Blue flash at TurnOn??

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Blue flash at TurnOn??

Postby GrooveGrinder » Sat Dec 13, 2008 5:26 pm

Hi all...

Turned on the 70 yesterday, and about 30-40
secs in, there was a blue flash and a pop thru
the spkrs.. well more like a "POfffT" than pop.
After that, it seemed fine, by the looks of it anyway.
I hesitated for a moment, wondering if it was a
passing phenomena and since nothing else bad
seemed to be happening, then thought the better
of it and switched off.
Didn't try listening, as I figured damage might be
caused and better to just switch off and figure out
what happened.
I WAS looking at it when it happened, and yet
couldn't tell exactly where it came from...
except that it *seemed* to be in the vicinity
of one EL34 pair.

Only other sign... and I'm not certain of it, is that
the 5AR4 seemed to maybe be a bit brighter
glowing than usual, but I could be wrong.

Some background info:
The amp is a total rebuild completed this past
May (thanks to help from the great folks at diytube!)
After putting around 300 hrs on it with heavy daily
use for "break-in", its been used almost exclusively
on sundays all day, since then.
Its been working great, making beautiful music
since then with no sign of trouble, including
the previous evening.
*I haven't adjusted bias since then*, after the
initial checks/adjustments in that initial period.

The amp consists of the following:
Tubes = JJ e34l and 5AR4, JAN-Phillips 7199s.
PC board is a modern replica PC3.
Can cap is the 40-> 80-> 30-> 20 from Dynakitparts.
CL-90 added.
Everything is new save for chassis and iron.
Plz let me know if other details would be helpful.

Any thoughts?
Thanks in advance.
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Postby TomMcNally » Sat Dec 13, 2008 9:50 pm

Tubes = JJ EL34 and 5AR4

In my opinion - JJ tubes are prone to
self destruction. The rectifier is
probably the culprit. Keep a spare handy !
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Postby GrooveGrinder » Sat Dec 13, 2008 11:32 pm

TomMcNally wrote:Tubes = JJ EL34 and 5AR4
In my opinion - JJ tubes are prone to
self destruction. The rectifier is
probably the culprit. Keep a spare handy !

Hmmm...
Ahh for the good 'ol days...

Hi Tom, thanks for the reply.
Should I try turning it on again and see what
happens?
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Postby WA4SWJ » Sun Dec 14, 2008 7:11 am

Hi Groovy,

If you haven't done it already, install resistors on the screens in series with the ultralinear taps from the output transformers. They can be 100 to 470 ohms or so. There is a good article linked below that explains the reasons, so no need to go into that here. I did this on my ST-70 that was having the same problem and no more issues for several years. Read the article - it's a good read and makes sense. As I said, no more problems with mine and I now do this as a matter of course when building new amps.

http://www.audioxpress.com/magsdirx/ax/ ... ie2544.pdf

Regards,
Ed Long
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By george, I think you've got it!

Postby GrooveGrinder » Sun Dec 14, 2008 8:31 pm

Hi Ed..
Thanks for the reply and the help!

Wow... that was indeed interesting reading.
Felt somehow like reading a 007 novel.
More importantly, I think thats it.

When you say you had the same problem, was
that a flash on startup?
Should I assume my output tube (pair) is fried?
Therefore not try powering up till replacing?

I should've mentioned that they're the blue glass
JJs... and the flash was blue.

And no, I hadn't done it already.. installing the
series screen resistors.
Didn't know its nescessary.. but now I do! lol.
I've leaned sooooooooo much from this project.
The gift that keeps on giving too..
Its a good thing in the end though.
The learning that is.

Would you mind a detailed description of just how
you implimented?
Thanks in advance if so..

What about the timer controlled B+ in the article?
Have you implimented that too, or not found it
nescessary for whatever reason?

Also, I might have noted more strongly that I haven't
re-biased since the first 300 or so hours
passed...
but I didn't want to "lead the witness" when asking
the question in the first post.

I'm realizing that using the larger-value can cap
is where most of my probs are comming from.
Starting with the rectifier flashovers upon initial
startup.
Best as I can figure, that was cured by the addition
of the CL-90 and brand new JJ 5AR4.

The can cap does help the sound... especially
the Bass... only very slightly loose, and pretty
"punchy", but I don't think its worth the
trouble.
Particularly given the cost today, of a good Mullard,
which seems the only thing robust enough.
Even then, you're at the ragged edge.

I'm not sure what I'd do differently if doing it
again
(which I likely will, since I've another 70 to re-do)

It would seem that even with the cap-boards
you'd run into the same dilemma.

While I'm at it....
On the subject of adding resistors I should've
in the first place:
What do you think about adding something like
a 20 ohm, 20 watt power resistor in series with
each of the EL34 filament windings?
Considering todays line voltages.
(Original power transformer on this 70)

Again, thanks so much!!!
Greg
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Postby Ty_Bower » Sun Dec 14, 2008 8:50 pm

WA4SWJ wrote:If you haven't done it already, install resistors on the screens in series with the ultralinear taps from the output transformers. They can be 100 to 470 ohms or so.


For what it's worth, all the talk put the fear into me a while ago. I installed ballast resistors between the screens and the UL taps in my Dynaco Mk3 project. I think I used carbon composition 82 ohm parts just because that's what I had laying around. 82 ohms is pretty close to nothing anyway, so I figure it can't hurt the sound any. I've never seen any arcing problems in my output tubes, but I do keep the B+ vacuum rectified and I've only had the amps working for a couple of months now.

Still, it seems like cheap insurance...
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby WA4SWJ » Mon Dec 15, 2008 6:48 am

Guys,

All I did was put a 100 ohm 2W resistor in series with the UL taps to the screen grids right at the tube socket. Easy to do. I attached one end of the resistor very close to the tube socket and then attached the UL lead from the OPT to the free end of the resistor. I do this on every amp I build now and since the ST-70, which was my second amp, I have had no more arcing problems at all.

Yes, I had flashes inside the power tubes on startup prior to the modification. They were JJ's also. In my ST-70 I use solid state rectification with fairly large filter caps so my power supply impedance is probably lower than a typical 5AR4 based power supply. I had trouble with the output tubes almost immediately. Once I installed the screen resistors I had no more problems. In my case the tube that arced was destroyed so I installed a new one, but you'll have to try yours and see if it survived. Usually arcing inside a power tube is close to fatal if not the end. But, if the arc was short-lived perhaps they might be OK. It won't hurt to try them - just keep the power switch close by. My feeling is that the ones that arced may be toast unfortunately. But give it a try.

Biasing would not cause this unless of course you have a runaway condition where the tube is biased so low that it draws excessive current and melts down. Of course it's good to check the bias occasionally. I like to check the bias balance which is easy to do with Shannon's boards - a nice feature.

As for timing the application of B+, I don't bother with that. I don't do that on any of the amps I have. However, I am thinking about a higher power amp that will use 866A mercury vapor rectifiers and those require preheat prior to application of high voltage.

I have not reduced the filament voltage on my amps either. I haven't seen the need to do that. To be honest I have not measured the filament voltage on my various tubes. I don't really see it as a problem. I'm sure there is an upper limit for filament voltage that will cause premature failure, but I don't have a guess at that. However, I have yet to have any tubes fail except the JJ that arced as mentioned above and a JJ GZ-34 (5AR4). If I recall correctly all of the power transformers I've used have 120VAC primaries so I doubt the filament voltage is more than +10% even on system voltage peaks. An increase like that would mean the line voltage would be 132 volts RMS, which is certainly possible, but probably is not a continuous situation.

Hope this helps. Best of luck and enjoy this fantastic hobby!
Ed Long
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Postby GrooveGrinder » Mon Dec 15, 2008 8:07 pm

Ed...

Clear, simple, and straight to the point!
Thanks!

Ya gotta respect a guy who "thinks about
higher power amps that will use 866A mercury
vapor rectifiers"!

Maybe Shannon will incorporate that into the Ikes?

BTW...
This is a fantastic hobby thanks to guys like you..
Otherwise it would be largely a study in frustration.

Greg
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Postby WA4SWJ » Tue Dec 16, 2008 6:36 am

Greg,

Glad I could help. I hope this fixes the problem for you.

Yeah, I used to mess around with this stuff when I was a teenager. Back then we barely had transistors like the CK722 or the 2N107 so vacuum tubes were just about the only choice. I used to love the blue glow of those mercury vapor rectifiers. I had some lead acid battery chargers that used them that I got as surplus from the telephone company - my dad worked for them. I have some 866A's and I'm just trying to decide what type of amp I'd like to build. There are problems (like noise) with this type of rectifier, but that can be over come. It's just fun to think about. I've seen some gorgeous amps online with 866's glowing away so I guess I've just got to have one. Or maybe it's just a second childhood!!

Regards,
Ed Long
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Re: Blue flash at TurnOn??

Postby mati1979 » Sat May 09, 2009 5:37 pm

GrooveGrinder wrote:Turned on the 70 yesterday, and about 30-40
secs in, there was a blue flash and a pop thru
the spkrs.. well more like a "POfffT" than pop.
.



I have a similar problem. In 2 hours of work , 2 times i see flash in it did in the tube GZ34 JJ. and i think a bias decrease to 0,85V (normal 0,95V)
:(

this is a problem for a low quality of tube ?
(maybe the fuse ? i m using a 10A 250V , a manual say : 2.5A slo-blo fuse but now i havent this fuse, only i have a 10A )
.
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Postby TomMcNally » Sat May 09, 2009 7:56 pm

JJ means Just Junk
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Postby kheper » Sat May 09, 2009 9:22 pm

TomMcNally wrote:JJ means Just Junk


Heh. What he said.

The 12au7 tubes I got sounded horrid. The EL-34s crapped out in short order.
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