Hi All,

I'm about to build a 4CX1500B amplifier for 2m.

Most, if not all, descriptions are blowing air into the enclosure on the
base side of the tube and the air goes through the socket and finaly
through the anode chimney.
For several reasons i would like to blow into the anode enclosure
directly onto the anode, the air would then go out through the anode
chimney. Only a small part of the air would cool the base of the tube
through small holes in the anode/cathode separation.

Is that a stupid idea ? Is there any reason not doing so ?

73, Patrick

"Patrick" wrote in message
...
Hi All,

I'm about to build a 4CX1500B amplifier for 2m.

Most, if not all, descriptions are blowing air into the enclosure on the
base side of the tube and the air goes through the socket and finaly
through the anode chimney.
For several reasons i would like to blow into the anode enclosure directly
onto the anode, the air would then go out through the anode chimney. Only
a small part of the air would cool the base of the tube through small
holes in the anode/cathode separation.

Is that a stupid idea ? Is there any reason not doing so ?

73, Patrick

Patrick -

I would suggest that you look at the 6M DXpedition Amplifier designed by
Chris, G3WOS.
Great photographs and commentary on desire to stay portable, cooling
requirements and
overall compartment design of the amplifier. It may give you a few ideas.
He used a Eimac 3CX1500A7 (8877) tube.
http://www.gare.co.uk/amplifier/

For your tetrode 4CX1500B amplifier, I would strongly suggest using Ian
White's (G3SEK) control board:
http://www.ifwtech.co.uk/g3sek/board.../tetrode-1.htm

Ian also has his 1997 QEX article available for reading on tetrode designs:
http://www.ifwtech.co.uk/g3sek/board.../tetrode-3.htm

w9gb

Patrick enscribed:
Hi All,

I'm about to build a 4CX1500B amplifier for 2m.

Most, if not all, descriptions are blowing air into the enclosure on the
base side of the tube and the air goes through the socket and finaly
through the anode chimney.
For several reasons i would like to blow into the anode enclosure
directly onto the anode, the air would then go out through the anode
chimney. Only a small part of the air would cool the base of the tube
through small holes in the anode/cathode separation.

Is that a stupid idea ? Is there any reason not doing so ?

73, Patrick

No, not a stupid idea at all. That's the way I do it on my current
4CX800 2m amp, and I have used that method on several 4cx250 class amps
in previous years. I think its was popularized, if not initiated, by
K2RIW some decades ago in his classic 432 amp.
Three things to watch for:
1) make sure there is a good seal from the outer part of the plate
resonator to the outside world; I use 25mil teflon sheet as a chimney
with clamps made from brass strip.
2) ensure there is an exit in the grid compartment so that the air can
flow through adequately.
3) put some mesh on the blower inlet to the plate cavity, otherwise the
resonance will be affected by the fan rotation. Gives a subtle, but
noticeable AM buzz.

Good luck with the project.

73 Alf NU8I (aka G4ABB,ON8KL,VK6GA,PZ5AG, etc)
Scottsdale AZ DM43an

On Sat, 09 Apr 2005 17:17:56 +0200, Patrick wrote:

Hi All,

I'm about to build a 4CX1500B amplifier for 2m.

snip
For several reasons i would like to blow into the anode enclosure directly
onto the anode, the air would then go out through the anode chimney.
snip

Not sure if this matters but on the larger tubes, the cool air was needed
to keep the tube contacts from heating. Also, there was an issue with heat
buildup on the base of the tube due to the filaments.

The tubes I am talking about were the 3cx15k and larger in a broadcast
environment so this could be moot.

Hi Al,

OK, thanks for the ideas. My first guess was that i will have
difficulties to find a blower that can give me enough pressure !

I think that there will be less back pressure than blowing all the air
through the socket...

I want to maximize the RF shielding between anode and cathode, by
leaving the smallest space possible in the socket. Doing this, the air
would have some difficulties to travel through... I could also drill
some holes at the other end of the cabinet, but that would reduce the
useful air flow also !

I will have a look on K2RIW's amps. Tnx for the info.

btw, i want to make my amp as small and simple as possible.. I will use
a cathode driven design with G2 grounded and plate resonator made by a
simple 1 turn coil, no line, no PFTE, etc ... (seen on YU1AW's "lazy
builder" page)

My actual concern is to find a simple but stable way to make a 10 pF HV
variable cap. Berylium copper and/or vacuum caps are not common here on
Corsica ! hi hi

73, Patrick. TK5EP.


Alfred Green a écrit :


No, not a stupid idea at all. That's the way I do it on my current
4CX800 2m amp, and I have used that method on several 4cx250 class amps
in previous years. I think its was popularized, if not initiated, by
K2RIW some decades ago in his classic 432 amp.
Three things to watch for:
1) make sure there is a good seal from the outer part of the plate
resonator to the outside world; I use 25mil teflon sheet as a chimney
with clamps made from brass strip.
2) ensure there is an exit in the grid compartment so that the air can
flow through adequately.
3) put some mesh on the blower inlet to the plate cavity, otherwise the
resonance will be affected by the fan rotation. Gives a subtle, but
noticeable AM buzz.

Good luck with the project.

73 Alf NU8I (aka G4ABB,ON8KL,VK6GA,PZ5AG, etc)
Scottsdale AZ DM43an

You need to have cool air moving across the tube base pins. The
conventional method of under-up cooling supplies the base pins
with safe cooling. If you blow into the anode, what happens to
the air reaching the base pins/contacts? The seal around the
tube base contacts also needs cooling and it should not be already
hot air from passing by the anode.

Smaller tubes are forgiving in cost and operation. Jump to
the 4cx1000/4cx1500 size and you had best not over heat the
tube, both base pins and anode.

cheers,
skipp

www.radiowrench.com/sonic I have a diagram of the Collins 30S-1 available
(as well as many others).



: Patrick wrote:
: Hi All,

: I'm about to build a 4CX1500B amplifier for 2m.

: Most, if not all, descriptions are blowing air into the enclosure on the
: base side of the tube and the air goes through the socket and finaly
: through the anode chimney.
: For several reasons i would like to blow into the anode enclosure
: directly onto the anode, the air would then go out through the anode
: chimney. Only a small part of the air would cool the base of the tube
: through small holes in the anode/cathode separation.

: Is that a stupid idea ? Is there any reason not doing so ?

: 73, Patrick

Hi Skipp,

Thanks for the advice.

Yes, in fact must the base be cooled as well !
I don't think the air that will go to the base will be hot, the air will
be under pressure in the anode cabinet and the hotest air will be
created on the anode cooler and be exhausted almost immediately.

I have seen a description where there are 2 Papst 3472 fans, one blowing
and one extracting the air. These fans can produce abt 36 CFM at a
very low noise level. Maybe a not too bad idea compared to a squirel
radial fan that produces the same volume but at a much higher noise level !!

Any suggestions ??

73 Patrick

Skipp a écrit :
You need to have cool air moving across the tube base pins. The
conventional method of under-up cooling supplies the base pins
with safe cooling. If you blow into the anode, what happens to
the air reaching the base pins/contacts? The seal around the
tube base contacts also needs cooling and it should not be already
hot air from passing by the anode.

Smaller tubes are forgiving in cost and operation. Jump to
the 4cx1000/4cx1500 size and you had best not over heat the
tube, both base pins and anode.

cheers,
skipp

www.radiowrench.com/sonic I have a diagram of the Collins 30S-1 available
(as well as many others).



: Patrick wrote:
: Hi All,

: I'm about to build a 4CX1500B amplifier for 2m.

: Most, if not all, descriptions are blowing air into the enclosure on the
: base side of the tube and the air goes through the socket and finaly
: through the anode chimney.
: For several reasons i would like to blow into the anode enclosure
: directly onto the anode, the air would then go out through the anode
: chimney. Only a small part of the air would cool the base of the tube
: through small holes in the anode/cathode separation.

: Is that a stupid idea ? Is there any reason not doing so ?

: 73, Patrick

The problem is back pressure on the fan. Most axial fans do not do
well with much back pressure. The flow drops sharply. Squirrel cage
type blowers are usually needed for those type tubes. For a 4cx1000
type tube you need a minimum of a 3 inch wheel on the blower at 3100
rpm and that is with a wide open socket and air path. A smaller blower
would need to be higher speed.

73
Gary K4FMX

On Mon, 18 Apr 2005 19:38:11 +0200, Patrick wrote:

Hi Skipp,

Thanks for the advice.

Yes, in fact must the base be cooled as well !
I don't think the air that will go to the base will be hot, the air will
be under pressure in the anode cabinet and the hotest air will be
created on the anode cooler and be exhausted almost immediately.

I have seen a description where there are 2 Papst 3472 fans, one blowing
and one extracting the air. These fans can produce abt 36 CFM at a
very low noise level. Maybe a not too bad idea compared to a squirel
radial fan that produces the same volume but at a much higher noise level !!

Any suggestions ??

73 Patrick

Skipp a écrit :
You need to have cool air moving across the tube base pins. The
conventional method of under-up cooling supplies the base pins
with safe cooling. If you blow into the anode, what happens to
the air reaching the base pins/contacts? The seal around the
tube base contacts also needs cooling and it should not be already
hot air from passing by the anode.

Smaller tubes are forgiving in cost and operation. Jump to
the 4cx1000/4cx1500 size and you had best not over heat the
tube, both base pins and anode.

cheers,
skipp

www.radiowrench.com/sonic I have a diagram of the Collins 30S-1 available
(as well as many others).



: Patrick wrote:
: Hi All,

: I'm about to build a 4CX1500B amplifier for 2m.

: Most, if not all, descriptions are blowing air into the enclosure on the
: base side of the tube and the air goes through the socket and finaly
: through the anode chimney.
: For several reasons i would like to blow into the anode enclosure
: directly onto the anode, the air would then go out through the anode
: chimney. Only a small part of the air would cool the base of the tube
: through small holes in the anode/cathode separation.

: Is that a stupid idea ? Is there any reason not doing so ?

: 73, Patrick

Hi gary,

You're right. But how does a combination of those fans work ?
One "pushing" and one "pulling" ?? The back pressure must certainly be
reduced ?! That's a problem for a hydraulic engineer !! hi hi

73, Patrick

Gary Schafer a écrit :
The problem is back pressure on the fan. Most axial fans do not do
well with much back pressure. The flow drops sharply. Squirrel cage
type blowers are usually needed for those type tubes. For a 4cx1000
type tube you need a minimum of a 3 inch wheel on the blower at 3100
rpm and that is with a wide open socket and air path. A smaller blower
would need to be higher speed.

73
Gary K4FMX

On Mon, 18 Apr 2005 19:38:11 +0200, Patrick wrote:


Hi Skipp,

Thanks for the advice.

Yes, in fact must the base be cooled as well !
I don't think the air that will go to the base will be hot, the air will
be under pressure in the anode cabinet and the hotest air will be
created on the anode cooler and be exhausted almost immediately.

I have seen a description where there are 2 Papst 3472 fans, one blowing
and one extracting the air. These fans can produce abt 36 CFM at a
very low noise level. Maybe a not too bad idea compared to a squirel
radial fan that produces the same volume but at a much higher noise level !!

Any suggestions ??

73 Patrick

Skipp a écrit :

You need to have cool air moving across the tube base pins. The
conventional method of under-up cooling supplies the base pins
with safe cooling. If you blow into the anode, what happens to
the air reaching the base pins/contacts? The seal around the
tube base contacts also needs cooling and it should not be already
hot air from passing by the anode.

Smaller tubes are forgiving in cost and operation. Jump to
the 4cx1000/4cx1500 size and you had best not over heat the
tube, both base pins and anode.

cheers,
skipp

www.radiowrench.com/sonic I have a diagram of the Collins 30S-1 available
(as well as many others).



: Patrick wrote:
: Hi All,

: I'm about to build a 4CX1500B amplifier for 2m.

: Most, if not all, descriptions are blowing air into the enclosure on the
: base side of the tube and the air goes through the socket and finaly
: through the anode chimney.
: For several reasons i would like to blow into the anode enclosure
: directly onto the anode, the air would then go out through the anode
: chimney. Only a small part of the air would cool the base of the tube
: through small holes in the anode/cathode separation.

: Is that a stupid idea ? Is there any reason not doing so ?

: 73, Patrick

You don't get much pressure relief pulling. Better to push.

73
Gary K4FMX

On Mon, 18 Apr 2005 22:42:03 +0200, Patrick wrote:

Hi gary,

You're right. But how does a combination of those fans work ?
One "pushing" and one "pulling" ?? The back pressure must certainly be
reduced ?! That's a problem for a hydraulic engineer !! hi hi

73, Patrick

Gary Schafer a écrit :
The problem is back pressure on the fan. Most axial fans do not do
well with much back pressure. The flow drops sharply. Squirrel cage
type blowers are usually needed for those type tubes. For a 4cx1000
type tube you need a minimum of a 3 inch wheel on the blower at 3100
rpm and that is with a wide open socket and air path. A smaller blower
would need to be higher speed.

73
Gary K4FMX

On Mon, 18 Apr 2005 19:38:11 +0200, Patrick wrote:


Hi Skipp,

Thanks for the advice.

Yes, in fact must the base be cooled as well !
I don't think the air that will go to the base will be hot, the air will
be under pressure in the anode cabinet and the hotest air will be
created on the anode cooler and be exhausted almost immediately.

I have seen a description where there are 2 Papst 3472 fans, one blowing
and one extracting the air. These fans can produce abt 36 CFM at a
very low noise level. Maybe a not too bad idea compared to a squirel
radial fan that produces the same volume but at a much higher noise level !!

Any suggestions ??

73 Patrick

Skipp a écrit :

You need to have cool air moving across the tube base pins. The
conventional method of under-up cooling supplies the base pins
with safe cooling. If you blow into the anode, what happens to
the air reaching the base pins/contacts? The seal around the
tube base contacts also needs cooling and it should not be already
hot air from passing by the anode.

Smaller tubes are forgiving in cost and operation. Jump to
the 4cx1000/4cx1500 size and you had best not over heat the
tube, both base pins and anode.

cheers,
skipp

www.radiowrench.com/sonic I have a diagram of the Collins 30S-1 available
(as well as many others).



: Patrick wrote:
: Hi All,

: I'm about to build a 4CX1500B amplifier for 2m.

: Most, if not all, descriptions are blowing air into the enclosure on the
: base side of the tube and the air goes through the socket and finaly
: through the anode chimney.
: For several reasons i would like to blow into the anode enclosure
: directly onto the anode, the air would then go out through the anode
: chimney. Only a small part of the air would cool the base of the tube
: through small holes in the anode/cathode separation.

: Is that a stupid idea ? Is there any reason not doing so ?

: 73, Patrick