[Alex]

Hello,

One of the two 6CA7 tubes in an amplifier is faulty. It has a bad grid
leakage reaching 50uA or so. Leakage current rises gradually as the tube
warms up.

My first question is what causes it, a flake of oxide lodged between the
grid and cathode or something else? (Visually the tube is very "clean" with
no signs of overheating, no dark or shiny deposits on the inside of the
envelope, no whiteness on the getter).

And the second question, of course, is it possible to "repair" this fault,
or the tube is destined to a rubbish bin.

Regards,
Alex

[Phil Allison[_3_]]

"Alex"


One of the two 6CA7 tubes in an amplifier is faulty. It has a bad grid
leakage reaching 50uA or so. Leakage current rises gradually as the tube
warms up.

My first question is what causes it,


** Poor insulation between the screen grid pin and the adjacent grid pin.


And the second question, of course, is it possible to "repair" this fault,
or the tube is destined to a rubbish bin.


** Can sometimes be cure by giving the bakelite base a very good wash in
methyl alcohol.

Soak it and drain it several times - then blow dry with hot air.

Worth a try.


...... Phil

[Bob Eld]

"Alex" wrote in message
...
Hello,

One of the two 6CA7 tubes in an amplifier is faulty. It has a bad grid
leakage reaching 50uA or so. Leakage current rises gradually as the tube
warms up.

My first question is what causes it, a flake of oxide lodged between the
grid and cathode or something else? (Visually the tube is very "clean"
with
no signs of overheating, no dark or shiny deposits on the inside of the
envelope, no whiteness on the getter).

And the second question, of course, is it possible to "repair" this fault,
or the tube is destined to a rubbish bin.

Regards,
Alex


The tube is likely gassy. Gas in vacuum tubes when bombarded by electrons
forms ions that are attracted to the most negative elements in the tube,
control grid and cathode. What you are observing is likely ion current on
the grid. A lower grid resistance or higher bias could reduce the effect,
but the only real remedy is a new tube. Sooner or later these ions will
sputter the cathode and ruin the tube's emission.

[Alex]

On Jan 29, 8:02*am, "Bob Eld" wrote:
"Alex" wrote in message

...





Hello,

One of the two 6CA7 tubes in an amplifier is faulty. It has a bad grid
leakage reaching 50uA or so. Leakage current rises gradually as the tube
warms up.

My first question is what causes it, a flake of oxide lodged between the
grid and cathode or something else? (Visually the tube is very "clean"
with
no signs of overheating, no dark or shiny deposits on the inside of the
envelope, no whiteness on the getter).

And the second question, of course, is it possible to "repair" this fault,
or the tube is destined to a rubbish bin.

Regards,
Alex

The tube is likely gassy. Gas in vacuum tubes when bombarded by electrons
forms ions that are attracted to the most negative elements in the tube,
control grid and cathode. What you are observing is likely ion current on
the grid. *A lower grid resistance or higher bias could reduce the effect,
but the only real remedy is a new tube. Sooner or later these ions will
sputter the cathode and ruin the tube's emission.- Hide quoted text -

- Show quoted text -

All those who replied sugest that it is some kind of a leakage from a
high potential to the grid: ionic current or even conductive surface
resistance inside or even outside in the base. I have no experience
with it, and my "theory" was it was resistance between the grid and
the cathode.

Anyway to prove one way or the other I will perform a simple
experiment:
- ground screen grid and plate;
- insert a uA-meter in the first grid circuit;
- warm up the tube;
- apply say +20V to the cathode;
- measure grid leakage current.

If my theory is correct, I will see some significant leakage current.
If you guys are correct I will see nothing.

If you are correct I will try to follow your advises. If I am correct,
I will be seeking for methods of burning out a speckle of material
between cathode and grid.

Regards,
Alex

[Bob Eld]

"Alex" wrote in message
...
On Jan 29, 8:02 am, "Bob Eld" wrote:
"Alex" wrote in message

...





Hello,

One of the two 6CA7 tubes in an amplifier is faulty. It has a bad grid
leakage reaching 50uA or so. Leakage current rises gradually as the tube
warms up.

My first question is what causes it, a flake of oxide lodged between the
grid and cathode or something else? (Visually the tube is very "clean"
with
no signs of overheating, no dark or shiny deposits on the inside of the
envelope, no whiteness on the getter).

And the second question, of course, is it possible to "repair" this
fault,
or the tube is destined to a rubbish bin.

Regards,
Alex

The tube is likely gassy. Gas in vacuum tubes when bombarded by electrons
forms ions that are attracted to the most negative elements in the tube,
control grid and cathode. What you are observing is likely ion current on
the grid. A lower grid resistance or higher bias could reduce the effect,
but the only real remedy is a new tube. Sooner or later these ions will
sputter the cathode and ruin the tube's emission.- Hide quoted text -

- Show quoted text -

All those who replied sugest that it is some kind of a leakage from a
high potential to the grid: ionic current or even conductive surface
resistance inside or even outside in the base. I have no experience
with it, and my "theory" was it was resistance between the grid and
the cathode.

Anyway to prove one way or the other I will perform a simple
experiment:
- ground screen grid and plate;
- insert a uA-meter in the first grid circuit;
- warm up the tube;
- apply say +20V to the cathode;
..- measure grid leakage current.

If my theory is correct, I will see some significant leakage current.
If you guys are correct I will see nothing.

If you are correct I will try to follow your advises. If I am correct,
I will be seeking for methods of burning out a speckle of material
between cathode and grid.

Regards,
Alex

Well OK, its fine to do an experiment. In fact it is possible to use the
measured grid current and plate voltage to estimate the amount of gas in a
tube (F. Terman, Electronic and Radio Engineering). Even if it is some
"speckle of material" one should still replace the tube for best
performance, at least I would. Let us know how your experiment turns out.

[Alex]

"Bob Eld" wrote in message
...

"Alex" wrote in message
...
On Jan 29, 8:02 am, "Bob Eld" wrote:
"Alex" wrote in message

...





Hello,

One of the two 6CA7 tubes in an amplifier is faulty. It has a bad grid
leakage reaching 50uA or so. Leakage current rises gradually as the
tube
warms up.

My first question is what causes it, a flake of oxide lodged between
the
grid and cathode or something else? (Visually the tube is very "clean"
with
no signs of overheating, no dark or shiny deposits on the inside of the
envelope, no whiteness on the getter).

And the second question, of course, is it possible to "repair" this
fault,
or the tube is destined to a rubbish bin.

Regards,
Alex

The tube is likely gassy. Gas in vacuum tubes when bombarded by electrons
forms ions that are attracted to the most negative elements in the tube,
control grid and cathode. What you are observing is likely ion current on
the grid. A lower grid resistance or higher bias could reduce the effect,
but the only real remedy is a new tube. Sooner or later these ions will
sputter the cathode and ruin the tube's emission.- Hide quoted text -

- Show quoted text -

All those who replied sugest that it is some kind of a leakage from a
high potential to the grid: ionic current or even conductive surface
resistance inside or even outside in the base. I have no experience
with it, and my "theory" was it was resistance between the grid and
the cathode.

Anyway to prove one way or the other I will perform a simple
experiment:
- ground screen grid and plate;
- insert a uA-meter in the first grid circuit;
- warm up the tube;
- apply say +20V to the cathode;
.- measure grid leakage current.

If my theory is correct, I will see some significant leakage current.
If you guys are correct I will see nothing.

If you are correct I will try to follow your advises. If I am correct,
I will be seeking for methods of burning out a speckle of material
between cathode and grid.

Regards,
Alex

Well OK, its fine to do an experiment. In fact it is possible to use the
measured grid current and plate voltage to estimate the amount of gas in a
tube (F. Terman, Electronic and Radio Engineering). Even if it is some
"speckle of material" one should still replace the tube for best
performance, at least I would. Let us know how your experiment turns out.

Well, you were right guys. It was not cathode-to-grid leakage. +20V on the
cathode with plate and screen grid disconnected did not generate any grid
current. Not a whistle.

It was indeed leakage resultant from poor vacuum. I fixed the problem by
heating the getter. See more detail in my reply to "diamondsouled" Larry.

[Phil Allison[_3_]]

"Alex is a DOPE "


It was indeed leakage resultant from poor vacuum. I fixed the problem by
heating the getter.

** ********.

Heating the getter is not possible without radio frequency heating gear.

You heated the whole tube.




..... Phil

[Phil Allison[_3_]]

"Alex"

All those who replied sugest that it is some kind of a leakage from a
high potential to the grid: ionic current or even conductive surface
resistance inside or even outside in the base. I have no experience
with it, and my "theory" was it was resistance between the grid and
the cathode.

Anyway to prove one way or the other I will perform a simple
experiment:


** Try heating the bakelite base with hot air gun while the tube is running.

Watch the grid leakage go through the roof almost immediately.



...... Phil

[Alex]

"Phil Allison" wrote in message
...

"Alex"

All those who replied sugest that it is some kind of a leakage from a
high potential to the grid: ionic current or even conductive surface
resistance inside or even outside in the base. I have no experience
with it, and my "theory" was it was resistance between the grid and
the cathode.

Anyway to prove one way or the other I will perform a simple
experiment:


** Try heating the bakelite base with hot air gun while the tube is
running.

Watch the grid leakage go through the roof almost immediately.

Heating the bakelite base does not make any noticeable difference -- thus
the leakage is not in the base.

[Phil Allison[_3_]]

"Alex"
"Phil Allison"
"Alex"

All those who replied sugest that it is some kind of a leakage from a
high potential to the grid: ionic current or even conductive surface
resistance inside or even outside in the base. I have no experience
with it, and my "theory" was it was resistance between the grid and
the cathode.

Anyway to prove one way or the other I will perform a simple
experiment:


** Try heating the bakelite base with hot air gun while the tube is
running.

Watch the grid leakage go through the roof almost immediately.

Heating the bakelite base does not make any noticeable difference


** How did you heat it ??

With a kiddies hair dryer set to low ??????

That is not a damn HOT AIR GUN !!!

Fact is - it ALWAYS makes a difference when you heat the base very hot.

Try the simple cleaning idea I posted.

You ****ing half wit ****** !!



...... Phil

[Phil Allison[_3_]]

"Bob Eld"


The tube is likely gassy.

** Nonsense.

A gassy tube will be leaky ( or even glow internally) from the moment you
apply power.



..... Phil

[diamondsouled[_2_]]

There is a way of waking up the getter flash in tubes such as this, it
is actually quite common in Tung Sol 6550s for them to test gassy/grid
leakage. Some people have built rigs where they can put enough voltage/
cuurent into the tube, gradually, that in will activate the getter
flash. Here's an alternative method that can work:

Here's how to bake one:

1. Put the cold tube(s) in a COLD oven; set the oven thermostat to 300
F.

2. Bake the tubes for 4-5 hours at this temp.

3. Turn the oven off and walk away. Don't peek! Let it cool very
gradually.

4. When the oven is completely cooled take the tubes out and retest.
They will likely be fine as the getter will have absorbed gas
molecules - but...

DO NOT SKIP THE RETEST!!

Some tubes may have some cathode poisoning too - the baking may help,
but just heating the cathode is often enough to remedy the situation.
For that a tube tester is fine - but you don't know what's going on
inside the tube so it's best to be safe. Let them sit in the tester
while after the baking process is complete, don't rush. .

Of course if there is little or no getter flash left there might be
nothing left to activate. ;^)

Cheers

Larry

[Engineer[_2_]]

On Jan 28, 5:33*pm, diamondsouled wrote:
There is a way of waking up the getter flash in tubes such as this, it
is actually quite common in Tung Sol 6550s for them to test gassy/grid
leakage. Some people have built rigs where they can put enough voltage/
cuurent into the tube, gradually, that in will activate the getter
flash. Here's an alternative method that can work:

Here's how to bake one:

1. Put the cold tube(s) in a COLD oven; set the oven thermostat to 300
F.

2. Bake the tubes for 4-5 hours at this temp.

3. Turn the oven off and walk away. Don't peek! Let it cool very
gradually.

4. When the oven is completely cooled take the tubes out and retest.
They will likely be fine as the getter will have absorbed gas
molecules - but...

DO NOT SKIP THE RETEST!!

Some tubes may have some cathode poisoning too - the baking may help,
but just heating the cathode is often enough to remedy the situation.
For that a tube tester is fine - but you don't know what's going on
inside the tube so it's best to be safe. Let them sit in the tester
while after the baking process is complete, don't rush. .

Of course if there is little or no getter flash left there might be
nothing left to activate. ;^)

Cheers

Larry

I hope it's not a silly question but how does baking the tube differ
from running the heater for an hour or two? You could even wrap the
tube in a cloth to get it hotter - be careful! Or just run it at a
modest plate current for a while.
Cheers,
Roger

[Alex]

"diamondsouled" wrote in message
...
There is a way of waking up the getter flash in tubes such as this, it
is actually quite common in Tung Sol 6550s for them to test gassy/grid
leakage. Some people have built rigs where they can put enough voltage/
cuurent into the tube, gradually, that in will activate the getter
flash. Here's an alternative method that can work:

Here's how to bake one:

1. Put the cold tube(s) in a COLD oven; set the oven thermostat to 300
F.

2. Bake the tubes for 4-5 hours at this temp.

3. Turn the oven off and walk away. Don't peek! Let it cool very
gradually.

4. When the oven is completely cooled take the tubes out and retest.
They will likely be fine as the getter will have absorbed gas
molecules - but...

DO NOT SKIP THE RETEST!!

Some tubes may have some cathode poisoning too - the baking may help,
but just heating the cathode is often enough to remedy the situation.
For that a tube tester is fine - but you don't know what's going on
inside the tube so it's best to be safe. Let them sit in the tester
while after the baking process is complete, don't rush. .

Of course if there is little or no getter flash left there might be
nothing left to activate. ;^)

Cheers

Larry

Thanks Larry,

Being a lazy person (living in Australia), I could not be bothered with 5
hour cooking plus probably another couple of hours of cooling. But once my
experiment proved that it was not cathode-to-grid leakage, I did the
following.

I blew a heat gun on the top of the tube envelope (the tube powered in
circuit with grid current being monitored) in attempt to activate the
getter. After approximately 20 seconds of blowing grid current shoots up by
50%, and at this stage I withdrew the heat gun. Surprisingly the current
dropped far below its initial value.

Then in a couple of minutes I would repeat the procedure. again current goes
up by 20%, then down by 50%. After several such themperature cyclings the
current dropped to nearly 150nA. (And it was above 50uA in the
beginning!!!). This is quite good. Again, can not be bothered trying
further.

Now the tube is simply running in normal mode (about 19W plate dissipation),
and the current is not rising. Instead it is even slowly sort of declining.

So your advice was very useful, since it obviously reflected correctly the
fault nature and the method of fixing by activating the getter by warming
it. I quite believe that your cooking technique would also give good
results, likely even better because of degassing all the elements of the
valve.

Thanks.
Regards,
Alex

[Phil Allison[_3_]]

"Alex"

I blew a heat gun

** Yet again, this LYING **** hides what he actually used.

on the top of the tube envelope (the tube powered in circuit with grid
current being monitored) in attempt to activate the getter.


** Beyond even the stupidest possible idea.

The getter is INSIDE a ****ING VACUUM !!

Vacuums DO NOT CONDUCT HEAT !!!!!!!!!!

IMBECILE !!!!!!!!!!!!!!

( Snip rest of this LYING MORONS abominable BULL**** !! )

FACT:

Heating the tube with hot air drives off any trapped moisture inside the
octal base.

Moisture is readily trapped in the residual flux use to solder the pins.

The flux then becomes highly conductive.

End of story.



...... Phil

[Bob Eld]

"Phil Allison" wrote in message
...

"Alex"

I blew a heat gun

** Yet again, this LYING **** hides what he actually used.

on the top of the tube envelope (the tube powered in circuit with grid
current being monitored) in attempt to activate the getter.


** Beyond even the stupidest possible idea.

The getter is INSIDE a ****ING VACUUM !!

Vacuums DO NOT CONDUCT HEAT !!!!!!!!!!

IMBECILE !!!!!!!!!!!!!!

( Snip rest of this LYING MORONS abominable BULL**** !! )

FACT:

Heating the tube with hot air drives off any trapped moisture inside the
octal base.

Moisture is readily trapped in the residual flux use to solder the pins.

The flux then becomes highly conductive.

End of story.



..... Phil

Hey Phyllis, It's time to go back on your meds. Leave the bloke alone, he
did the experiments and the results are as predicted. Your Tourettes
outbursts are not particularly helpful and say a lot more about you that the
people you call vile names. Now crawl back under your rock!

[Phil Allison[_3_]]

"Bob Eld"

** **** off and die to you stupid ******.

[Ian Iveson]

Phil Allison wrote:

I blew a heat gun

** Yet again, this LYING **** hides what he actually
used.

Looks to me like he used a heat gun.

on the top of the tube envelope (the tube powered in
circuit with grid current being monitored) in attempt to
activate the getter.


** Beyond even the stupidest possible idea.

The getter is INSIDE a ****ING VACUUM !!

The getter *was* inside the valve before it was flashed onto
the inside of the glass.

Vacuums DO NOT CONDUCT HEAT !!!!!!!!!!

Glass conducts heat. The getter flash gets hot.

......!!!!!!!!!!!!!!...

Snipped out of spite

Heating the tube with hot air drives off any trapped
moisture inside the octal base.

Moisture is readily trapped in the residual flux use to
solder the pins.

The flux then becomes highly conductive.

End of story.

Nice story, but not very well told. The punch line would be
better at the end instead of the beginning.

Alex's experiment doesn't prove anything, especially as the
valve was operating at the time and so getting heated at
both ends, and in the middle.

For whoever asked why heating in an oven is better than
normal valve operation, it's because gassy valves don't *do*
normal operation. By the time the getter material gets hot,
the valve may have been damaged by the flow of ions, at a
guess.

Ian

[Lord Valve]

Phil Allison wrote:

Vacuums DO NOT CONDUCT HEAT !!!!!!!!!!

Yeah - that's why when the sun comes up in the morning, it gets colder
outside...

IMBECILE !!!!!!!!!!!!!!

Do tell...

Lord Valve
Lying criminally insane autistic septic ****wit (thought I'd
save you a few keystrokes, Phillykins...).

[Phil Allison[_3_]]

"Lord Valve is a MASSIVE ******"


Phil Allison wrote:

Vacuums DO NOT CONDUCT HEAT !!!!!!!!!!

Yeah - that's why when the sun comes up in the morning, it gets colder
outside...


** So Willie boy thinks the sun's radiation reaches earth by CONDUCTION
????

Via the mysterious EATHER no doubt !!!!

Or that the OP used a radiant heat source and NOT hot air as he claimed ?

Wot a ****ing, tenth witted BULL****TING moron.



..... Phil

[Alex]

"Alex" wrote in message
...
I blew a heat gun on the top of the tube envelope (the tube powered in
circuit with grid current being monitored) in attempt to activate the
getter. After approximately 20 seconds of blowing grid current shoots up
by 50%, and at this stage I withdrew the heat gun. Surprisingly the
current dropped far below its initial value.

Then in a couple of minutes I would repeat the procedure. again current
goes up by 20%, then down by 50%. After several such themperature cyclings
the current dropped to nearly 150nA. (And it was above 50uA in the
beginning!!!). This is quite good. Again, can not be bothered trying
further.

Now the tube is simply running in normal mode (about 19W plate
dissipation), and the current is not rising. Instead it is even slowly
sort of declining.

So your advice was very useful, since it obviously reflected correctly the
fault nature and the method of fixing by activating the getter by warming
it. I quite believe that your cooking technique would also give good
results, likely even better because of degassing all the elements of the
valve.

I retested the tube next day, and the current increased to 1uA, then in 15
minutes went back down by a decimal order. It might suggest that gasses are
constantly leaking into the tube. Long periods of non operation might make
the tube troublesome again. I guess it needs to be discarded and replaced.

Alex

[Ian Iveson]

Alex said:

I retested the tube next day, and the current increased to
1uA, then in 15 minutes went back down by a decimal order.
It might suggest that gasses are constantly leaking into
the tube. Long periods of non operation might make the
tube troublesome again. I guess it needs to be discarded
and replaced.

Seems to me more likely that, if gas is the problem, then it
is escaping from the metal bits inside the valve. If enough
entered the valve from outside overnight to cause a problem,
then surely the valve would be full of air by now? I suppose
it could be that it only gets in when the valve is hot.

Some metals can hold *lots* of gas, in one form or another.
The oven treatment works, presumably, because the gas is
driven out of the metal and consumed by the getter flash at
that temperature, without the complications introduced by an
electric field or the damage done by ion bombardment.

If the outgassing declines sufficiently before the getter is
done getting, then the valve may be OK.

Phil's hypothesis might be worth testing. It would be easy
to try considering you have the requisite heat gun.

Ian

[Farticus]

"diamondsouled" wrote in message
...
There is a way of waking up the getter flash in tubes such as this, it
is actually quite common in Tung Sol 6550s for them to test gassy/grid
leakage. Some people have built rigs where they can put enough voltage/
cuurent into the tube, gradually, that in will activate the getter
flash. Here's an alternative method that can work:

Here's how to bake one:

1. Put the cold tube(s) in a COLD oven; set the oven thermostat to 300
F.

2. Bake the tubes for 4-5 hours at this temp.

3. Turn the oven off and walk away. Don't peek! Let it cool very
gradually.

4. When the oven is completely cooled take the tubes out and retest.
They will likely be fine as the getter will have absorbed gas
molecules - but...

DO NOT SKIP THE RETEST!!

Some tubes may have some cathode poisoning too - the baking may help,
but just heating the cathode is often enough to remedy the situation.
For that a tube tester is fine - but you don't know what's going on
inside the tube so it's best to be safe. Let them sit in the tester
while after the baking process is complete, don't rush. .

Of course if there is little or no getter flash left there might be
nothing left to activate. ;^)

Cheers

Larry

.... add a teaspoon of bicarbonate of soda and a pinch of salt to taste!

[Patrick Turner]

On Jan 29, 6:09*pm, "Alex" wrote:
Hello,

One of the two 6CA7 tubes in an amplifier is faulty. It has a bad grid
leakage reaching 50uA or so. Leakage current rises gradually as the tube
warms up.

My first question is what causes it, a flake of oxide lodged between the
grid and cathode or something else? (Visually the tube is very "clean" with
no signs of overheating, no dark or shiny deposits on the inside of the
envelope, no whiteness on the getter).

And the second question, of course, is it possible to "repair" this fault,
or the tube is destined to a rubbish bin.

Regards,
Alex

If you have 50uA of grid current at idle then you'd see +5V across a
100k biasing resistance.

Phil mentions it could be socket leakage currents, but I have found it
usually isn't the cause because when you plug a new tube into the same
socket there is not +5V across the 100k bias R, and in fact its usual
to see a slight negative voltage across the 100k. ie, a current flow
opposite to what you are seeing.

Leaky coupling caps can also cause the same problem, and a replacement
new tube will give the same results as seen on the old tube if this is
the problem.

Mostly the "positive grid" syndrome is in mainly old tubes nearing the
end of their life. Typically, soon after turn on there is a slow
increase in positive voltage at the grid which then settles to a
value. The 5 volt increase in grid voltage might increase anode
current by 25mA if the effective working gm of the tube is 5mA/V. If
Ea = 400V, then the increase in Pda might be 400V x 25mA = 10W which
may begin toturn the anodes cherry red. The more the tubes heat up,
the more of this "reverse grid current" one may see at idle. Sooner
rather than later the tube will begin to rebel against the grid's
control of anode current and when that happens the tube will go into
thermal runaway and maybe melt its glass or warp its internal
electodes causing a short and all during this trauma maybe the PT and
OPT can become overheated and windings can develop very expensive
faults.

If you have Eg at +5V more than the voltage bias source, then the tube
is about stuffed, ****ed, worn out, and in need of replacement. I bet
that if you look at the gettering it will have "smokey dull patches"
or brown edges or be faded, and in general much less shiny and bright
like it is in a new tube; ie, the tube is a bit gassy.

I recommend always replacing OP tubes where the Ig 20uA. Where you
have cathode biasing, the regulation of bias by the Rk does not save
the tube, and Ek just can rise alarmingly as the grid begins to loose
control of Ia.

In many old Quad-II amps which are owned by old curmudgeonly misers
who hate to ever spend any money on maintenance on anything there are
KT66 tubes Eg might be over +4V above the bias voltage. And the bias
Rk in Quad-II are a rather too high 680k, and 50uA Ig would generate
34Vdc across Rk. But where I see 4V across the 680k, or where Ig is
thus only 6uA, the tubes are not healthy. I have altered Quad-II amps
to use separate RC cathode biasing networks as exist in Quad-40, and
this helps the problem a little, but really Rg should NEVER be more
than 220k in any large octal output tube, and preferably 100k max. Of
course where Rg is just lowered from say 680k to say 100k, you may
upset the working parameters of the driving tubes especially if they
are gutless wonders like EF86 with only 0.5 mA of idle current.
Reducing the Rg to 100k in a Quad-II amp would more than halve the
gain of the EF86 and thus more than halve the effective applied NFB
and may cause distortion because you limit the amount of un-clipped
voltage able to be produced to below what is needed to drive the OP
KT66, AND you shorten the time constant between the 0.iuF caps driving
the Rg, so maybe LF stability is ruined.
However, if driver circuitry is altered to cope with the lower Rg
value, then Eg will be much lower than where Rg was originally say
680k, and the tube may be sreviceable for a few more months or years.

One may wonder why grids go positive. Some say positive ions in the
tube collect and attach themselves after the tube warms up and thus
the grid becomes an electron absorber like the anode, albeit in a
rather diminished manner.
Some say its leakage current across the mica support structure within
the tube, especially between the screen support rods and grid support
rods. The distance is minimal. Some makers have a cut slot between
these rods in the mica to minimise leakage.

But I am not aware of any way to improve the action of the gettering
once it becomes "filled up" with gass molecules.

Input signal tubes like 6SN7 can also have the problem of positive
grids.
All the samples of such tubes I have tested were also noisy, often
microphonic due to loosening of the metal structures within the mica
discs and there is only one place for them, into the rubbish bin.

Patrick Turner.

[Phil Allison[_3_]]

"Patrick Turner"


If you have 50uA of grid current at idle then you'd see +5V across a
100k biasing resistance.

Phil mentions it could be socket leakage currents,

** No he did not - you pathetic trouble maker.

Phil said it was often the result of leakage in the TUBE's OWN BASE !!


but I have found it
usually isn't the cause because when you plug a new tube into the same
socket there is not +5V across the 100k bias R,

** Irrelevant.


.... Phil

[Lord Valve]

Phil Allison snarled:

Phil said it was often the result of leakage in the TUBE's OWN BASE !!

Does this negatively impact base response?

Lord Valve
Pathetic wanking criminally insane autistic
bull****ting septic ****wit...

[Bob Eld]

"Lord Valve" wrote in message
...
Phil Allison snarled:

Phil said it was often the result of leakage in the TUBE's OWN BASE !!

Does this negatively impact base response?

Lord Valve
Pathetic wanking criminally insane autistic
bull****ting septic ****wit...

Yes, since the base leaks it obviously can't get to the woofer so the base
response must be affected. Duh! It probably leaks out and lays around on the
floor.

Bob the ******

[Ian Iveson]

Phil Allison wrote?

If you have 50uA of grid current at idle then you'd see
+5V across a
100k biasing resistance.

Phil mentions it could be socket leakage currents,

** No he did not - you pathetic trouble maker.

Phil said it was often the result of leakage in the TUBE's
OWN BASE !!


but I have found it
usually isn't the cause because when you plug a new tube
into the same
socket there is not +5V across the 100k bias R,

** Irrelevant.

Patrick may lack the empathy to bother reading properly, but
he has a point all the same, as you know. Current flowing
through the valve would have consequences different from
that flowing externally between pins.

The effects would depend partly on the grid, screen and
cathode resistors. Alex is good enough at the maths to
invent a test that would distinguish between the two mooted
causes, I'm sure. Don't know why he hasn't already.

Neither do I know why he is considering binning the valve
without even trying your remedy. Are you both in the
Antipodes? Maybe if you live close enough, each to the
other, he could give it to you instead?

Alex, it only takes a moment to put the valve in the oven.
It's not like you have to stand there 'til it's cooked.

Ian

[Alex]

"Ian Iveson" wrote in message
...
Phil Allison wrote?

If you have 50uA of grid current at idle then you'd see +5V across a
100k biasing resistance.

Phil mentions it could be socket leakage currents,

** No he did not - you pathetic trouble maker.

Phil said it was often the result of leakage in the TUBE's OWN BASE !!


but I have found it
usually isn't the cause because when you plug a new tube into the same
socket there is not +5V across the 100k bias R,

** Irrelevant.

Patrick may lack the empathy to bother reading properly, but he has a
point all the same, as you know. Current flowing through the valve would
have consequences different from that flowing externally between pins.

The effects would depend partly on the grid, screen and cathode resistors.
Alex is good enough at the maths to invent a test that would distinguish
between the two mooted causes, I'm sure. Don't know why he hasn't already.

Neither do I know why he is considering binning the valve without even
trying your remedy. Are you both in the Antipodes? Maybe if you live close
enough, each to the other, he could give it to you instead?

Alex, it only takes a moment to put the valve in the oven. It's not like
you have to stand there 'til it's cooked.

Ian

Hello Ian,
Actually it was Larry who suggested cooking the valve at 300C for 4-5 hours
in the oven. I have not done that, but was heating the top of the valve by a
heat gun when the tube was working. In this case instead of runaway I saw
the grid current declining. Several "cycles" reduced grid current 100 times.

[Ian Iveson]

Alex said:

Hello Ian,
Actually it was Larry who suggested cooking the valve at
300C for 4-5 hours in the oven. I have not done that, but
was heating the top of the valve by a heat gun when the
tube was working. In this case instead of runaway I saw
the grid current declining. Several "cycles" reduced grid
current 100 times.

Yes, but it was Phil who suggested heating the base as a
cure for the hypothetical grid/screen leak.

Do try to keep up :-)

Just switched my amps on for the first time in a while coz
I've been using my PC speakers whilst looking for new stuff
to listen to. Two of the output valves on the same PP side
of one channel are much more extensively and brightly blue
than they were, so I guess I should find out why. One
problem arising from my over-engineered power supply is that
unbalanced bias current doesn't result in hum, so there is
no obvious audible early warning of valve failure.

AFAICS, the blue is still clinging to the glass, but thicker
than it was, and the space between the electrodes is still
clear. Info on this phenomenon is quite scarce, but my
limited experience suggests that it increases with anode
voltage, and has no effect on current. The 6CH6 in
particular can be quite spectacular because it has big
cut-outs in the anode that let electrons out. I guess the
faster ones reach the glass.

Anyway, why might the blue have got brighter? Low current
would increase the voltage a bit, but then they should all
be glowing more brightly. Perhaps they are, and the
brightest then are still the brightest now, but more so.

The best parts of my amps by far are the output
transformers, so I'd better check anyway...

Ian

[Patrick Turner]

On Feb 1, 11:24*am, "Ian Iveson"
wrote:
Phil Allison wrote?





If you have 50uA of grid current at idle then you'd see
+5V across a
100k biasing resistance.

Phil mentions it could be socket leakage currents,

** No he did not *- *you pathetic trouble maker.

Phil said it was often the result of leakage in the TUBE's
OWN *BASE !!

but I have found it
usually isn't the cause because when you plug a new tube
into the same
socket there is not +5V across the 100k bias R,

** *Irrelevant.

Patrick may lack the empathy to bother reading properly, but
he has a point all the same, as you know. *Current flowing
through the valve would have consequences different from
that flowing externally between pins.

The effects would depend partly on the grid, screen and
cathode resistors. Alex is good enough at the maths to
invent a test that would distinguish between the two mooted
causes, I'm sure. Don't know why he hasn't already.

Neither do I know why he is considering binning the valve
without even trying your remedy. Are you both in the
Antipodes? Maybe if you live close enough, each to the
other, he could give it to you instead?

Alex, it only takes a moment to put the valve in the oven.
It's not like you have to stand there 'til it's cooked.

Ian- Hide quoted text -

- Show quoted text -

If the tube base has become partially carbonised due to heat cycles of
age and use then what difference will cooking the tube make?

I would think that if one were to just clean around the grid pin with
wet&dry sand paper then any surface pollution or carbon layer would be
removed and the low resistance between sat pin 4 and 5 would go high.

If there was a 400Vdc difference between pin 4 and 5, then with 0.05mA
the R = 400 / 0.05 = 8,000k or 8 megohms.

Most new octal tube bases ( and the sockets ) have resistances between
pins much higher than 8M.

Thanks to Phil and others here I have learnt something. So next time I
have to fiddle around with an amp which uses octal output tubes I will
plug a couple of old 6550 into the amp. I have a few here which
*definately* have the very problem the OP described, ie, a +ve few
volts appearing at the grid relative to the bias supply.

[Patrick Turner]

On Jan 31, 4:01*pm, "Phil Allison" wrote:
"Patrick Turner"

If you have 50uA of grid current at idle then you'd see +5V across a
100k biasing resistance.

Phil mentions it could be socket leakage currents,

** No he did not *- *you pathetic trouble maker.

Phil said it was often the result of leakage in the TUBE's *OWN *BASE !!

but I have found it
usually isn't the cause because when you plug a new tube into the same
socket there is not +5V across the 100k bias R,

** *Irrelevant.

... *Phil

Ok, point taken.

But what does one do if the leakiness is due to pollution *inside* the
bakelite base?
Probably this is unlikely because the volume between the base inside
and tube is sealed.

It would be very difficult to remove the base and clean it and then
replace it.

I'd have to say that not many tube bases become leaky.

I don't recall that cleaning up the bottom of bases of tubes removes
the problem.

But pin 4 IS CLOSE to pin 5 so the leaky path is most likely between
these pins.

I have found the only tubes that have become leaky as the OP suggested
are OLD tubes with many hours and with gettering that shows oxidation
and discolor, and that the problem is more likely something happening
inside the tube.

Many ppl have blamed sockets for the phenomenon. The sockets remain
uncleaned after several tube replacements.

So when a new tube is inserted, and there is no large grid current,
then it shows the socket is not to blame.

Patrick Turner.

[Phil Allison[_3_]]

"Patrick Turner"
"Phil Allison"

Phil mentions it could be socket leakage currents,

** No he did not - you pathetic trouble maker.

Phil said it was often the result of leakage in the TUBE's OWN BASE !!

but I have found it
usually isn't the cause because when you plug a new tube into the same
socket there is not +5V across the 100k bias R,

** Irrelevant.

Ok, point taken.

But what does one do if the leakiness is due to pollution *inside* the
bakelite base?

** That is what I posed about.

Probably this is unlikely because the volume between the base inside
and tube is sealed.

** Nope.


It would be very difficult to remove the base and clean it and then
replace it.

** EL34s not usually have an airtight seal between the base and the glass
plus there are holes where pins are missing in the base.

I'd have to say that not many tube bases become leaky.

** Seen it with some new EL34s sold under various brands.

The fix is to soak the tube, base down, in a cup of methylated spirits to
remove the solder flux that the makers left behind. This may take a couple
of times using fresh metho to get it really clean.

Then dry with hot air.

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


..... Phil

[mick]

On Wed, 03 Feb 2010 10:14:57 +1100, Phil Allison wrote:

snip

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



Is this a relatively recent problem, Phil? 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 could well believe that it's something to do with
the production methods & materials that are being used now.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

[Alex]

"Patrick Turner" wrote in message
...
On Jan 29, 6:09 pm, "Alex" wrote:
Hello,

One of the two 6CA7 tubes in an amplifier is faulty. It has a bad grid
leakage reaching 50uA or so. Leakage current rises gradually as the tube
warms up.

My first question is what causes it, a flake of oxide lodged between the
grid and cathode or something else? (Visually the tube is very "clean"
with
no signs of overheating, no dark or shiny deposits on the inside of the
envelope, no whiteness on the getter).

And the second question, of course, is it possible to "repair" this fault,
or the tube is destined to a rubbish bin.

Regards,
Alex

If you have 50uA of grid current at idle then you'd see +5V across a
100k biasing resistance.

Alex:
In fact I had 15V across 300K. I stopped the "test", asked question at this
forum and later managed to reduce this current 100 times by several thermal
cyclings with a heat gun. I feel it is not a permanent fix. After some
period of inactivity the tube develops some smaller grid current, but now it
does not go into runaway, but goes down back to 500nA or less during 10
minutes. I explain it by the getter warming up and doing its job better.

Partrick:
If you have Eg at +5V more than the voltage bias source, then the tube
is about stuffed, ****ed, worn out, and in need of replacement. I bet
that if you look at the gettering it will have "smokey dull patches"
or brown edges or be faded, and in general much less shiny and bright
like it is in a new tube; ie, the tube is a bit gassy.

Alex:
The tube is surprisingly clean and clear with no anode burnt spots and no
deposits neither dard nor silvery on the glass.

Patrick:
I recommend always replacing OP tubes where the Ig 20uA. Where you
have cathode biasing, the regulation of bias by the Rk does not save
the tube, and Ek just can rise alarmingly as the grid begins to loose
control of Ia.

Alex:
I would never use such a dude myself. But this is an old 50W public address
amplifier which I am selling. I undertook the tube "fixing" out of
curiosity. I will warn a buyer to test and/or replace tubes if he intends to
use the amp other than for salvaging the transformers (Ferguson /
Henderson). (By the way it has got a microphone step up transformer as
well.)