Mike Knudsen wrote:

Can someone explain how a tube can show good cathode emission, but not be up to
snuff in transconductance? A tube's performance depends on teh geometry and
structure of its grids and plate and their spacing. If hte tube hasn't been
physically knocked around, the geometry hasn't changed. So if the cathode is
putting out the electroncs, how can the tube not be up to snuff?

Let's say you have a beam power tube that was used on an amplifier with
a screen grid supply problem, and the screen grid drew current and burned
completely away. There's nothing left any more.

That tube will test just fine on an emission tester. The cathode is putting
out plenty of electrons, but the tube Gm and mu are way off because the
geometry itself has been radically altered. It tests fine, but it will
not even come close to working in a circuit.

Okay, it's an extreme example, but plenty of things go wrong with tubes
other than the cathode getting stripped.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article , (Scott Dorsey)
writes:

Let's say you have a beam power tube that was used on an amplifier with
a screen grid supply problem, and the screen grid drew current and burned
completely away. There's nothing left any more.

Agreed. Worse yet, a 6146 that got overdriven and the control grid has bruned
away. No gain at all, but good emission.

But I did ask "other than physical abuse", and maybe "electrical abuse" should
have been included as well. Is it possible for a non-abused (in any way) tube
to lose gain and performance, while its emission remains good?

Not including gas, which I believe the cheap tube testers can catch.

BTW, I once owned a little grid emission tester, whihc I have seen mentioned
here once or twice. Grid emission will kill your AGC line. --Mike K.

Oscar loves trash, but hates Spam! Delete him to reply to me.

Mike Knudsen wrote:
In article , (Scott Dorsey)
writes:

Let's say you have a beam power tube that was used on an amplifier with
a screen grid supply problem, and the screen grid drew current and burned
completely away. There's nothing left any more.

Agreed. Worse yet, a 6146 that got overdriven and the control grid has bruned
away. No gain at all, but good emission.

But I did ask "other than physical abuse", and maybe "electrical abuse" should
have been included as well. Is it possible for a non-abused (in any way) tube
to lose gain and performance, while its emission remains good?

Sure, because you can get less dramatic examples of that sort of thing.
Grids that sag a little bit, for instance. Yes, it's a mechanical failure,
but it's a common one and it's not necessarily due to abuse. Deformed plates
on power tubes are common too.

Not including gas, which I believe the cheap tube testers can catch.

Yes.

BTW, I once owned a little grid emission tester, whihc I have seen mentioned
here once or twice. Grid emission will kill your AGC line. --Mike K.

Somewhere I have a free tube tester that Chicago Transformer gave me as a
promotional item. It puts the tube filament across the AC line, with a
neon bulb and resistor in series. If the filament is good, the neon bulb
lights up, and the lamp doesn't pass enough current to light up, let alone
damage, the filament. Totally useless, but a nifty conversation piece.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Mike Knudsen wrote:
In article , (Scott Dorsey)
writes:

Let's say you have a beam power tube that was used on an amplifier with
a screen grid supply problem, and the screen grid drew current and burned
completely away. There's nothing left any more.

Agreed. Worse yet, a 6146 that got overdriven and the control grid has bruned
away. No gain at all, but good emission.

But I did ask "other than physical abuse", and maybe "electrical abuse" should
have been included as well. Is it possible for a non-abused (in any way) tube
to lose gain and performance, while its emission remains good?

Sure, because you can get less dramatic examples of that sort of thing.
Grids that sag a little bit, for instance. Yes, it's a mechanical failure,
but it's a common one and it's not necessarily due to abuse. Deformed plates
on power tubes are common too.

Not including gas, which I believe the cheap tube testers can catch.

Yes.

BTW, I once owned a little grid emission tester, whihc I have seen mentioned
here once or twice. Grid emission will kill your AGC line. --Mike K.

Somewhere I have a free tube tester that Chicago Transformer gave me as a
promotional item. It puts the tube filament across the AC line, with a
neon bulb and resistor in series. If the filament is good, the neon bulb
lights up, and the lamp doesn't pass enough current to light up, let alone
damage, the filament. Totally useless, but a nifty conversation piece.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article , (Scott Dorsey)
writes:

Let's say you have a beam power tube that was used on an amplifier with
a screen grid supply problem, and the screen grid drew current and burned
completely away. There's nothing left any more.

Agreed. Worse yet, a 6146 that got overdriven and the control grid has bruned
away. No gain at all, but good emission.

But I did ask "other than physical abuse", and maybe "electrical abuse" should
have been included as well. Is it possible for a non-abused (in any way) tube
to lose gain and performance, while its emission remains good?

Not including gas, which I believe the cheap tube testers can catch.

BTW, I once owned a little grid emission tester, whihc I have seen mentioned
here once or twice. Grid emission will kill your AGC line. --Mike K.

Oscar loves trash, but hates Spam! Delete him to reply to me.