Tube Sparking

[lawbadman]

Hello all,
I had a frightening experience a few days ago when I was testing my aikido circuit. I switched on my power supply and was waiting for the tubes to warm up and the voltage to rise. All of a sudden I saw some big blue sparks in my rectifier tube. I immediately turned it off. My brother said that he heard some scary popping and buzzing from the computer speakers.


I have a feeling that I might have too much capacitance. What also confuses me is that this is not the first time that I have powered up the circuit (although not for long periods).

Here is the configuration:
power transformer is 300-0-300 100mA

5Y3GT--470R--40uf--2490R--220uf--11700R--320uf--Aikido circuit



Does anybody have any idea what might have caused the sparking? Can the tube still be used?

[anatech]

Hi lawbadman,
Your filter caps are new, not NOS - right? Also, your 5Y3 is only rated for 20uF assuming a 50 ohm impedance in the AC supply (from RC-27).

The tube may still be okay, but lose the 40 uF input cap. 10~20 uF should be fine.

-Chris

[ray_moth]

I agree, that's a typical symptom of using too high as smoothing cap for the rectifier concerned. The 470 ohm might protect the rectifier from the 40uF smoothing cap to some extent but 16uF would be safer.

[lawbadman]

So if I replace the 40uf with a 10uf I will be safe?
I have had this configuration (excluding the 11.7K and the 320uf) for a long time now with different circuits and I have never had a problem. Is this the tube telling me it has had enough?

[anatech]

Hi lawbadman,

Is this the tube telling me it has had enough?

I'd say so. Replace the first cap with a 10uF. Your tube may like you again. Certainly other 5Y3's won't hide in the darkness when you come looking.

-Chris

[tubelab.com]

This is the classic symptom of too much capacitance. I have one of my TubelabSE amplifiers that uses a 100uF input cap on a 5AR4. Yes this is too much. It was the only cap that I could find when I built this one, and I am too busy to change it. About once a month there is fireworks in the 5AR4 when I turn it on. The tube is an old RCA that I took out of another amp. I look at this as a controlled experiment to see how long it takes it to blow up. It has been in there for about almost two years, and hasn't fried yet. DON"T do this with an expensive tube, you know that it will blow up the first time you turn it on. This amp also has an undersized power transformer that probably helps the rectifier survive.

[anatech]

Hi tubelab,

This amp also has an undersized power transformer that probably helps the rectifier survive.

Yup, I'd say so. But then again, you know what's going on.

-Chris

[Nature B0y]

From RC-30, the max. hot-switching transient current for the 5Y3 is 2.5 amps per plate. The 470R resistor limits hot-switching current to less than one amp in this circuit, even if the capacitor is discharged as it would be at power up. As such, there shouldn't be any problem running a first capacitor of 40uF. As an aside, RC-14 specifically talks about using a 40uF first capacitor with a total effective plate-supply impedence of a meager 10R per plate, suggesting such a configuration is ok. I've been running a 5Y3 in my stereo SE 6V6 amp with 150R + 100uF for years with no problems. I suppose I should admit that amp has a somewhat anemic power transformer as well, but I don't think it matters all that much in this case...

Of course, this *particular* tube seems to be having problems with the capacitance of the circuit, but I don't believe that the circuit is at fault. Dropping the value of the first capacitor will probably drop your B+ voltage a bit, which you may or may not care about. If you want to continue using *that* 5Y3 without otherwise altering the circuit, you could add a switch in series with the 470R (which you would leave off until the circuit has equalized) and another, much larger, resistor (say 10K) from the 5Y3 to the 40uF cap. My personal opinion, however, is that you should just get another 5Y3, and leave the circuit alone.

[lawbadman]

OK, here is an update on my power supply.

Today I finished building my 12b4 pre amp so I needed to test it. I replaced the 40uF with a 22uF cap, and took off a few other stuff which left:
5Y3GT----470ohm------22uF------3490ohm------220uf-----3300----82uF

I connected everything together and fear fully flipped the switch for a smoke test. Well everthing started out great, but then something happened which was quite unexpected. When the voltage (at the 82uF cap) got to about 190volts @ 35mA the transformer started buzzing, I turned it off as soon as I heard it. Good news is that there was no smoke.

So the question now is what caused the buzzing in the trafo?
I measured the secondarys and they measured 83 and 75 ohm, so I dont think that it burnt out. The weird thing about the trafo is that it is always hot (but not too hot that I can't touch it) even at low loads. The trafo is a Hammond 272BX by the way.

This power supply is getting to me now I figure I will just buy a new trafo and move to a nice SS power supply.

Any thoughts as to what might have happened?
Any suggestions are welcome.



Thanks all
Lawrence

[lndm]

There's buzzing and there's buzzing. With a new filter setup, the current waveform will be different. Maybe the peaks are causing the buzz but it may still be ok. (it isn't a hard thumping buzz is it?)

As far as the earlier diagnosis, a rectifier that is worn will arc. Lowering the capacitance may help because the rectifier sees easier going, but such a rectifier is still worn. It just has more trouble giving out the current.

Could you look at the ps output on an oscilloscope?