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THE AM BULLETIN BOARD => Technical Forum => Topic started by: W0PWE on March 18, 2020, 06:58:06 PM



Title: Cooling a Pair of 4CX250B
Post by: W0PWE on March 18, 2020, 06:58:06 PM
I am in the process of re-building an old amp that my dad built in the 70’s. My goal is to use it at the 100 watt carrier level. It uses a pair of 4CXxxx tubes and I have both 4CX250B and 4CX350A tubes as options. The sockets, squirrel cage fan, 4CX250’s and a few other parts came out of an old military VHF transmitter that is now long gone.

The grid compartment is roughly 5” wide, 11” long and 4” deep with the fan blowing air in the top rear and the tube sockets on top towards the front. Tube sockets look to be proprietary but probably not much different than the SK610 in terms of airflow design. Dad had made a shroud to contain the air coming up around the screen rings and help it to go through the anode fins. The fan is a 1/8 horsepower squirrel cage type with a wheel that is 2.5” diameter and 2.75” wide.
 
I don’t feel that the tubes are getting enough cooling airflow. With just limited testing I softened the solder on the anode connectors. My anode connectors are homebrew from ¼ inch brass strip and similar to the those of the EJ Johnson 124-116 socket. With the tubes removed I can feel good air flow coming up through sockets but with tubes inserted, the tube base and screen ring severely limit the flow.
 
I made a test chamber with a cardboard box all sealed up, the blower, holes for a pair of tubes and a homebrew monometer. With a pair of tubes stuck in their holes air flow through the fins seems good. The monometer does not move though, even with the tube holes restricted or closed off completely.

My 1987 ARRL Handbook has a nice section on the subject. It shows some example blower data and the Dayton 4C440 looks like it would do the job (the 4CX350’s need about 12CFM at 0.6” water. ) But that fan is roughly equivalent or maybe a little smaller than the one I have and mine won’t even wiggle the monometer.
 
I would appreciate any suggestions on how to cool these tubes. It seems there are plenty of fans or blowers out there with the necessary specs but I get the feeling I’m missing something obvious here. Thanks and 73.


Title: Re: Cooling a Pair of 4CX250B
Post by: KD6VXI on March 18, 2020, 08:54:04 PM
The pitch of the blades makes a big difference in airflow.  Especially when forcing air through the restrictive sockets and fins.

You need to make sure you have needed cfm with the rated backflow.

IE, double the necessary cfm of a single tube at the same cfm.

I don't remember the model number of the two tube 250B box I built is.  It's at home, and I'm in the parking lot at a store stocking up.

We go on lock down tomorrow, so I get to play antennas and radios!

--Shane
KD6VXI


Title: Re: Cooling a Pair of 4CX250B
Post by: KK4YY on March 18, 2020, 10:34:09 PM
Is the amplifier running efficiently? You should get about 60% peak efficiency if everything is in order. If efficiency is poor, the tubes will have to dissipate extra heat which requires extra cooling (or makes then run extra hot).

What is the input vs.output power when properly tuned?


Title: Re: Cooling a Pair of 4CX250B
Post by: kc2we on March 19, 2020, 06:20:37 AM
Remember, 4CX350's are class AB1 only. No grid current. For VHF FM or HF class C, stick with 250's. See the thread, "Need Some Help With My Amp Project" ST


Title: Re: Cooling a Pair of 4CX250B
Post by: WBear2GCR on March 19, 2020, 08:02:22 AM
Let's see some pictures please?

Tubes in and out (need to see the sockets).

The "plumbing" needs to be correct.

From your description the air is blown down from the anode end?
Not up from a pressurized sub chassis that the tube sockets are
mounted in?

There needs to be a chimney if it is the second method.



Title: Re: Cooling a Pair of 4CX250B
Post by: W0PWE on March 19, 2020, 01:04:09 PM
Thanks for the response and thanks for pointing me to the thread  "Need Some Help With My Amp Project”. I read the whole thing and found some great info there. I will try to get some pictures of the sockets etc. but first let me provide a little more info.

It is a pressurized grid chamber so flow is up through the sockets and then through the anode fins. I do not have chimneys but I have ordered materials to make them.

I intend to run class AB1 and I have separate meters for both control grid and screen. I have built new power supplies and both the grid and screen supplies are regulated and have bleeders so that I can see negative current on the meters. The screen has a 12.5K bleeder to provide the roughly 15ma per tube as described in the Eimac data sheet.

After much fooling around with my NanoVNA I believe I have the Pi network output set up properly. The screen current peaks at resonance and dips with loading per Eimac newsletter AS13 – “Understanding Tetrode Screen Current” by W6EMD.
 
Input circuit is a 100 ohm swamping resistor. With the 4CX350’s I need to keep the drive below 4 watts or I see negative grid current and positive screen current. Low drive requirement is fine with me as I intend to eventually drive it with some type of homebrew low level exciter.

In AB1 my efficiency is about 50%. With 800 watts DC input the output is pretty close to 400W.

Things may not be as bad as I first thought. Last night I ran a test and monitored the anode temperature with an IR thermometer. Sitting with zero signal plate current (200ma) it took about 10 minutes for the temperature to stop climbing and it settled in at about 280F (138C). Keydown at 350 watts output for 2 minutes warmed them up to 330F (165C). I wasn’t brave enough to run key down longer than 2 minutes but it looked like the temp was starting to stabilize.

I know I need to stay well away from the datasheet maximum of 250C but I don’t know what kind of temperature I should expect under normal operating conditions. Again, my goal is to run 100 watts carrier – 400W P-P.


Title: Re: Cooling a Pair of 4CX250B
Post by: KK4YY on March 19, 2020, 01:37:47 PM
With an efficiency of 50% at 400W output you should see about 25% efficiency when the drive is reduced to yield 100W output. Figure on 300W of Pd with a dead carrier and 400W Pd at modulation peaks or about 350W Pd on average. Running it tuned to 350W output, as you have, is probably a good test as to how much it will heat up when in the full AM service you have planned.

Good luck with your dad's amp. Few of us get the chance to run a home brew hand-me-down.


Title: Re: Cooling a Pair of 4CX250B
Post by: WBear2GCR on March 19, 2020, 05:01:32 PM
You can NOT run them without chimneys. The air won't pass through the fins with enough
velocity and flow.

Probably you can get away with some PVC pipe for testing only.

Teflon will work.
The originals were ceramic.
If you have a local ceramic making place, just make some out of pottery ceramic.
Actually you can make it by merely casting the right grade of gypsum.
It can be fired, or not.
Fired, you can glaze it.
Ought to work unfired.
Super strong stuff, stronger than concrete, btw.

The only critical dimension is the ID of the tube.
That ought to be easy enough to do with any number of formers... the outside
is non critical.

If you make a "dummy" chimney out of say plastic or wood - you can use the latex mold making
stuff from a hobby store to make the mold, and then just pour it... wait for it to dry.

Again, this stuff can be poured or gooped depending on the type.


Title: Re: Cooling a Pair of 4CX250B
Post by: DMOD on March 19, 2020, 05:20:33 PM
I am in the process of re-building an old amp that my dad built in the 70’s. My goal is to use it at the 100 watt carrier level. It uses a pair of 4CXxxx tubes and I have both 4CX250B and 4CX350A tubes as options. The sockets, squirrel cage fan, 4CX250’s and a few other parts came out of an old military VHF transmitter that is now long gone.

The grid compartment is roughly 5” wide, 11” long and 4” deep with the fan blowing air in the top rear and the tube sockets on top towards the front. Tube sockets look to be proprietary but probably not much different than the SK610 in terms of airflow design. Dad had made a shroud to contain the air coming up around the screen rings and help it to go through the anode fins. The fan is a 1/8 horsepower squirrel cage type with a wheel that is 2.5” diameter and 2.75” wide.
 
I don’t feel that the tubes are getting enough cooling airflow. With just limited testing I softened the solder on the anode connectors. My anode connectors are homebrew from ¼ inch brass strip and similar to the those of the EJ Johnson 124-116 socket. With the tubes removed I can feel good air flow coming up through sockets but with tubes inserted, the tube base and screen ring severely limit the flow.
 
I made a test chamber with a cardboard box all sealed up, the blower, holes for a pair of tubes and a homebrew monometer. With a pair of tubes stuck in their holes air flow through the fins seems good. The monometer does not move though, even with the tube holes restricted or closed off completely.

My 1987 ARRL Handbook has a nice section on the subject. It shows some example blower data and the Dayton 4C440 looks like it would do the job (the 4CX350’s need about 12CFM at 0.6” water. ) But that fan is roughly equivalent or maybe a little smaller than the one I have and mine won’t even wiggle the monometer.
 
I would appreciate any suggestions on how to cool these tubes. It seems there are plenty of fans or blowers out there with the necessary specs but I get the feeling I’m missing something obvious here. Thanks and 73.


According to tube specs you need about minimum of 8 cfm for two tubes. If you can fit a modern muffin fan to the chassis I think sufficient airflow will result.

Mechatronics has both DC and AC axial fans:

https://www.mechatronics.com/products/dc-axial-fans.php

https://www.mechatronics.com/products/ac-axial-fans.php

https://www.mechatronics.com/products/dc-high-airflow-fans/MR4020/


Phil



Title: Re: Cooling a Pair of 4CX250B
Post by: KD6VXI on March 19, 2020, 06:01:43 PM
I am in the process of re-building an old amp that my dad built in the 70’s. My goal is to use it at the 100 watt carrier level. It uses a pair of 4CXxxx tubes and I have both 4CX250B and 4CX350A tubes as options. The sockets, squirrel cage fan, 4CX250’s and a few other parts came out of an old military VHF transmitter that is now long gone.

The grid compartment is roughly 5” wide, 11” long and 4” deep with the fan blowing air in the top rear and the tube sockets on top towards the front. Tube sockets look to be proprietary but probably not much different than the SK610 in terms of airflow design. Dad had made a shroud to contain the air coming up around the screen rings and help it to go through the anode fins. The fan is a 1/8 horsepower squirrel cage type with a wheel that is 2.5” diameter and 2.75” wide.
 
I don’t feel that the tubes are getting enough cooling airflow. With just limited testing I softened the solder on the anode connectors. My anode connectors are homebrew from ¼ inch brass strip and similar to the those of the EJ Johnson 124-116 socket. With the tubes removed I can feel good air flow coming up through sockets but with tubes inserted, the tube base and screen ring severely limit the flow.
 
I made a test chamber with a cardboard box all sealed up, the blower, holes for a pair of tubes and a homebrew monometer. With a pair of tubes stuck in their holes air flow through the fins seems good. The monometer does not move though, even with the tube holes restricted or closed off completely.

My 1987 ARRL Handbook has a nice section on the subject. It shows some example blower data and the Dayton 4C440 looks like it would do the job (the 4CX350’s need about 12CFM at 0.6” water. ) But that fan is roughly equivalent or maybe a little smaller than the one I have and mine won’t even wiggle the monometer.
 
I would appreciate any suggestions on how to cool these tubes. It seems there are plenty of fans or blowers out there with the necessary specs but I get the feeling I’m missing something obvious here. Thanks and 73.


According to tube specs you need about minimum of 8 cfm for two tubes. If you can fit a modern muffin fan to the chassis I think sufficient airflow will result.

Mechatronics has both DC and AC axial fans:

https://www.mechatronics.com/products/dc-axial-fans.php

https://www.mechatronics.com/products/ac-axial-fans.php

https://www.mechatronics.com/products/dc-high-airflow-fans/MR4020/


Phil



Not sure about that.

CPI shows more airflow required.

Double the tubes, double the cfm.  Static pressure stays the same.  6 cfm is needed at 250 watts PDiss.

--Shane
KD6VXI


Title: Re: Cooling a Pair of 4CX250B
Post by: W0PWE on March 19, 2020, 07:05:08 PM
Thanks for the continued support and ideas. I never thought of making my own ceramic parts (chimneys) but I have a friend with a kiln so that is possible. Actually it sounds like fun. I have ordered some 0.010" Nomex paper and I will start with that but perhaps will do some testing with PVC as suggested.

It had a 4.25 inch muffin fan in it but I went back to the squirrel cage thinking it would work better with the relatively high static pressure. There are some high pressure muffin fans available now days. I think they were developed for pushing air through PC heatsinks. If I buy one I will remember the comment I read somewhere that said "your fan is too big if the tubes won't stay in their sockets" hi.

I think I figured out how to put pictures in the gallery and use attachments but it looks I can only attach 3 to the message. A few more are in the gallery.

Phil - It was great to meet you at the Perry RF Fest in Feb. Doesn't sound like we will be gathering at hamfests anytime in the near future.  :(


Title: Re: Cooling a Pair of 4CX250B
Post by: DMOD on March 19, 2020, 08:58:35 PM

Not sure about that.

CPI shows more airflow required.

Double the tubes, double the cfm.  Static pressure stays the same.  6 cfm is needed at 250 watts PDiss.

--Shane
KD6VXI

RCA tube specs for the 4CX250B/7203 showed 3.8 cfm per tube X2 = 7.6 cfm or rounded up 8 cfm, which is why a 10 cfm fan or larger was suggested and links given to possible fans.

https://frank.pocnet.net/sheets/079/7/7203.pdf


Phil


Title: Re: Cooling a Pair of 4CX250B
Post by: WBear2GCR on March 19, 2020, 10:02:55 PM
Imho, centrifugal blower is the only option for this application.

Muffin fans are not designed to work into a static air column.

The tubes should not be run without chimneys installed.
PVC will work for short tests, or as long as the operating temp at
the outer fins remains below the melting point of the PVC.

Take a look at the pix of the Eimac chimneys - they're tapered,
but there is no actual need for that. Note that they clip onto those
clips facing out on the socket assembly you have, and the brighter area
is where they formerly sat.

                        _-_-


Title: Re: Cooling a Pair of 4CX250B
Post by: W0PWE on March 20, 2020, 12:03:23 AM
OK, I'm a little slow but I'm beginning to understand. My first mistake was in thinking a chimney is a cylinder that takes the hot air exhausted from the tube and conveys it out of the enclosure - like a fireplace chimney. I have looked at pictures of the SK606 chimney many times but didn't really understand what they were made of or how they worked. To further confuse me I have a similar part that I thought was a SK606 but it not at all the same animal. After looking closely at SK606 pictures and info I understand and I can see why what I have won't work.

What I have is the box or shroud shown in the attached picture. So I believe that when the air comes up from below it expands into this large volume and looses the pressure it needs to blow through the fins at the correct velocity. My thermodynamics might not be completely accurate but this is clearly a different flow situation than that of the SK606 chimneys.

I found a drawing of the SK606 with dimensions. I can create a 3D model, print a dummy on my 3D printer and then make a mold and gypsum or ceramic parts as you described above. If I happen to have a piece of PVC or something of the right ID I will play with that first.


Title: Re: Cooling a Pair of 4CX250B
Post by: KD6VXI on March 20, 2020, 12:04:10 AM

Not sure about that.

CPI shows more airflow required.

Double the tubes, double the cfm.  Static pressure stays the same.  6 cfm is needed at 250 watts PDiss.

--Shane
KD6VXI

RCA tube specs for the 4CX250B/7203 showed 3.8 cfm per tube X2 = 7.6 cfm or rounded up 8 cfm, which is why a 10 cfm fan or larger was suggested and links given to possible fans.

https://frank.pocnet.net/sheets/079/7/7203.pdf


Phil

I believe I'd go with the higher flow rate., just to be safe.  As I stated, that was the Eimac spec downloaded from the Eimac website.

Having built enough 250b amps to fill a truck, I've found a lot higher airflow needed with both Eimac and Amperex  tubes.

That spec sheet is from 61.  This one is 74.

https://frank.pocnet.net/sheets/140/7/7203.pdf



To each his own, but I use a higher cfm blower with mine.

--Shane
KD6VXI


Title: Re: Cooling a Pair of 4CX250B
Post by: KF7WWW on March 20, 2020, 12:07:42 AM
Why not just buy the correct chimneys instead of spending so much time building your own? They are plentiful on the market and can usually be had for $20 or so.


Title: Re: Cooling a Pair of 4CX250B
Post by: K1JJ on March 20, 2020, 12:38:09 AM
There's one thing to know about running external anode tubes...   If they are small, like 4CX-250s, they tend to make a lot of air and fan noise before they get enough air to run cool as they should.   I had a TMC 4CX-350 amp with a pair and it sounded like a machine shop when put on the operating table next to me.  I ended up changing out to a larger blower with a Variac to slow it down. Even then, I needed to move it away to the other side of the room.

In contrast, a bigger tube like an 8877 (or larger) can use a bigger blower that is running slower rpm. The air does not make as much noise. The tube fins are wider spaced with less turbulence. I have a big amp that runs quieter than that old 4CX-350 TMC amp.

Do a mock-up first and see if you can live with the noise; especially paying attention to your on-air transmit S/N -  and mixing of the air noise with your audio.  A reasonably quiet shack is very important, especially for AM.

I think the loudest "small" rig I ever heard is a (R-392 /)  T195  ... good gawd!  (Collins design, single 4CX-150)

T


Title: Re: Cooling a Pair of 4CX250B
Post by: Opcom on March 20, 2020, 03:20:58 PM
One thing was said and it was not addressed. It was said that with the tube holes sealed off the manometer does not move. This is very important to look at.

Are you sure the manometer is built correctly? The free end must be open for it to indicate pressure by lifting a column of water x inches. The tubes anodes exhaust to open air therefore the manometer end is open to air. Sometimes the handbooks are not totally clear.

Even the simplest manometer will move for 0.6 inch of water.
What is the inside diameter of your manometer tubing?

Whether it is 12CFM (for a pair?) at 0.6" of pressure water column, or 6.4 CFM per tube at 0.82 inch of water column (pick one) the manometer will show it. As it was mentioned, more pressure = more flow = more cooling. Nothing wrong with more pressure except noise.

(I ran an NCL-2000 sitting on a "wind chest" I made from an old 12" bud rack, top door removed, with a big dual squirrel cage rackmount blower in it, and it was able to run for several hours at 100% output to power an experiment. When later shut off and discharged, the anodes and power transformer were cool to the touch. That's what extreme airflow gets you. I never bothered to measure the pressue on that.)

The truth is that once the tubes are in the sockets and there are chimneys in place sealing the anodes to the chassis, the manometer reading is all you need to know, provided the air exiting the tops of the anodes can escape freely. Eimac et al. have kindly given tables for this because it is easy to measure.

The reason for this is because the pressure is directly lost according to the airflow for that tube. You can make chimney temporarily from paper and electrical tape for the manometer/pressure test.

Here is a video of one I built in action.
http://bunkerofdoom.com/3cx3000/100_2086a.MOV

It shows just over 2&7/16" (2.4375") of water, half on each leg of the clear manometer tube, with one side moving down and the other up.

Does yours do this or not move? It should clearly be able to indicate 0.3" (5/16") on each side. No need to split hairs but there will be some movement. If not, there is something wrong.

More accuracy can be had by tilting the manometer. The column may move 4 or 5 inches horizontally but will still rise vertically only the distance proportional to the pressure. Dwyer makes those, and uses a quite thin tubing. Special thin fluid is used in them but note the expanded scale from 0-2". Where uptime is important you can make your own more compact one, use or make the thin fluid, and incorporate it into the front panel. OK that is way overkill but the topic deserved the info.


Title: Re: Cooling a Pair of 4CX250B
Post by: DMOD on March 20, 2020, 04:45:50 PM
...What I have is the box or shroud shown in the attached picture. So I believe that when the air comes up from below it expands into this large volume and looses the pressure it needs to blow through the fins at the correct velocity. My thermodynamics might not be completely accurate but this is clearly a different flow situation than that of the SK606 chimneys.

I found a drawing of the SK606 with dimensions. I can create a 3D model, print a dummy on my 3D printer and then make a mold and gypsum or ceramic parts as you described above. If I happen to have a piece of PVC or something of the right ID I will play with that first.

In terms of thermodynamics, most of the heat generated within the tube is conducted to the fins where cooling is to then take place via forced convection.

It appears from the picture that the airflow first cools the pins, as it should, and then blows up through fins. It also appears the chimney would further funnel the airflow in sort of a venturi effect toward the top, increasing air velocity slightly.


Phil  


Title: Re: Cooling a Pair of 4CX250B
Post by: WBear2GCR on March 20, 2020, 06:22:34 PM
That "shroud" ought work just fine, as long as it is an air tight enclosure to the chassis and
around the sides.

You should feel a pretty good stream of air when the blower is powered up and no B+
on the tube... So if the plate current is within normal bounds, it ought to be fine.

The expansion of the air in the shroud ought not make any difference.
Of course the air input and the chassis also need to not leak.

This is the first time you showed a "shroud".

Imho, of course...

EDIT: A second look at the top of the shroud shows burning on the edges of the
top around the tube cutouts. Looks like it was green fiberglass pcb
material at one time. Very overheated. Which is why Eimac used ceramic. So, on a
second look, NG. The tube anode clamps can be modified and used, I expect. Unless the
sockets grab the anodes...


TMC PAL350 amp example below (same as the same amp using 4CX250B tubes)

That setup does not permit any "parasitic suppressors" on each tube... but you could
cut that in half... need be.

A non-contact laser thermometer from Harbor Freight will tell you the temp rise...


Title: Re: Cooling a Pair of 4CX250B
Post by: w4bfs on March 20, 2020, 08:29:39 PM
if you don't like the noise of air cooling .... oil cooling is a very reasonable alternative if you figure out how to keep it sealed up ... afaik oil has a specific heat figure of hundreds of times better than air .... a gallon per minute is VERY generous at the kilowatt level


Title: Re: Cooling a Pair of 4CX250B
Post by: W0PWE on March 20, 2020, 10:38:28 PM
I have rebuilt the manometer and it works a little better. The right side is inclined at about a 30 degree angle so I have a little better resolution. My tubing is 1/8” ID. Running a few of my fans in my closed off test box I can see a very slight movement in the water when I connect the manometer. Too small to measure even on the inclined side. The only thing that really moves it is my hot air gun. It shows about 2” of H2O pressure. Maybe I will disconnect the heating element and duct tape that thing into the amp (hi).
 
When connected to the grid chamber of my amp it also barely moves. That is with the squirrel cage fan. I will probably take the fan out again and try it in the closed test box but I imagine results will be less than stellar. Wish it would do ½ as well as the one in your video. I guess I need a different fan although a ham friend of mine has the exact same fan in a 4CX250 VHF amp and he is having no problems.

I tried makeshift chimneys made of paper towel roll. Waving my hand over the tubes (with B+ off) I can now feel air coming out of the fins. There isn’t much air moving but I was surprised to see that there was enough to blow out the flame on a stick match when I put it over the fins. With Idle current of 200ma the tubes still warmed right up to 300F in about 3 minutes. That’s as far as I wanted to go with cardboard chimneys.
 
You are right, that shroud or whatever we want to call it was made of green fiberglass PCB material and those brown spots are burns. That happened many years ago. I have not let it get anywhere near that hot. But those burns marks made me realize that I needed to pay attention to cooling when I got the thing running. I have parasitic suppressors and intended to install them when I made the anode connectors. But then I forgot. It will be easy enough to add them when (if) I wrap this thing up.


Nice to see the picture of the TMC PAL 350. I looked at a lot of amplifier designs before I started on this project but I did not run across the PAL 350 until you pointed me to Glenn’s post. That is a pretty impressive example of amplifier design.

Oil cooling is a neat idea and fun to think about but I think I have already gone too far with this old amp. I’m beginning to believe it should have stayed in the garage attic where it has been for the last 25 years. Thank you all for the ideas and support.


Title: Re: Cooling a Pair of 4CX250B
Post by: W0PWE on March 21, 2020, 06:39:50 PM
Read this if you are bored. Long story short - I did some testing and I still need a bigger fan.  ;)

In my junk I found a CPU cooler with a Delta brand fan. The data sheet for this fan shows Max air flow (at zero pressure) of 68CFM and Max pressure (at zero airflow) of about 0.3” H2O.) This fan is inadequate for cooling my amp but at least it is a fan with known (or assumed) performance and I can use it to evaluate my test setup. So I mounted it on my test box, sealed up anything that might leak with duct tape, powered it up with maximum allowed voltage and checked the manometer. The inclined side moved maybe 1/16” but it is hard to say. Certainly not what I would expect with this fan. Manometers are pretty simple and it’s hard to mess one up. But it looks like I have succeeded. The tubing is used and there are stains inside from the water that was previously in it. Perhaps that impacts it’s ability to respond to these very low pressure changes. Normally I would run out to the home improvement center and get a fresh piece of tubing but I’m voluntarily isolated so I’m making do with what I have.

OK I can’t test pressure so how about flow? I saw a guy on youtube testing a batch of CPU fans timing how long it takes to inflate a 30 gallon garbage bag. Hmmm, we could start a whole new thread on why that is an invalid test but lets try it. I fixed a 30 gallon bag to my test fixture and my CPU fan inflates it in roughly 10 seconds and by keeping all conditions as consistent as possible the results are pretty repeatable after 3 runs. Converting that number we get about 24CFM. Well, that seems like a ballpark number. At least it’s within the operating range of the fan. So then I removed the squirrel cage fan from the amp and tested it in the same way. It was about the same, maybe 9 seconds.

So then I made a collector out of cardboard that allows me to catch the anode exhaust air in the garbage bag. With the fan re-installed and cardboard chimneys taped in place it took 23 seconds to fill the bag. So that is about 10.5CFM. For a ballpark number that’s believable.


Title: Re: Cooling a Pair of 4CX250B
Post by: W4DNR on March 22, 2020, 12:04:53 PM
In the 70s, I built an  80-20 amp in a Bud box similar to the Collins 30L-1 amp.
I used four Eimac sockets with the built-in screen bypass capacitors.
I stacked two 4 inch Rotron 100cfm  axial flow fans  ( for improved back pressure performance )
Under SSB conditions, the airflow temperature out of the top of the amp was acceptable.

A friend built a similar cooled PAIR of 4CX250b tubes and the amp ran hot with reduced airflow,
although he used the same two stacked Rotrons.

I'm sure his amp had more back pressure and stalled the Rotrons. 

This is where a large diameter slower rpm blower beats a smaller faster rpm blower.

Remote the blower in a wooden box and use a flex duct to the amp for lower noise.

Don W4DNR


Title: Re: Cooling a Pair of 4CX250B
Post by: KK4YY on March 22, 2020, 01:48:34 PM
One time I tried to remote a noisy blower to the outside of my house. Originally, the blower was mounted right behind a 4-400A with an Eimac air system socket. I extended the run with a hose of like size diameter. The hose was from a swimming pool filter. It was flexible due to the spiral shape of the hose.

When I turned it on I could hardly hear the blower outside the house. What I did hear was an amazingly loud howl caused by the spiral shape of the hose. It was louder than the blower had been and even more annoying. And all that turbulence and friction slowed the air output to a crawl. :'(

Maybe a hose with a smooth surface on its interior would have been better. Maybe a larger diameter hose would have been better. Maybe a shorter hose would be better. But maybe testing it out before I made a hole in the wall of my house would have been best. ;)


Title: Re: Cooling a Pair of 4CX250B
Post by: KD6VXI on March 22, 2020, 02:09:32 PM
Everyone likes to use spiral flex because it's easy to use.

The turbulence it creates about halves the airflow and puts a beating on the blower.

If you can use non-spiraled (is that a thing???) then it's a lot better.

The 250B is a hard beast to cool, as the anode is so small and the fin spacing is minute. 

When I'm out in the shop today, I'll pull the blower model number off the 2 x 250 box I'm getting ready to ship out.

--Shane
KD6VXI


Title: Re: Cooling a Pair of 4CX250B
Post by: K1JJ on March 22, 2020, 02:24:00 PM
Shane:

You are absolutely right about spiraled, clothes dryer, plastic hose.  I learned the hard way and mention it sometimes. In addition to backpressure increases, turbulance adds additional air noise.

Look on the web for fiberglass, flexible ducting.  It is smooth inside and strong. The spiral is on the outside where it belongs. No turbulance here.


Also, for runs longer than 6", stay away from metal or PVC ducting. It makes a blower sound like an amplified trumpet. You want accoustical deadening material only.

T



Title: Re: Cooling a Pair of 4CX250B
Post by: K8DI on March 22, 2020, 02:26:19 PM
I did some testing and I still need a bigger fan.  ;)

So I'm both curious and bored...

Maybe something like this
https://www.digikey.com/product-detail/en/mechatronics-fan-group/UF190APA12-H1C2F/1570-1184-ND/5209881 (https://www.digikey.com/product-detail/en/mechatronics-fan-group/UF190APA12-H1C2F/1570-1184-ND/5209881)

data sheet page attached.

Note that sound drops 3db for 200 rpm (50 vs 60 Hz rating). You can probably slow it down with a variac or traic dimmer or whatever and get the sound lower.  This model has a flange. Many of the other similar ones on Digikey or Mouser do not, which makes mounting them a project in itself.  You could mount this in a box with a hose and put it outside the room, too.

Like Shane said, spiral tubing is horrible for airflow.  If you can mount it remotely, in a fixed relation to the amp, you can use ordinary PVC drain pipe -- smooth low loss interior -- or at least for most of the run.

Good luck and don't give up.

Ed



Title: Re: Cooling a Pair of 4CX250B
Post by: KD6VXI on March 22, 2020, 03:30:28 PM
OK.

Went out to the shop and completely disassembled the covers...

The blower on the pair of 250s is....

Sony SSF22C.

It's a 38CFM 10 volt blower.  It's running on a 14 volt power supply in this amp, and has run for a few years.

So it's outputting quite a bit MORE cfm in this application, but unless your blowing the tube out of the socket, you need more CFM.

Incidentally, this amplifier only runs in AM service.  It runs 2.5kv on the plates and output capability is well over a kw pep....  So tubes are run hard by the owner.

Original pair of tubes in it.  It came into my shop to have a switchable transformer or swamped grid input (so 10 or 100 Watt pep in) and variable regulated g1 and g2 supplies.

be prepared, it's not cheap.  I found it for a whopping 6 bucks!

https://www.surpluscenter.com/Electrical/Blowers-Fans/DC-Centrifugal-Blowers/38-CFM-10-Volt-DC-Sony-Blower-16-1505.axd

--Shane
KD6VXI

**edited to include a url and price.


Title: Re: Cooling a Pair of 4CX250B
Post by: Opcom on March 22, 2020, 06:17:48 PM
I have rebuilt the manometer and it works a little better. The right side is inclined at about a 30 degree angle so I have a little better resolution. My tubing is 1/8” ID. Running a few of my fans in my closed off test box I can see a very slight movement in the water when I connect the manometer. Too small to measure even on the inclined side. The only thing that really moves it is my hot air gun. It shows about 2” of H2O pressure. Maybe I will disconnect the heating element and duct tape that thing into the amp (hi).
 

1/8" seems small for a water manometer. Due to capillary action and surface tension and being dirty, it could be erratic at low pressures, but it is moving and maybe it's just the diameter. The tubing should be at least 1/4" and clean too. Mine's dirty as you can see for I let the old water evaporate from it, instead of draining it.

Someone told me that a tiny amount of detergent breaks the surface tension, not sure if it's worth a try.

Fans. some people use them. Blowers are much better at overcoming twists and turns and weird restrictions.

My manometer and video - this was a larger blower removed from a transmitter that used a 4CX5000 and a 4-1000. I am going to use it for a 3CX3000 amplifier. I like a little overkill.


Title: Re: Cooling a Pair of 4CX250B
Post by: W0PWE on March 22, 2020, 07:08:28 PM
Wow there is a bunch of good news here since I last checked! Thanks for all the input.

When scrounging around this morning I found a nice new piece of 3/8” I.D. tubing. I built a new manometer with it and it works great. Easy to fill, easy to read, and now I believe it. So you were correct to question the hose size Patrick – thanks. In my amp I’m still only seeing a lousy 1/8” of pressure but at least I have a way to measure it.
 
Don – I had some fun with your suggestion of stacking two axial fans. I never would have thought of that even though I actually had two of them on my bench sitting one on top of the other. These fans rotate in opposite directions and I think that is desirable. I tried them out with my new manometer and test chamber and they produce about ½” of pressure at zero flow. That’s the best fan I have so far and I’m going to install it in the amp to see what it does for me.
 
But I won’t need to install it permanently because Shane has the solution for me. Gosh Shane that was very nice of you to go out to the shop and tear into that amp to get the info for me. And you found a supplier with an awesome price! I really appreciate that. I have been fan/blower shopping on-line the last few hours and getting depressed. That Sony blower looks like one I saw in a commercial amp and if it works in the beast you have there it ought to cool my little amp just fine. Awesome!

Follow up on some earlier discussions about chimney’s etc. A ham friend has a pair of Eimac sockets and SK606 chimneys that he is going to send me in a few days. If the chimneys don’t work with my sockets I will swap them out the Eimacs. Or I might just swap them anyway.
Thanks again for all the support.  73-Jerry


Title: Re: Cooling a Pair of 4CX250B
Post by: KD6VXI on March 22, 2020, 07:16:24 PM
You're welcome bud.

Should be more than enough to cool an amp that is run at sane levels.

Patrick:  Plain old dish detergent is GREAT at breaking surface tension.  It also breaks droplets up and allows for complete cover of whatever you are spraying, if using as a spray.

Don't need anything special, plain old dawn or palmolive works great.

--Shane
KD6VXI


Title: Re: Cooling a Pair of 4CX250B
Post by: Opcom on March 23, 2020, 12:49:49 PM
Perfect! glad the cooling issue's resolved.

Even though this is done now, just going to throw a related or maybe silly idea on the table here, mostly applying to gear having a long-ish duct.

Longer ducts are common enough in ham-scale transmitters. The KWM-2 uses a quite long 1.5" diameter rubberized duct. Homebrews are anything goes.

The old respectable BC transmitters like the BTA-5F and BTA-10F used a cloth duct from the blower to the tube.

There were not any turns in the ducting, but the cloth would have isolated blower vibration and mechanical noises from the tranmitter or tube. The cloth should have been quiet as well. Anyone ever been around that old a transmitter? I've only seen inside one. The duct was apparently "duck cloth" or some sort of canvas, and was a little slack but not sagging and very well formed to the fit.

One possibility for this today may be to cut off part of the leg of some old "skinny jeans" (all skinny jeans should die) and use that as a duct itself if there's enough pressure, or just the lining for a duct and places where the air flows past. The thrift store may be a source.

Having made a vibration-isolating mount for a blower here, the only failing is the above mentioned metal tubing ductwork, which could have been plastic or anything - this is not yet set in stone at all.

Curves in the duct would require some cutting and sewing of the cloth to fit the shape whether as a lining or the duct itself. I do not know if 0.8 to 1" or even 2" pressure would inflate a cloth duct or if some sort of external plastic-like ribs running its length would be required.

There are not many pictures of the cloth ducting except in the old catalogs.


Title: Re: Cooling a Pair of 4CX250B
Post by: Detroit47 on March 23, 2020, 01:28:44 PM
Hey Shane how is the left coast? What are you working on a Telestar or a John Boy? I like what Bill Orr said and I will quote. That is from the 1979 Armature newsletters. It is on line as a PDF I can post it here I don't know if that is allowed. It is on the Bunker of doom http://www.bunkerofdoom.com/lit/Eimac_amateur_service_newsletters1979.pdf

"No simple rules can be given to accommodate all tube installations in all possible equipments. Tubes
can be damaged by lack of air, but never by too much air, unless the blast is strong enough to lift the
tube out of the socket and smash it against your ceiling! Use the largest blower you can afford. A great
deal can be learned about air flow by puffing cigarette smoke into the blower and observing the path it
takes in quitting the amplifier."

Johnathan N8QPC


Title: Re: Cooling a Pair of 4CX250B
Post by: WD5JKO on March 23, 2020, 03:41:59 PM

OK I can’t test pressure so how about flow? I saw a guy on youtube testing a batch of CPU fans timing how long it takes to inflate a 30 gallon garbage bag. Hmmm, we could start a whole new thread on why that is an invalid test but lets try it. I fixed a 30 gallon bag to my test fixture and my CPU fan inflates it in roughly 10 seconds and by keeping all conditions as consistent as possible the results are pretty repeatable after 3 runs. Converting that number we get about 24CFM. Well, that seems like a ballpark number. At least it’s within the operating range of the fan. So then I removed the squirrel cage fan from the amp and tested it in the same way. It was about the same, maybe 9 seconds.

So then I made a collector out of cardboard that allows me to catch the anode exhaust air in the garbage bag. With the fan re-installed and cardboard chimneys taped in place it took 23 seconds to fill the bag. So that is about 10.5CFM. For a ballpark number that’s believable.


Jerry, you have an impressive QRZ page.

I was reading through your comments, and the CFM tests you did intrigued me. For those that are mentally challenged (me), could you highlight the calculations required to get CFM from the time it takes to fill a 30 gallon garbage bag? Then maybe point out tips on getting repeatable data.

Thanks,
Jim
Wd5JKO


Title: Re: Cooling a Pair of 4CX250B
Post by: KD6VXI on March 23, 2020, 04:13:38 PM
Jonathan,

Neither.  I like Telestar and can't day enough bad things about John Boy. Lol.

This is one built by Chris in Alabama. No regulation of any ki d....  Not even a zener in G1.

It's clean now.  Does respectable IMD.  I pick up my Hermes Lite today (hopefully) so I might be able to run some imd tests on it.

Before, G2 voltage would be 350 unloaded, 330 keyed and 320 full out.

Nice.....  Not.

It's now adjustable from 320 to 200 volts. 

--Shane
KD6VXI


Title: Re: Cooling a Pair of 4CX250B
Post by: WBear2GCR on March 23, 2020, 04:32:16 PM
Patrick,

I suspect the "cloth" in the RCA ad was actually fiberglass... or something like it.

Nomex might work today??

Also I think you meant to say KWS-1 not KWM-2? :D

                 _-_-


Title: Re: Cooling a Pair of 4CX250B
Post by: W0PWE on March 23, 2020, 11:36:47 PM
Jim asked about my goofy flow measurement technique so I will try to answer here. I'm sure this method has plenty of measurement errors but it at least gives me a ballpark idea of flow. I made a "collector" out of cardboard that is basically just a rectangular box with the ends cut out. The collector fits over the tubes so that one of the open ends "seals" to the chassis and all the air coming out the anode fins has to go up the collector and out the other open end. I then attach garbage bag to the top of the collector with a rubber band.

With everything in place in the amp I turn on the fan (HV off of course) and use my phone as a stopwatch to measure the amount of time required to fill the garbage bag with air. Google tells us that one gallon is 0.134 cubic feet so if it takes 26 seconds to fill the bag we can calculate airflow as follows:
(30 Gal/26 sec) x (60 sec/min) x (0.134 Cubic Feet/Gal) = 9.28 Cubic Feet per second
Or simply 241.4 divided by time to fill in seconds.

To help try and achieve consistent results I always squeeze the air out of the bag before I start and I always try to lay the bag out in the same way when deflated. I stop timing just before the bag puffs up like a balloon (as in the picture) because at that point the flow is zero. With a flow rate around 10CFM it takes 24 seconds to fill so if you don't have lightning fast reflexes on the stop watch it doesn't matter too much.

I ordered my fan today. That source Shane gave me is just 3 hours down the interstate from me so it should be here in a day or two. It was so cheap I ordered 2. They also had some handy looking little transformers so I ordered a couple of those too.
73 - Jerry


Title: Re: Cooling a Pair of 4CX250B
Post by: W2BX on March 24, 2020, 08:51:55 AM
Jonathan,

Neither.  I like Telestar and can't day enough bad things about John Boy. Lol.


Are those CB amplifiers?


Title: Re: Cooling a Pair of 4CX250B
Post by: W2BX on March 24, 2020, 09:04:06 AM
Jerry, what a great photo of that garbage bag filled with air sitting on top of your amp!
 :) :)


Title: Re: Cooling a Pair of 4CX250B
Post by: KD6VXI on March 24, 2020, 11:37:19 AM
Jonathan,

Neither.  I like Telestar and can't day enough bad things about John Boy. Lol.


Are those CB amplifiers?


John Boy is.  Telestar made multiband amplifiers.

--Shane
KD6VXI


Title: Re: Cooling a Pair of 4CX250B
Post by: kc2we on March 24, 2020, 03:08:33 PM
Bernoulli's principal. Increase diameter of supply hose to reduce the velocity and noise. Create a plenum box inside to transition to a smaller size hose. U might find some parts and pieces at a shop / store that sells HVAC stuff like Ferguson Plumbing Supply , HD or Lowe's. You might have to play with this to make sure the flow is adequate. Use a manometer to measure the back pressure in the underside of the chassis. U can make one out of some clear tubing if you don't have a commercial instrument. There's another post in this thread about that. ST


Title: Re: Cooling a Pair of 4CX250B
Post by: K8DI on March 24, 2020, 04:23:09 PM
(http://amfone.net/Amforum/index.php?action=dlattach;topic=45702.0;attach=64229;image)

so are AM'ers truly windbags, now?

 ;D ;D

Ed
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands