The rectifier tube (5U4-G) on one of my new KT88 monoblocks has started briefly flashing white light on power-up. I saw it first two days ago and have powered it up twice since then. Each time it flashes then all seems normal. Voltages within normal range (within a percent or so of the other monoblock) and the sound is great. No fuse blowing or any other symptom than this white flash.
I've ordered two new tubes and don't intend to power it up again until I replace the tube, but as a newbie to tube rectification I'm wondering if this is a common harmless phenomenon or the harbinger of a magic smoke leak?
BTW years ago I was witness to a rather spectacular tube failure involving a flash, small explosion of the envelope and flame, something I'd rather not witness again!
I've ordered two new tubes and don't intend to power it up again until I replace the tube, but as a newbie to tube rectification I'm wondering if this is a common harmless phenomenon or the harbinger of a magic smoke leak?
BTW years ago I was witness to a rather spectacular tube failure involving a flash, small explosion of the envelope and flame, something I'd rather not witness again!
Hi,
If it's a current production 5U4 then it doesn't come as a surprise, there have been reports of flakey ones for awhile....
However if this is a problem that only developped recently and only happens at turn on_when the PS caps charge up_ then it could well be that the cap's value is to high fro what the tube can handle.
The amount of max. capacitance for a cap input type filter is sometimes stated in the manufacturer's datasheet and is always directly proportional to the current drawn.
It's not unthinkable you're exceeding one or worse, both limits.
Cheers,
If it's a current production 5U4 then it doesn't come as a surprise, there have been reports of flakey ones for awhile....
However if this is a problem that only developped recently and only happens at turn on_when the PS caps charge up_ then it could well be that the cap's value is to high fro what the tube can handle.
The amount of max. capacitance for a cap input type filter is sometimes stated in the manufacturer's datasheet and is always directly proportional to the current drawn.
It's not unthinkable you're exceeding one or worse, both limits.
Cheers,
fdegrove said:Hi,
If it's a current production 5U4 then it doesn't come as a surprise, ..
... if this is a problem that ... only happens at turn on... it could well be that the cap's value is to high fro what the tube can handle...
Cheers,
Frank,
Thanks for the info. They are current production so that may be it. The ones I've ordered are also new production but will be Sovtek and my current pair are Chinese.
The data sheet says that for a cap input PS the first cap can be 40uF which is the value I've installed. The 5U4 on the other monoblock isn't showing any signs of this (yet
) and of course it is set up the same way. However I'll investigate purchasing some lower value ASC caps just in case.40 uF for a 5U4 tube seems like a perfectly safe value. I've seen production amps from 20 to 30 years ago that exceeded the specification by 10 or 20 uFs that still have the original rectifier tube in them and are going strong. It IS an issue when you grossly exceed the maximum, such as with 100 uF or so. Your tube is most likely flakey. I'm not sure if the flash is just a harmless "thing", or if it is the precurser to an explosion of some sort. I have heard some bad things about the reliability of Chinese tubes. I have never used Chinese tubes, so i can't comment from personal experience. I use vintage tubes salvaged from new equipment, along with some SOVTEKs, JJ Teslas, and some of those russian marked 6(pi)14(pi) (Russian Military EL84, same as SOVTEK EL84M). SOVTEKs are very reliable tubes.
Dr. Photon said:
I figured that 40uF would be OK. It is listed in the data sheet as the max and given that values vary +/- 20% in a lot of caps, especially when this tube was designed, I assumed there was some safety margin in that spec. I hope I don't have to replace them.
This is my first time (and probably last) using Chinese tubes. As it turns out the Sovteks are only $1 more than the Chinese tubes I originally bought! I should have gone that way at the beginning.
I already miss listening to the system so I hope the tubes show up early this week!
Sherman said:
I figured that 40uF would be OK. It is listed in the data sheet as the max and given that values vary +/- 20% in a lot of caps, especially when this tube was designed, I assumed there was some safety margin in that spec. I hope I don't have to replace them.
The specs of yesteryear hardly bear relation to current production tubes of today.......in another thread it was mentioned that manufacturers can't spend acres of time producing the NOS quality of earlier years. So with this in mind, the only solution is derate recent made tubes and operate well within limits.....if you want'em to have a decent life. That goes for rectifiers......so many rel new ones are fault-found casulties on my MI service bench .....simply because they can't take the stick. The worst is the DHeat 5U4G......and believe it or not the latter GZ34. My only solution is to reduce the Cin cap value....by a 1/3......i.e 40uF to 33uF and to put resistors equal to the secondary resistance per sec winding in each anode rect leg..that works for cathode bias fine. The case of fixed bias is to simply parallel another ident rectifier with another.....so long the heater can supply the guts.
richj
richwalters said:
Rich,
Thanks for the feedback. As I stated before I ordered new tubes. Today for the first time- shame on me- I noticed the tubes are slightly different inside. The envelope and bases are exactly the same but inside one has dish-shaped plates and the one that is flashing has smaller ones with a bump on them.
I also found a place to get smaller caps of the same type so if I need to lower those values I will do that as well. WHen I measured voltage and current after the tube all was within limits but I don't have a capacitance meter.
The problem I have is that the PS seems to be so darn clean right now that I hate to change it. If I do though I think I'll simply add the smaller cap, move my choke connections to the new smaller cap and the old 40uF cap and then make an RC network for the last 100uF cap. Should be even cleaner that way and I think I have the real estate on the chassis to add a third cap. No worries about too low voltage as my target B+ was 400V and I'm getting 429. I can lose a little across the resistor.
Hi there.... this thingy about longevity of power tubes in general (+ rects) ..the strangest thing is I don't have any longevity problems with the minor stuff.....recent Svet ECL82's, EF86's and most of the 6SN7 class....they seem to take what comes.. year on year....I presume material costs aren't so much of an issue when compared to the large tubes.
The surge voltage from hv transformers is ghasty espec when a flash over occurs is stressful......and may contain ringing.......when using a center tapped secondary i.e bi-phase, an anti surge fuse placed in the center tap to grnd can offer protection if a flash over occurs. But it is a warning shot to do something about it.
The 5U4 or equiv U52
heater 5V @ 2.25A
max resevoir cap of 16uF (MOV spec)
min series resistance in each anode leg of 180 ohms non inductive.....(hard to get these days)
max Iout 250mA
Vin max AC rms 500-0-500
It looks that the same tube made by clone manuf will accept differing values of res cap values...The cue is really to seek an indirect rect capable of taking the same AC volts/ current...the GZ34 would give a lower I out and could be overrun. The GZ34 is quoted at
Iout = 160mA; (would be overrun in your circuit)
max resevoir cap of 60uF
series resistor each anode leg 175 ohms
Vin max AC rms 550-0-550
richj
The surge voltage from hv transformers is ghasty espec when a flash over occurs is stressful......and may contain ringing.......when using a center tapped secondary i.e bi-phase, an anti surge fuse placed in the center tap to grnd can offer protection if a flash over occurs. But it is a warning shot to do something about it.
The 5U4 or equiv U52
heater 5V @ 2.25A
max resevoir cap of 16uF (MOV spec)
min series resistance in each anode leg of 180 ohms non inductive.....(hard to get these days)
max Iout 250mA
Vin max AC rms 500-0-500
It looks that the same tube made by clone manuf will accept differing values of res cap values...The cue is really to seek an indirect rect capable of taking the same AC volts/ current...the GZ34 would give a lower I out and could be overrun. The GZ34 is quoted at
Iout = 160mA; (would be overrun in your circuit)
max resevoir cap of 60uF
series resistor each anode leg 175 ohms
Vin max AC rms 550-0-550
richj
richwalters said:
One of the reasons I selected the 5U4G was its relatively high current out. I'm running a 6N1P driving a KT88 in a single ended design. The KT88 is biased at about 75mA. I wanted to have a safety margin over the max draw.
I could have gone with SS rectification in a lot less space and for less money but darn it, tubes just look nice, and the 5U4Gs have a nice size and shape so they fit well with the KT88.
In the end I don't want to blow anything up so I'll keep a very close eye on things when the new tubes arrive. If it happens again I'll have to examine my options including a different rectifier tube.
fdegrove said:Hi,
Sherman,
Since you're in the U.S., why not get your hands on some NOS 5U4GBs?
I hear the Russian ones suffer from similar problems as the Chinese ones anyway...
MAybe the probelm was solved in the meantime, I don't know....the rumour mill has it, so........
Cheers,
Frank,
You are right, I may have ordered the Sovteks too hastily (though I really do miss listening to the new SETs). It appears I can get NOS for only about $30 each. I'll probably go ahead and order a pair. I am glad I spread the purchases for these amps over a few months. Makes it harder for the wife to nail down exactly what they cost!!
Also I have finished the bases for them so I suppose they are "complete". Now I need to take a couple photos for the gallery. (Proud papa syndrome here even with the flashing rectifier!)
$30 is WAY too much for a 5U4. this is a tube that was in a billion devices.
prices from AES:
5U4GB/5AS4 New, domestic, $14
5U4G, New, Domestic, $24
5U4G chinese - $10
so.. if you dont care about the shape of the tube, a NOS us made one is $14. If you do its $24
I'm not sure how the new sovtek 5U4's look, but the old soviet ones are tough as nails, iirc. they have a different filament than american ones.. it goes up-down 5x per section, instead of once.. Old SSSR name is 5C3S
eg. Link to ebay auction for 5C3S
this is an auction of 5C3S, ST shape, svetlana manufacture.... notice how filament goes up down many times.. this is how soviet 5U4's were made for some reason. looks cool at least
prices from AES:
5U4GB/5AS4 New, domestic, $14
5U4G, New, Domestic, $24
5U4G chinese - $10
so.. if you dont care about the shape of the tube, a NOS us made one is $14. If you do its $24
I'm not sure how the new sovtek 5U4's look, but the old soviet ones are tough as nails, iirc. they have a different filament than american ones.. it goes up-down 5x per section, instead of once.. Old SSSR name is 5C3S
eg. Link to ebay auction for 5C3S
this is an auction of 5C3S, ST shape, svetlana manufacture.... notice how filament goes up down many times.. this is how soviet 5U4's were made for some reason. looks cool at least
Colt45 said:
Thanks!
I had priced them locally (there is an old TV repair place that has a limited stock of tubes. I didn't check Angela which is what I should have done first!
It appears FedEx tried to deliver them today but of course no one was here! I'm very interested now in how the Sovteks perform. I should know the basics within 24 hours.
anatech said:
Chris,
It does make me feel better! It is good to hear the Sovteks are solid. Having a small collection of Soviet/Russian cameras I know that the Russians are capable of turning out built-like-a-tank solid stuff (though never state-of-the-art in my experience).
I'm concerned now about the Chinese tube thing though because my KT88s are Valve Arts which are Chinese.
On the positive side they sound great and I know that Bottlehead uses Valve Art tubes for their 2A3 amp so I've got my fingers crossed.
Hi Sherman,
I have been using Electroharmonix as much as possible lately. These seem to be better still. As an example, the 6L6EH can stand up to a Fender Twin. Their 12AX7EH is about as quiet as any and non-microphonic. So where a tubeis offered in both brands, go for Electroharmonix.
The biggest problem I have with tube rectifiers are the happy souls who don't read the tube manual and hang too much capacitance on them (you are fine that way).
There is one brand of preamp that makes the Cetron version of a 5U4 arc at turn on. That is just stupid, actually the entire preamp is just that.
-Chris
I have been using Electroharmonix as much as possible lately. These seem to be better still. As an example, the 6L6EH can stand up to a Fender Twin. Their 12AX7EH is about as quiet as any and non-microphonic. So where a tubeis offered in both brands, go for Electroharmonix.
The biggest problem I have with tube rectifiers are the happy souls who don't read the tube manual and hang too much capacitance on them (you are fine that way).
There is one brand of preamp that makes the Cetron version of a 5U4 arc at turn on. That is just stupid, actually the entire preamp is just that.
-Chris
Well, the new Sovteks were delivered yesterday and I plugged them in and powered up. No flashing or arcing or any weird behavior. Good voltages and current and the amps sound good. Of course that is after powering up only twice with the new tubes so I'll have to wait before making any real judgements.
I will say that the Sovteks are identical (the Chinese tubes had some differences inside) and seem pretty solid. The envelopes are clear (one of the Chinese tubes, not the flashing one, had a large bubble in the envelope).
I will say that the Sovteks are identical (the Chinese tubes had some differences inside) and seem pretty solid. The envelopes are clear (one of the Chinese tubes, not the flashing one, had a large bubble in the envelope).
Sherman said:
Hi there.........I still think it's a close run thing.....you might have tried the other rect tubes on a mains voltage high and you have problems......depends how much AC line variation you have where you are. I'm only mentioning this because I had a case where nom I'm 230V but can slip unawares above 240V and below 210V.
On 500V AC tranny that can go above 525V....
.
Examine carefullyrichj
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