[Phil Allison[_3_]]

"mick"

Is this a relatively recent problem, Phil?

** Depends what you call " recent ".

It's something I've never come
across (or, at least, not spotted) in spite of quite a few years playing
with octal valves.

** I don't play with valves.

I could well believe that it's something to do with
the production methods & materials that are being used now.

** Or simply a careless stuff up in some Russian valve plant.

At some stage, the flux used to solder the pins on an octal base is cleaned
off - likely by passing the bases through a solvent bath. If the solvent
is not regularly replaced, then a thin film of a conductive contaminant is
left all over the base when it dries - just conductive enough to ruin the
insulation quality when the base gets hot.

As I said, a few soak and rinse cycles in clean methylated spirits did the
trick on several hundred Sovtek EL34s that would otherwise have had to be
binned. This was about 9 years ago.


...... Phil

[Patrick Turner]

On Feb 4, 3:47*pm, "Phil Allison" wrote:
"mick"

Is this a relatively recent problem, Phil?

** Depends what you call " recent ".

It's something I've never come
across (or, at least, not spotted) in spite of quite a few years playing
with octal valves.

** I don't play with valves.

I could well believe that it's something to do with
the production methods & materials that are being used now.

Yes, but unlikely. I have witnessed the gradual decline of 12 x
GE6550A which I bought new in 1995.
All ended up with positive grids; some were worse than others.

One might think the tube BASES would have become conductive between
pin4 and pin5 muchsooner if it was a flux /soldering problem.

Tubes from Russia and made since 1995 have not been any worse than the
USA made GE6550A.


** Or simply a careless stuff up in some Russian valve plant.

Stuff ups have occurred in all countries where tube manufacturing was/
is done.
Not a single factory has 100% perfect quality.


At some stage, the flux used to solder the pins on an octal base is cleaned
off - *likely by passing the bases through a solvent bath.

"likely" ? so you assume this to be so. I've never known all the
approximately 300 steps you need to take to make a tube.

*If the solvent
is not regularly replaced, then a thin film of a conductive contaminant is
left all over the base when it dries - just conductive enough to ruin the
insulation quality when the base gets hot.

So would you not think that the "conductive contaminent" would become
active from the first time the tube was used?

And BTW, isn't flux non conductive? if you probe a lump of dried flux
with probes close, you get an OL resistance reading, no? AFAIK, it is
of great importance for flux to be NON conductive, lest conductive
paths occur across tracks in boards and from point to point wiring
etc..

As I said, a few soak and rinse cycles in clean methylated spirits did the
trick on several hundred Sovtek EL34s that would otherwise have had to be
binned. *This was about 9 years ago.

The problem may not have been flux; maybe something else uknown.

I still have several old tubes which develop the positive grid
syndrome so when I test an amp which takes octal tubes I can see what
happens when I clean the bases up.

I've never seen the problem in a new tube.

In cases where the +ve grids occur, the tubes still give full power at
low THD, but one feels nervous about leaving them in a circuit because
of the risk of thermal runaway.

Patrick Turner.



..... *Phil

[Phil Allison[_3_]]

"Patrick Turner"


You have not seen the problem nor tried the cure so will not believe a damn
thing - as usual.

Go drop dead.



..... Phil

[Ian Iveson]

Patrick wrote:

Is the 6CH6 an octal or nine pin mini without a
bakelite/plastic
base?
You can get +ve grids in tubes like 6BM8, 6GW8, 6BQ5 etc.
And also in
signal tubes like 6CG7.
I've measured it, and usually the tubes are damn old, noisy,
and
microphonic

***9 pin, he

http://www.tubecollector.org/cv4055.htm

***Considerably shorter than an EL84. Anode is just a
cylinder. You can see all the gubbins through the cutouts,
so when they're on they look like little illustrations of
How Valves Work.

***Mine are Brimar. Cheap, and all of good quality so far:
they've been going for years and I've not needed to replace
one.

I am not sure exactly why you get different levels of the
dark blue
glow even on a new bunch of matched tubes.

***No, neither am I. Small details of alignment I suppose.
Maybe the composition of the glass has something to do with
it.

But if the leakage is at the base of a tube, and was between
say a
screen pin voltage and grid1 pin, then as I said in an
earlier post
the resistance path is quite a high value, 8M for where you
have 400V
difference and 0.05mA.
The noise voltage across 8M would be quite high, and this is
in series
with the grid input resistance which is many megohms, so the
leakage
path resistance noise will impart its noise into the grid.

Conclusion? = Keep the bottoms of your tubes as clean as
you can!!!

***Sounds good to me. Watch you don't wash off those
priceless labels though.

And don't forget tube sockets and coupling caps.

***What solvent is best for cleaning sockets?

Ian