[Roger Jones]

Hi, Vacuumlanders,
I just came across a very dead EL84/6BQ5 (from a P-P audio amplifier
with a leaky coupling cap to its g1 - take note!) Just for interest, I
cut the glass off just above the base with a Dremel-type cutting disk
(use glasses and gloves!) Then, I pulled off the plate assembly to
look at the electrodes.
The RCA tube manual calls this tube a "Beam Power Tube" but there are
NO beam plates, rather just a lose spiral grid 3. (Indeed, the word
tetrode is NOT used by RCA) But to me it looks like a power pentode.
Perhaps the rather open g3 spiral and supports may cause electron
beams, but it's not clear to me how this would happen.
Any comments on this and the name?
Cheers,
Roger

[[email protected]]

On Sunday, March 18, 2012 11:28:10 PM UTC-4, Roger Jones wrote:
Hi, Vacuumlanders,
I just came across a very dead EL84/6BQ5 (from a P-P audio amplifier
with a leaky coupling cap to its g1 - take note!) Just for interest, I
cut the glass off just above the base with a Dremel-type cutting disk
(use glasses and gloves!) Then, I pulled off the plate assembly to
look at the electrodes.
The RCA tube manual calls this tube a "Beam Power Tube" but there are
NO beam plates, rather just a lose spiral grid 3. (Indeed, the word
tetrode is NOT used by RCA) But to me it looks like a power pentode.
Perhaps the rather open g3 spiral and supports may cause electron
beams, but it's not clear to me how this would happen.
Any comments on this and the name?
Cheers,
Roger

Who is the maker/date of this specimen?

[Phil Allison[_3_]]

"Roger Jones"

I just came across a very dead EL84/6BQ5 (from a P-P audio amplifier
with a leaky coupling cap to its g1 - take note!) Just for interest, I
cut the glass off just above the base with a Dremel-type cutting disk
(use glasses and gloves!) Then, I pulled off the plate assembly to
look at the electrodes.

The RCA tube manual calls this tube a "Beam Power Tube" but there are
NO beam plates, rather just a lose spiral grid 3. (Indeed, the word
tetrode is NOT used by RCA) But to me it looks like a power pentode.


** It's a patent issue.

The EL34 and EL84 are both "power pentodes" with original patents held by
Mullard and Philips.

RCA made a beam tube equivalent of the EL34 and called it the 6CA7.

Later, other makers labelled EL34s as 6CA7s or used both numbers - eg AWV
here in Australia.

Makers other than RCA labelled EL84s as 6BQ5s - to increase sales.

If you ever find an actual RCA brand 6BQ5 from the 1950s - lemme know if it
has beam plates.

Whacky.



.... Phil

[Roger Jones]

On Mar 19, 5:19*am, wrote:
On Sunday, March 18, 2012 11:28:10 PM UTC-4, Roger Jones wrote:
Hi, Vacuumlanders,

(snip)

Roger

Who is the maker/date of this specimen?

Can't tread it on the (now broken) glass! I can just see "6BQ5" and
"Canada".
Phil, thanks for the EL34/6CA7 comments.
Cheers,
Roger

[Alex Pogossov]

"Roger Jones" wrote in message
...
Hi, Vacuumlanders,
I just came across a very dead EL84/6BQ5 (from a P-P audio amplifier
with a leaky coupling cap to its g1 - take note!) Just for interest, I
cut the glass off just above the base with a Dremel-type cutting disk
(use glasses and gloves!) Then, I pulled off the plate assembly to
look at the electrodes.
The RCA tube manual calls this tube a "Beam Power Tube" but there are
NO beam plates, rather just a lose spiral grid 3. (Indeed, the word
tetrode is NOT used by RCA) But to me it looks like a power pentode.
Perhaps the rather open g3 spiral and supports may cause electron
beams, but it's not clear to me how this would happen.
Any comments on this and the name?
Cheers,
Roger

If I understand correctly, the main feature of a beam tetrode is the same
pitch and alignment of the control grid and screen grid. This creates the
beams and helps keep screen current low.

On the other hand, to keep the screen grid electron interception small
enough, the screen grid can not be very dense. Therefore the control grid is
also not dense. Thus the transconductance is not that high (per each mA of
plate current) -- probably about 10% (mA/V) of the plate current (mA): about
5mA/V per 50mA plate current.

If you want a high efficiency high-gm tube, the control grid has to be
tight, but the screen still can not be too tight. So the hi-gm beam tetrode
is impossible. It will be a pentode, though some pentodes can have
suppression plates or frame instead of a suppression grid.

EL84 has 11mA/V, a tight control grid and never can be a true beam tetrode,
even if some manufacturer fits supression plates instead of the third grid.

[Phil Allison[_3_]]

"Alex Pogossov"

If I understand correctly, the main feature of a beam tetrode is the same
pitch and alignment of the control grid and screen grid.


** That is a design feature of the POWER PENTODE too
- you retarded imbecile.



.... Phil

[Alex Pogossov]

"Phil Allison" wrote in message
...

"Alex Pogossov"

If I understand correctly, the main feature of a beam tetrode is the same
pitch and alignment of the control grid and screen grid.


** That is a design feature of the POWER PENTODE too
- you retarded imbecile.

Alex:

The above statement is either bull**** or incorrect.
In EL84, pitch of the screen grid is about 3 times larger than that of the
control grid.

But 6V6 and 6L6 have the same pitch.

[Phil Allison[_3_]]

"Alex Pogossov"
"Phil Allison"
"Alex Pogossov"

If I understand correctly, the main feature of a beam tetrode is the
same pitch and alignment of the control grid and screen grid.


** That is a design feature of the POWER PENTODE too
- you retarded imbecile.

The above statement is either bull**** or incorrect.


** Fraid it is an ABSOLUTE FACT - you retarded jerk.


In EL84, pitch of the screen grid is about 3 times larger than that of the
control grid.

** No way.

You must be looking at the *SUPPRESSOR* grid - internally connected to
the cathode.

In a power pentode, the screen has to sit in the "shadow" of the control
grid or else screen current goes through the roof.

As it happens, the screen current of the EL84 is particularly low.

Plus the EL34 and EL84 are very constructed very much alike.



..... Phil

[Alex Pogossov]

"Phil Allison" wrote in message
...

"Alex Pogossov"
"Phil Allison"
"Alex Pogossov"

If I understand correctly, the main feature of a beam tetrode is the
same pitch and alignment of the control grid and screen grid.


** That is a design feature of the POWER PENTODE too
- you retarded imbecile.

The above statement is either bull**** or incorrect.


** Fraid it is an ABSOLUTE FACT - you retarded jerk.


In EL84, pitch of the screen grid is about 3 times larger than that of
the control grid.

** No way.

You must be looking at the *SUPPRESSOR* grid - internally connected to
the cathode.


Alex:
I am looking at the screen grid. The suppressor grid has yet larger pitch
(1.5 ... 2 of the screen grid).

In a power pentode, the screen has to sit in the "shadow" of the control
grid or else screen current goes through the roof.

As it happens, the screen current of the EL84 is particularly low.

Alex:
It is low only because the screen grid is not that dense. If you look into a
hi-gm power video power pentode with gm = 20...30mA/V, it would have a frame
grid -- so fine, it is almost invisible. But the screen grid is quite widely
wound. Some of such pentodes would have only suppressor plates, not the
suppressor grid. It is because the distance between the screen and the plate
is long -- almost enough to suppress secondary emission by space charge
alone -- without or almost without suppressor electrodes.


Plus the EL34 and EL84 are very constructed very much alike.

[Phil Allison[_3_]]

"Alex Pogossov"

If I understand correctly, the main feature of a beam tetrode is the
same pitch and alignment of the control grid and screen grid.


** That is a design feature of the POWER PENTODE too
- you retarded imbecile.

The above statement is either bull**** or incorrect.


** Fraid it is an ABSOLUTE FACT - you retarded jerk.


In EL84, pitch of the screen grid is about 3 times larger than that of
the control grid.


** Post some pics.

Looks like the ratio is exactly 2:1 on the samples I have.

So the shadowing idea still applies.


In a power pentode, the screen has to sit in the "shadow" of the control
grid or else screen current goes through the roof.

As it happens, the screen current of the EL84 is particularly low.


It is low only because the screen grid is not that dense.


** Your ****ing mad assumption - not fact.

The EL84 simply has no beam plates, it is constructed like a power pentode.

End of story.



..... Phil

[Roger Jones]

On Mar 21, 9:40*am, "Phil Allison" wrote:
"Alex Pogossov"











If I understand correctly, the main feature of a beam tetrode is the
same pitch and alignment of the control grid and screen grid.

** That is a design feature of the *POWER PENTODE *too
- *you retarded imbecile.

The above statement is either bull**** or incorrect.

** *Fraid it is an ABSOLUTE FACT * - *you retarded jerk.

In EL84, pitch of the screen grid is about 3 times larger than that of
the control grid.

*** Post some pics.

* Looks like the ratio is exactly 2:1 on the samples I have.

* *So the shadowing idea still applies.

In a power pentode, the screen has to sit in the "shadow" of the control
grid or else screen current goes through the roof.

As it happens, the screen current of the EL84 is particularly low.

It is low only because the screen grid is not that dense.

** Your ****ing mad assumption *- *not fact.

*The EL84 simply has no beam plates, it is constructed like a power pentode.

*End of story.

.... *Phil

Jeez, guys, I did not intend to ignite WW3!
I still have the naked electrode assy. floating around on the bench...
will go and look for it... be right back!
OK, g3 (suppressor) is very open... a wide spiral, no more than 10
effective turns (but they bunch at the ends so total number is more...
say 15.)
g2 (screen) is much tighter than g3, but it's hard to see if it
shadows the g1 wires... No, it does not. g1 is wound much tighter
(more turns, shorter pitch.)
That's all. Hope this helps. Peace!
Cheers,
Roger.

[Phil Allison[_3_]]

"Roger Jones"
"Phil Allison"

The EL84 simply has no beam plates, it is constructed like a power
pentode.

End of story.


Jeez, guys, I did not intend to ignite WW3!


** But your original claim was false:

RCA say the 6BQ5 is a " power pentode ".

Here are 4 pages from the RCA 1959 data book.

http://www.classiccmp.org/rtellason/tubedata/6BQ5.PDF




..... Phil

[Patrick Turner]

On Mar 20, 7:13*pm, "Alex Pogossov" wrote:
"Roger Jones" wrote in message

...

Hi, Vacuumlanders,
I just came across a very dead EL84/6BQ5 (from a P-P audio amplifier
with a leaky coupling cap to its g1 - take note!) Just for interest, I
cut the glass off just above the base with a Dremel-type cutting disk
(use glasses and gloves!) Then, I pulled off the plate assembly to
look at the electrodes.
The RCA tube manual calls this tube a "Beam Power Tube" but there are
NO beam plates, rather just a lose spiral grid 3. (Indeed, the word
tetrode is NOT used by RCA) But to me it looks like a power pentode.
Perhaps the rather open g3 spiral and supports may cause electron
beams, but it's not clear to me how this would happen.
Any comments on this and the name?
Cheers,
Roger

If I understand correctly, the main feature of a beam tetrode is the same
pitch and alignment of the control grid and screen grid. This creates the
beams and helps keep screen current low.

There is alignment of screen wires and grid wires in both power beam
tetrodes and power pentodes. This alignment is means Ig2 is low
compared to Ia because the g2 wires lie in the shadows in the stream
of electrons created by the g1 wires. The positive charge on the
screen thus does not attract many electrons, although when Ea is
pulled low by high Ia acting on RLa, Ig2 then goes high itself. Have a
look at the curves, its all there.

The EL85 or 6BQ5 has a supressor grid to prevent secondary emission
and to thus prevent the "kink" in pure tetrode tubes without a
supressor or beam forming plates.
Where a suppressor grid is used, there is no beam forming action due
to the suppressor grid.

The power beam tetrodes don't have a suppressor grid, but have a pair
of plates located between screen and anode and connected to cathode so
that their relative negative voltage field repels electrons heading to
anode, and from anode to screen when Ea is low. The fields force the
electrons into becoming beams, with their own increased negative
field, helping to repel electrons bouncing off the anode back to the
anode insted of to the screen when they have lost enough velocity and
energy. The use of beam forming plates in beam tetrodes removed the
kink in tetrode Ra curves seen in old data sheets for early tetrodes.
Hence we got what were sometimes called "kinkless tetrodes" 6V6 and
6L6 are classic examples.

In general, pentodes have higher screen current than do beam tetrodes,
but Ig2 varies hugely depending on the value of Eg2 compared to Ea.


On the other hand, to keep the screen grid electron interception small
enough, the screen grid can not be very dense. Therefore the control grid is
also not dense. Thus the transconductance is not that high (per each mA of
plate current) -- probably about 10% (mA/V) of the plate current (mA): about
5mA/V per 50mA plate current.

Just exactly what g1 gm and g2 gm are depends very much on Ia, and
relative values of Ea and Eg2.

Rarely ever does one ever see any power beam tetrode or power pentode
used in Ea Eg2 and Ia conditions quoted in data sheets which then have
the gm quoted.
Most music most people listen to is generated by power tubes operating
with maybe 1/3 of the Ia stated for where the g1 gm is valid. So for
example, a 6550 beam tetrode may idle at 45mA, and the g1 gm might
only be 5mA/V, not the more better figure of 11mA/V quoted in data
sheets. I suggest you examine the Ra curves for such tubes to
understand just how little meaning the basic quoted figures for gm, Ra
and µ have when the Ia and Ea and Eg2 are not the same as those used
to get the data.

If you want a high efficiency high-gm tube, the control grid has to be
tight, but the screen still can not be too tight. So the hi-gm beam tetrode
is impossible. It will be a pentode, though some pentodes can have
suppression plates or frame instead of a suppression grid.

I suggest you examine curves, rather than postulate vague assumptions.


EL84 has 11mA/V, a tight control grid and never can be a true beam tetrode,
even if some manufacturer fits supression plates instead of the third grid.

Tube makers might strongly disagree. But the use of beam forming
plates were invented to get around the european patents on pentodes.

KT120 seem to have slightly higher gm than usual.

Patrick Turner.

[Alex Pogossov]

"Patrick Turner" wrote in message
...
On Mar 20, 7:13 pm, "Alex Pogossov" wrote:
"Roger Jones" wrote in message

...

Hi, Vacuumlanders,
I just came across a very dead EL84/6BQ5 (from a P-P audio amplifier
with a leaky coupling cap to its g1 - take note!) Just for interest, I
cut the glass off just above the base with a Dremel-type cutting disk
(use glasses and gloves!) Then, I pulled off the plate assembly to
look at the electrodes.
The RCA tube manual calls this tube a "Beam Power Tube" but there are
NO beam plates, rather just a lose spiral grid 3. (Indeed, the word
tetrode is NOT used by RCA) But to me it looks like a power pentode.
Perhaps the rather open g3 spiral and supports may cause electron
beams, but it's not clear to me how this would happen.
Any comments on this and the name?
Cheers,
Roger

If I understand correctly, the main feature of a beam tetrode is the same
pitch and alignment of the control grid and screen grid. This creates the
beams and helps keep screen current low.

There is alignment of screen wires and grid wires in both power beam
tetrodes and power pentodes. This alignment is means Ig2 is low
compared to Ia because the g2 wires lie in the shadows in the stream
of electrons created by the g1 wires. The positive charge on the
screen thus does not attract many electrons, although when Ea is
pulled low by high Ia acting on RLa, Ig2 then goes high itself. Have a
look at the curves, its all there.

The EL85 or 6BQ5 has a supressor grid to prevent secondary emission
and to thus prevent the "kink" in pure tetrode tubes without a
supressor or beam forming plates.
Where a suppressor grid is used, there is no beam forming action due
to the suppressor grid.

The power beam tetrodes don't have a suppressor grid, but have a pair
of plates located between screen and anode and connected to cathode so
that their relative negative voltage field repels electrons heading to
anode, and from anode to screen when Ea is low. The fields force the
electrons into becoming beams, with their own increased negative
field, helping to repel electrons bouncing off the anode back to the
anode insted of to the screen when they have lost enough velocity and
energy. The use of beam forming plates in beam tetrodes removed the
kink in tetrode Ra curves seen in old data sheets for early tetrodes.
Hence we got what were sometimes called "kinkless tetrodes" 6V6 and
6L6 are classic examples.

In general, pentodes have higher screen current than do beam tetrodes,
but Ig2 varies hugely depending on the value of Eg2 compared to Ea.


On the other hand, to keep the screen grid electron interception small
enough, the screen grid can not be very dense. Therefore the control grid
is
also not dense. Thus the transconductance is not that high (per each mA of
plate current) -- probably about 10% (mA/V) of the plate current (mA):
about
5mA/V per 50mA plate current.

Just exactly what g1 gm and g2 gm are depends very much on Ia, and
relative values of Ea and Eg2.

Rarely ever does one ever see any power beam tetrode or power pentode
used in Ea Eg2 and Ia conditions quoted in data sheets which then have
the gm quoted.
Most music most people listen to is generated by power tubes operating
with maybe 1/3 of the Ia stated for where the g1 gm is valid. So for
example, a 6550 beam tetrode may idle at 45mA, and the g1 gm might
only be 5mA/V, not the more better figure of 11mA/V quoted in data
sheets. I suggest you examine the Ra curves for such tubes to
understand just how little meaning the basic quoted figures for gm, Ra
and µ have when the Ia and Ea and Eg2 are not the same as those used
to get the data.

If you want a high efficiency high-gm tube, the control grid has to be
tight, but the screen still can not be too tight. So the hi-gm beam
tetrode
is impossible. It will be a pentode, though some pentodes can have
suppression plates or frame instead of a suppression grid.

I suggest you examine curves, rather than postulate vague assumptions.


EL84 has 11mA/V, a tight control grid and never can be a true beam
tetrode,
even if some manufacturer fits supression plates instead of the third
grid.

Tube makers might strongly disagree. But the use of beam forming
plates were invented to get around the european patents on pentodes.

KT120 seem to have slightly higher gm than usual.

Patrick Turner.

---------------------------------------------
Alex:
What I was saying is:
1. In a true beam tetrode g1 and g2 have the same pitch (No of turns) and
they are aligned. In pentodes g1 has a finer pitch than g2.

2. In a true beam tetrode gm (in mA/V) is about 10% of the Ia (in mA). For
example, 6V6 has Ia=45mA and gm=4.5mA/V, KT120 has Ia=130mA and gm=12mA/V,
etc. In pentodes gm can be as high as 20...100% of Ia. for example, EL84 has
11mA/V at Ia=45mA. (We are not talking about old lousy pentodes like 6F6).

3. In a prue beam tetrode Ig2 is about 10% of Ia (if Va is of the order of
Vg2). In pentodes Ig2 is higher, 15...20% of Ia, even with some kind of
grids alignment.

4. The grids structure is of primary importance for the above distinction
between beam tetrodes versus pentodes, not the beam plates, or suppressor
grid or frames or just a large distance to the plate.

[John L Stewart]

Originally Posted by Roger Jones

Hi, Vacuumlanders,
I just came across a very dead EL84/6BQ5 (from a P-P audio amplifier
with a leaky coupling cap to its g1 - take note!) Just for interest, I
cut the glass off just above the base with a Dremel-type cutting disk
(use glasses and gloves!) Then, I pulled off the plate assembly to
look at the electrodes.
The RCA tube manual calls this tube a "Beam Power Tube" but there are
NO beam plates, rather just a lose spiral grid 3. (Indeed, the word
tetrode is NOT used by RCA) But to me it looks like a power pentode.
Perhaps the rather open g3 spiral and supports may cause electron
beams, but it's not clear to me how this would happen.
Any comments on this and the name?
Cheers,
Roger

For the curious & others the following is lifted from "History of the British Radio Valve to 1940" by Keith R Thrower. As excellant resource for anyone interested in vacuum tubes.

...

It is generally not known that the beam tetrode was invented in Britain by two EMI engineers, Cabot S Bull & Sidney Rodda, in an attempt to find an alternative to the pentode, the patent for which was owned by Philips. Unfortunately MOV, which was half owned by EMI, did not believe the valve could be manufactured & the design was passed to RCA under an exchange agreement between the two companies. The invention is clearly set out in a patent application made in Aug 1933.

(Description of the claim)

An interesting alternative to the beam tetrode was the critical distance tetrode that was introduced by Hivac in 1935 & was the brainchild of JH Owen Harries. It was found that if the spacing between the anode & screen was increased, there was a critical separation (about 3 cm) where there was no kink in the anode characteristic & the valve had exceptionally good linearity.

END OF QUOTE

Cheers to all, John