed that you wrote picofarads, not microfarads.  22000pF = 0.022uF.  If they are all in parallel after the choke then 0.022uF X 37 = 0.814uF. That should not be a problem after the choke should it? I would try to identify how much capacitance is directly after the rectifier now. Most 5U4 rectifiers like to see less than 40uF. I don't know about the 5AR4 or 274B, but I'll bet they are similarly low. Maybe someone can chime in with that info.

In any event, I think Steve summed it up the best and I would just stick with the rectifier type that came with your DAC until you can get more specific information on the design, or maybe even a schematic. It is interesting that the DAC uses such a large rectifier in the first place, because a 6X5 is usually big enough to run a couple of line stage tubes.

BTW, direct from sophiaelectric.com they state : "The maximum current capability for Sophia Electric 274B mesh plate tube is 90ma, suitable for tube pre-amp and mono-block 45 single ended and 2A3 mono-block single ended.

We would recommend Sophia Electric 274B rigid plate tube for 300B amplifiers. The maximum current capability for this 274B rigid plate tube is 160ma."

That doesn't solve the capacitance issue, but at least you know how much current they are rated for.

[Dracule1]

That doesn't sound right.  These are electrolytic caps a little bigger than the size of a D cell battery.  I think they're 22 microF each.

[Steve]

Thanks Steve,  I understand the problem better now.  It can be difficult understand the designer of my DAC because English is not his first language.  So are you saying the  5AR4 is a better choice for my DAC than the 5U4 or 274B?

I have to admit I also just saw the 22,000 and figured 22,000uf. I also just saw on Ak forum that someone is using 11,000uf (if I remember correctly) in his CD player. Anyway, from your description it sounds like the filter capacitance is fairly high.

The 5ar4 rectifier is indirectly heated. As the cathode slowly warms up, current starts to gradually flow and increases over many seconds (maybe 12 seconds or more) the filter capacitors are charging. Of course the peak surge is lessened. I would still like to see the filter capacitors limited to a certain UF to be on the safe side. But yes, I would venture the 5AR4 is a better choice in this situation.

Cheers.

Steve

[Dracule1]

Hi Steve, I take it the 5U4 tube is directly heated?

[Steve]

Hi Steve, I take it the 5U4 tube is directly heated?

You are correct. The 5U4 family is directly heated cathode.

To general public, I did a slight mod in my previous post which I underlined. Hope it is easier to understand.

Cheers.

[Dracule1]

Thanks again Steve.  Not sure what to do about the stock 5U4 I'm using now.  It arcs for about 1/2 second every time I power up the DAC and then it's fine.  But my system sounds more shrill (higher freq is more gritty and tilted up), less dimensional, and bass level and impact seems less since this started to happen.  Not sure if I can blame it on the rectifier tube and arcing. 

[Steve]

Thanks again Steve.  Not sure what to do about the stock 5U4 I'm using now.  It arcs for about 1/2 second every time I power up the DAC and then it's fine.  But my system sounds more shrill (higher freq is more gritty and tilted up), less dimensional, and bass level and impact seems less since this started to happen.  Not sure if I can blame it on the rectifier tube and arcing.

I would remove the 5U4 type as soon as possible. It is already damaged.

Cheers.

[Dracule1]

Sh*#t!   Do you think it may have damaged my DAC or other components downstream?

[Steve]

Sh*#t!   Do you think it may have damaged my DAC or other components downstream?

I doubt it hurt the dac.

[Dracule1]

Steve, do you have any 5AR4 tubes you would recommend?  I'm looking for dynamic open sound, extended bass, and smooth detailed sound. 

[Steve]

Steve, do you have any 5AR4 tubes you would recommend?  I'm looking for dynamic open sound, extended bass, and smooth detailed sound.

I sure don't. Sorry Dracule1.

Steve