Fireworks in my valves!

[Infinitely Baffled]

Hi. What exactly is going on when you see arcing inside the glass envelope of a valve? The 5U4G rectifier in my 300BSET has developed a tendency to flicker for a moment or two upon switch-on. Then it settles down and works fine. I have tried three others and two of them do the same. Is it most likely the valve, or something gone wrong in the amp itself? Is it serious? Infinitely Baffled.

[Ali Tait]

What's the amp?

[andrew ivimey]

Let me guess, its a Chinese amplifier!

The rectifier can't take the in rush of voltage from the secondary winding of the mains transformer so arcs over and it could be because it is a cheap and crappy 5U4G, or the inrush of voltage is too high because the mains transformer (being Chinese - an assumption I admit) is wound to take 220 volts input and not the 235-40 volts which is what most UK mains is most of the time.

But for 300Bs you need somewhere between 300-400 volts depending on how you design the amplifier. And 5U4gs are very happy at this operating point so, I would guess there is nasty solid state bridge rectification (which will give a peak in rush of 450 - 750volts or so, which for a cold started i.e. no warmed through cathode / heater is a very very harsh way of being treated. If I were that 5U4g I would be very rude to my uncaring owner) If so, the
5U4g, the valve rectifier is just for show - and what a show!

It is bad news for the seriously stressed rectifier because they really don't like this happening and will eventually, well quite soon actually, go out on strike and it is probably bad news for the amplifier as having seriously weakened the rectifier, the rest of the amplifier has to put up with this. The amplifier will continue to (assumption that it is a cheap and cheerful chinese amplifer - could be wrong, I admit but ...) sound less good and less good as a consequence.

What HT peak gets through to the 300Bs through the arcs and when there is no warmed through cathodes either can damage them too (300Bs are not cheap) - it would be interesting to measure what is exactly happening when the 5U4G arcs.

[Richard]

Andrew, cor blimey, if poor Infinitely Baffled wasn't baffled before he certainly will be now. In fact I'll join him as I can't make sense of your middle 2 paragraphs the 5u4 valve rect is flashing, what would that have to do with a ss bridge and why would the rect be a dummy

Over-voltage from an incorrect mains tx possibly or more likely (since it was ok before and new rect valves haven't cured it) a failing first ht cap (after the rect) would be my guess.

Rich

[Ali Tait]

It would seem a good few Chinese amps use SS rectification,but stick a valve rectumfryer in too in the assumption that it makes the amp more sexy and saleable.Assuming we are talking about a Chinese amp here of course!

[andrew ivimey]

okay, ... let's assume the 5U4Gs are the finest ever manufactured and not crummy russian or chinese ones, the like of which I have come across from time to time.

So, my thesis is that the mains transformer is pumping out too may volts to the rectifier anyway because it has been wound for 220 volts. This is a guess, I may be wrong - one in the eye for Andy Pandy then!

Now I have from time to time also come across valve amps from a certain country, that use a bridge rectifier of diodes and then put a 5U4G for example because it looks good. I do not believe this valve rectifier makes the HT any smoother, softer or more manageable for whatever comes next in the amplifier. I could be wrong, I have been wrong before, so maybe that's two in the eye for Android the prophet.

The bridge rectumfriar puts the HT at 1.4 times the AC voltage from the secondary of the mains transformer, depending also in what caps are placed where - This means of course that the mains transformer in the first place can put out fewer HT AC volts - cheepniss! The mighty A.Zwarkwon could be wrong here too. Running HT from a SS bridge rectifier you get immediate wonderful peak in HT far far above the running voltage - hope the caps can take it, phew.

we still don't know anything about this amp because our brother here never told us what kind of amp it was in the first place, which is a shame really.

Now, whether or not all the above or some of it is true, we still have the arcing. Now what is that going to do to the valve rectifier? wreck it.
I am right about this, I've done this and worse to various valves. Hooray for Andy the experimenter!

If that nasty arcing electricity flashing across from cold heater winding (cathode) to anode in the rectumfriar then makes its way out to the amplifier interesting things could happen and I'd like to measure it just to see what is actually happening. Ah, Doktor Android, the scientist - hooray for empirical method.

So, do I make myself sprkling crystal clear, as clear as a sky of deepest azure etc etc, perhaps.

Do you know for example how the Universe began?

Happy to be wrong, but we really do need to know more about this spawm of Satan and measure a few things, if it isn't just cheap rectifiers from less relaible reputable manufacturers.

I'm off to eat!

[Richard]

Ali,

Really? I can see the possibility of a dummy valve rect for a high voltage amp but surely not for a 300B amp when they only need a single 5u4 anyway? But then surely they wouldn't put the ht through it, just light the heaters. HT through a 5u4 wouldn't cause it to flash anyway as 5u4 is rated way above rectified 400v from a ss rect or not. It might do if the load was too high but that would be caused by something further down the line.

Rich

[andrew ivimey]

I don't disagree, brother Richard, but I have come across the scenario I describe a few times; utterly pointless in my book but who reads from the book of a prophet in these benighted times?

'... something wrong down the line....' ?

very probably, I'd agree.

[Ali Tait]

Well the same thing is happening in the 5u4 in my (Ex Andrew's) 813 amp.Andrew,have you used winged c svet 5u4's? Any good? Just got 4 of these nos 1972 for 15 quid.Thought it was worth a punt.Won't be able to try them in the amp till tomorrow night.

[andrew ivimey]

I'm afraid I take a dim view of russsian 5U4s be they svetlana or any of the other factories. The valves themselves look so fragile compared to the old American equivalents.

I've moved away from 5u4g types and only have a couple of GZ34s only I can't think why. When Ongaku rects go boiiiiinggg, I'll do a very different PSU.

Otherwise its 6D22s types or old mercury vapours for very slow turn on and cool looks.

If INfBaff can tell us more, someone will have and answer and I'll stop being silly - promise.