[[email protected]]

How hot is too hot for an EL84?

I've got a second Caliphone phono, this one is "newer" and uses an EL84
in the single-ended output. And man, does it run hot. A minute after
turn-on it smells like an Easy-Bake oven and is way way way too hot to
touch even after cooling off for a few minutes.

I'm mostly used to older larger octal output tubes, where even if the
plate is glowing dull red the glass is cooler than this EL84.

Will check for proper bias etc. tonight I guess. What would be a
nominal plate current, 40 or 50mA at 250V? That'd be 10 or 12 watts and
this smells way hotter!

Tim.

[Eeyore]

wrote:

How hot is too hot for an EL84?

I've got a second Caliphone phono, this one is "newer" and uses an EL84
in the single-ended output. And man, does it run hot. A minute after
turn-on it smells like an Easy-Bake oven and is way way way too hot to
touch even after cooling off for a few minutes.

I'm mostly used to older larger octal output tubes, where even if the
plate is glowing dull red the glass is cooler than this EL84.

Will check for proper bias etc. tonight I guess. What would be a
nominal plate current, 40 or 50mA at 250V? That'd be 10 or 12 watts and
this smells way hotter!

If the plate's red hot, it's certainly too much !

Graham

[Paul Sherwin]

On Thu, 30 Nov 2006 09:42:05 -0800, shoppa wrote:

How hot is too hot for an EL84?

I've got a second Caliphone phono, this one is "newer" and uses an EL84
in the single-ended output. And man, does it run hot. A minute after
turn-on it smells like an Easy-Bake oven and is way way way too hot to
touch even after cooling off for a few minutes.

I'm mostly used to older larger octal output tubes, where even if the
plate is glowing dull red the glass is cooler than this EL84.

Will check for proper bias etc. tonight I guess. What would be a
nominal plate current, 40 or 50mA at 250V? That'd be 10 or 12 watts and
this smells way hotter!

They do run hot, but it sounds as if you may have a fault condition. Check
the grid coupling capacitor. If you have +ve volts on the EL84 grid the
cap is probably bad and the tube is being overrun.

Paul

[John Stone]

On 11/30/06 11:42 AM, in article
,
" wrote:

How hot is too hot for an EL84?

I've got a second Caliphone phono, this one is "newer" and uses an EL84
in the single-ended output. And man, does it run hot. A minute after
turn-on it smells like an Easy-Bake oven and is way way way too hot to
touch even after cooling off for a few minutes.

I'm mostly used to older larger octal output tubes, where even if the
plate is glowing dull red the glass is cooler than this EL84.

Will check for proper bias etc. tonight I guess. What would be a
nominal plate current, 40 or 50mA at 250V? That'd be 10 or 12 watts and
this smells way hotter!

They run pretty hot normally, but plates or screens absolutely shouldn't be
glowing. I'm assuming a cathode biasing scheme so it's simple enough to
check the current via the voltage drop through the cathode resistor. 40-50ma
is a fair guess. German sets using an SE EL84 are in this range. Pull the
output tube and make sure you read 0 volts at the grid pin. A leaky coupling
cap can cause this problem.

[Superhet]

In normal operation it would be way to hot to touch unless you have a
large letter S on your shirt. Make sure the coupling cap to the grid is
a new Mylar type, not an old paper cap. And by all means check the idle
plate current.

wrote:
How hot is too hot for an EL84?

I've got a second Caliphone phono, this one is "newer" and uses an EL84
in the single-ended output. And man, does it run hot. A minute after
turn-on it smells like an Easy-Bake oven and is way way way too hot to
touch even after cooling off for a few minutes.

I'm mostly used to older larger octal output tubes, where even if the
plate is glowing dull red the glass is cooler than this EL84.

Will check for proper bias etc. tonight I guess. What would be a
nominal plate current, 40 or 50mA at 250V? That'd be 10 or 12 watts and
this smells way hotter!

Tim.

[[email protected]]

Superhet wrote:
In normal operation it would be way to hot to touch unless you have a
large letter S on your shirt. Make sure the coupling cap to the grid is
a new Mylar type, not an old paper cap.

In fact the cap is a ceramic :-). This is not exactly hi-fi as we would
define it today...

And by all means check the idle
plate current.

It's dropping 10V across the 150ohm cathode resistor, for 66mA. No
leakage in the grid cap. Schematic says there is supposed to be 8V
across the cathode resistor.

320V on the plate*66mA = 21W, probably not the most reliable operating
point. Even if it was 8V on the cathode resistor it would be nearly 20W
dissipation in that tube.

I didn't know that I was buying an Easy-Bake oven at the same time I
bought this. You really could bake muffins next to that EL84!

Tim.

[Superhet]

320v on the plate is on the high side. Tube manual calls for 300v max
and typ op at 250v with 48 ma zero signal plate current. I would
increase the cathode resistor to lower the plate current. That said the
max ratings are 300v plate and 65 ma cathode current so if you want to
run this at max you are there.

wrote:
Superhet wrote:
In normal operation it would be way to hot to touch unless you have a
large letter S on your shirt. Make sure the coupling cap to the grid is
a new Mylar type, not an old paper cap.

In fact the cap is a ceramic :-). This is not exactly hi-fi as we would
define it today...

And by all means check the idle
plate current.

It's dropping 10V across the 150ohm cathode resistor, for 66mA. No
leakage in the grid cap. Schematic says there is supposed to be 8V
across the cathode resistor.

320V on the plate*66mA = 21W, probably not the most reliable operating
point. Even if it was 8V on the cathode resistor it would be nearly 20W
dissipation in that tube.

I didn't know that I was buying an Easy-Bake oven at the same time I
bought this. You really could bake muffins next to that EL84!

Tim.

[[email protected]]

Superhet wrote:
320v on the plate is on the high side. Tube manual calls for 300v max
and typ op at 250v with 48 ma zero signal plate current. I would
increase the cathode resistor to lower the plate current. That said the
max ratings are 300v plate and 65 ma cathode current so if you want to
run this at max you are there.

The schematic calls for 320V on the plate. In real life I'm getting
closer to 340V at idle. Probably something to do with 120VAC in instead
of a design target of 110VAC.

In any event, I just tweaked the screen grid dropping resistors (they
had drifted down from 6.8K to closer to 4K, unusual for carbon comps)
to about 8K and now I am getting closer to 40mA plate current, a number
that I'm much happier with. Washing off some of the dust probably
helped get rid of the Easy-Bake oven smell too :-).

Tim.

[Superhet]

The proper screen current will be kinder to your tube and power supply
as well as audio quality.

wrote:
Superhet wrote:
320v on the plate is on the high side. Tube manual calls for 300v max
and typ op at 250v with 48 ma zero signal plate current. I would
increase the cathode resistor to lower the plate current. That said the
max ratings are 300v plate and 65 ma cathode current so if you want to
run this at max you are there.

The schematic calls for 320V on the plate. In real life I'm getting
closer to 340V at idle. Probably something to do with 120VAC in instead
of a design target of 110VAC.

In any event, I just tweaked the screen grid dropping resistors (they
had drifted down from 6.8K to closer to 4K, unusual for carbon comps)
to about 8K and now I am getting closer to 40mA plate current, a number
that I'm much happier with. Washing off some of the dust probably
helped get rid of the Easy-Bake oven smell too :-).

Tim.

[thomas]

wrote:
How hot is too hot for an EL84?

I've got a second Caliphone phono, this one is "newer" and uses an EL84
in the single-ended output. And man, does it run hot. A minute after
turn-on it smells like an Easy-Bake oven and is way way way too hot to
touch even after cooling off for a few minutes.

What is the model number on your califone?

[robert casey]

How hot is too hot for an EL84?

I've got a second Caliphone phono, this one is "newer" and uses an EL84
in the single-ended output. And man, does it run hot. A minute after
turn-on it smells like an Easy-Bake oven and is way way way too hot to
touch even after cooling off for a few minutes.


The tube itself or the cabinet of it?